All about sulfur hexafluoride

Sulfur hexafluoride (SF₆) is a synthetic gas obtained by reacting sulfur with gaseous fluorine. The molecule has an octahedral shape, with six fluorine atoms at the vertices and a sulfur atom in the center. In this compound, sulfur has its highest valence. This structure is the reason for the gas’s extraordinary stability and exceptional chemical inertness, as significant energy is required for its decomposition. SF₆ decomposition due to temperature generally begins only around 500°C. However, in the presence of certain metals, especially metals and their alloys containing silicon, decomposition can occur at 180–200°C. SF₆ is poorly soluble in water, slightly more so in alcohol. Pure gas does not react with hydrogen or metals, and reacts with oxygen only in the presence of electrical discharges. It is colorless, non-toxic, odorless, and non-flammable.

The molecular weight of SF₆ is 146.06, and its density at 20°C and 1 bar pressure is 6.16 g/l, approximately five times that of air. It is thus one of the heaviest known gases.

The thermodynamic properties of SF₆ are described by the Mollier diagram. The critical point of SF₆ is at 37.46 bar and 45.58°C, which allows it to be liquefied by compression for transport and storage.

The application of SF₆ in electrical equipment is due to its excellent electrical properties. It is known that the dielectric strength of gases depends on several factors: the mean free path of molecules, their cross-section, the occurrence of inelastic collisions, and the ability to capture and store electrons during these collisions. Electronegative gases like SF₆ can capture electrons by forming negative ions, significantly increasing dielectric strength by slowing down free electrons.

The dielectric strength of SF₆ exceeds that of air by 1.8–3.0 times depending on testing conditions. In a uniform field, it is about 2.4 times higher. At a pressure of about 3 bar, SF₆ reaches 75% of the dielectric strength of insulating oil in a uniform field, and in a non-uniform field, it may even show better insulating properties than oil.

Sulfur hexafluoride cannot serve as the sole insulating material in a switchgear or circuit breaker — support and bushing insulators must be made of solid insulating materials. These materials operate in the SF₆ atmosphere, so it is important to understand the effect of SF₆ on solid insulating materials. It is necessary to differentiate between materials stressed only electrically in pure SF₆ (e.g., busbar insulators) and those operating in breaker chambers, which are also exposed to SF₆ decomposition products. Surface strength of these insulators in SF₆ is particularly important. Most manufacturers use epoxy resins with special fillers for insulators, fully meeting electrical and mechanical strength requirements. Studies of samples coated with fluoropolymer layers (Teflon) showed a significant increase in surface flashover voltage in the gas. There are also other insulating materials that may come into contact with SF₆ in electrical devices. These materials can show significant property variations depending on the manufacturer, even with similar compositions, so every solid insulating material intended for use in SF₆ devices must be tested (especially during maintenance).

Many studies have been conducted to determine heat transfer capabilities in SF₆. Although the molar specific heat of SF₆ is lower than air, per unit volume, it is 3.7 times higher than air. The thermal conductivity of SF₆, 1.26×10^-4 W/cm×K, is over twice lower than that of air (2.86×10^-4 W/cm×K), but considering convection, the heat transfer capability of SF₆ is higher than air and approaches that of helium or hydrogen. This allows higher current density in conductors (e.g., busbars) in an SF₆ atmosphere compared to air.

A separate issue is the thermal conductivity of SF₆ at high temperatures during arc quenching. Studies show that SF₆ dissociation intensifies at 2000–2100 K and completes around 4000 K, resulting in F and S atoms and a small fraction of diatomic compounds. Under these conditions, the ratio of specific heat to thermal conductivity can be considered constant. Tests under these conditions demonstrated effective heat removal from the arc, reducing its diameter and increasing arc resistance.

The use of SF₆ in breaker chambers is due to its excellent arc-quenching properties. Early tests (1954, USA) showed that its quenching capability exceeds that of air by about 100 times. AC current interruption, especially at low power factors, depends more on the rate of recovery of dielectric strength than the cold gas dielectric strength. The rate of dielectric recovery depends on the thermal and electrical properties of the plasma, including thermal conductivity, temperature distribution, dissociation, voltage drop, arc power and energy, and the arc time constant. Studies in SF₆ confirm the advantages of this gas regarding these factors.

The main quenching properties of SF₆ are related to its dissociation. Dissociation starts at about 2000 K and occurs in stages at varying ionization energies, with dielectric strength increasing rapidly as temperature drops. The arc core conducts practically all current; outside the core, electron density and electrical conductivity are very low.

SF₆ being electronegative also reduces free electron density, equivalent to lowering gas temperature by ~500 K.

A major advantage of SF₆ is that the arc core disappears only at current zero, preventing premature core destruction. Arc column dissipation occurs abruptly 6–7 ms before current zero. This provides an advantage over air or vacuum breakers — there are virtually no overvoltages even when interrupting small currents.

SF₆ must meet quality requirements regarding impurities (see Table 2.1), as these affect gas properties.

Table 2.1. Requirements for technical SF₆ (PN-EN IEC 60376)

Impurities must be limited so they do not jeopardize device operation. For example, water, acidic contaminants, and oxygen together may cause corrosion and improper operation. Water with acidic contaminants can condense at low temperatures and high operating pressures, threatening electrical safety. Impurity levels also affect the type and quantity of secondary chemical compounds formed during thermal SF₆ decomposition (after an arc).

It is recommended to fill and top up devices only with verified SF₆ gas, ideally from a single supplier.

Most often, the state of SF₆ in the device differs from the state of the gas at the time of its filling. It contains impurities that appear at various stages of preparing the device for operation and use [34].

Impurities in the gas within the device are caused to varying degrees by:

• improper selection of the device’s construction materials, which may lead to desorption of moisture into the gas or the formation of impurities through secondary chemical reactions with decomposed SF₆,
• factory assembly errors,
• errors during on-site installation,
• leakage of enclosures and errors in refilling losses,
• gas decomposition due to electrical discharges and switching arcs,
• chemical reactions occurring after discharges,
• operation of internal device mechanisms [31, 32].

Of course, the state of the gas in an operating device fundamentally depends on its intended function. It is different in the enclosed compartments of a shielded switchgear than in a high-voltage circuit breaker or another switching device (e.g., a disconnector). We will discuss these issues primarily based on the typical “product life cycle diagram” shown below, which is most often analyzed according to the ecological procedures of “Cleaner Production” [44].

In such a “product life cycle diagram,” attention is paid to how the state of impurities and associated hazards may be affected at all stages of the product’s existence: from production to disposal.

The device designer can influence not only the functional properties of the designed device but also the condition of the gas contained within it. Proper selection of construction and insulating materials is crucial. This involves both their chemical reactivity with SF₆ (especially with decomposition products) and the elimination of porous materials – which absorb moisture and air before assembly and release these substances into SF₆. Choosing an appropriate sealing design – with high efficiency and durability – limits gas losses and the possibility of introducing impurities during refilling [33, 34].

Table 3.1. Product life cycle phases and the state of SF₆ impurities

In the case of circuit breakers, optimizing the quenching chamber is extremely important. This involves reducing arc duration (decreasing the energy supplied by the arc) and limiting the gas to the necessary amount. The selection of the type and volume of adsorbent is also important – ensuring effective performance throughout the operational period. These issues should be resolved during the design tests of the device.

The manufacturer should ensure proper technology for producing components (surface smoothness) and dry, clean assembly at all stages. During assembly, contact of components with atmospheric moisture should be minimized, components prepared for assembly must be sealed in foil and stored in a dry room. At the final stage of assembling switching devices, the adsorbent is installed – its quality has a decisive impact on the subsequent state of the gas. In all SF₆ devices, the state of the gas depends on excellent assembly tightness, achieving a high vacuum (drying the interior) before filling, and adherence to gas filling procedures.

The user has virtually no influence on the above quality assurance conditions (except for selecting the supplier). Their role begins with the installation and commissioning of the devices. Even if the installation is performed on-site by the supplier, the recipient should ensure proper storage conditions (as brief as possible) and supervise the assembly process. Gas refilling (according to procedures) and acceptance tests (according to acceptance conditions) must be performed correctly. Following the operating instructions of the device is crucial during further use. Proper compensation of gas losses – i.e., refilling – is very important for the durability of the device and the level of impurities in the gas.

High-voltage SF₆ circuit breakers present a separate issue regarding the state of the gas. Regardless of technological impurities introduced into the interior of the circuit breaker, as with switchgear compartments, decomposition products and their secondary chemical compounds appear. During operation, it is necessary to ensure that the maximum breaking current and the maximum number of operations (according to the switching capability diagram in the operating instructions) are not exceeded. Gas decomposition due to electrical discharges and arcs is the main cause of toxic compound formation [35].

The primary method for checking the state of the gas – especially in circuit breakers – is to take an SF₆ sample and perform diagnostics, preferably chromatographic. The inspection frequency should be determined by the device manufacturer. In principle, gas condition inspection is carried out every 1 to 5 years during periodic maintenance.

Monitoring the gas condition during operation should confirm that impurity levels do not exceed permissible values.

Table 3.2. Types and permissible amounts of impurities during gas operation according to PN-EN IEC 60480 [41]

If, during inspection, it is found that the impurity concentration exceeds the permissible level, a gas replacement procedure should be carried out.

The final phase – dismantling the device after full operation or damage – is the most critical stage in terms of personnel and environmental hazards, especially for the highest-voltage circuit breakers. This work should be performed by a specialized team under proper procedures.

Decommissioning a circuit breaker does not necessarily require dismantling the poles into parts at the substation. It is always necessary to reduce the gas pressure to a slight overpressure relative to atmospheric pressure. This should be done by pumping the gas into cylinders.

Maintaining slight overpressure in the circuit breaker poles prepared for transport (e.g., to the manufacturer) aims to prevent moisture ingress. Moisture causes:

• change of the nature of powdery residues from unbound deposits to sticky products adhering to the internal elements of the quenching chamber. In this case, more hydrolysis products also appear,
• formation of aggressive gaseous secondary reaction products that are corrosive to structural elements and highly toxic.

Enclosed switchgear for the highest voltages is the primary reason for using sulfur hexafluoride in power equipment. As mentioned earlier, this was determined by the good insulating properties of this gas. Once this gas was introduced, it became possible to design high-voltage switchgear in enclosures — modeled after low-voltage switchgear — commonly referred to as metal-clad.SF₆-based technology has several significant advantages [20]:

• by utilizing the excellent insulating and arc-interrupting capabilities of sulfur hexafluoride, equipment dimensions have been significantly reduced, which in turn allows for:
• reducing the footprint of the installation and improving the layout of the power substation,
• a significant reduction in the number of components, which is associated with lower consumption of raw materials and energy in production, manufacturing processes, machining, operation and disposal,
• the hermetic enclosure of high-voltage busbars in grounded housings makes the SF₆-based system independent of atmospheric pollution and degradation processes, and also enables:
• a significant extension of equipment reliability lifetime,
• greatly reduced maintenance, inspection and repair requirements, resulting in higher reliability, durability and availability — meaning constant readiness for operation,
• reduction of energy losses and fire hazards.

The above statements demonstrate that, apart from the excellent technical parameters, operational reliability and economic aspects, SF₆ insulation has no alternative solutions that would surpass it from an ecological point of view. This allows for the most favorable solutions when environmental circumstances throughout the entire service life cycle and total costs are taken into account [20].The development of enclosed high-voltage switchgear began with the construction of switchgear rated at 123 kV. A hermetically enclosed high-voltage gas-insulated switchgear (GIS) with SF₆ insulation resembles a conglomerate of metal pipes and tanks of considerable dimensions [10, 11].In order to evaluate the overall design of such a structure, a number of specific classification criteria must be introduced — criteria not generally used for other types of switchgear construction. These classification criteria fall within several key groups of issues. Omitting secondary aspects, the classification of switchgear can be presented as follows:

• with regard to the method of busbar insulation:
  • switchgear with single-phase insulated busbars,
  • switchgear with three-phase insulated busbars,
• with regard to the busbar arrangement:
  • single busbar system,
  • double busbar system,
• with regard to the switching capability of the switching device used:
  • circuit-breaker switchgear (bays),
  • disconnector switchgear (bays),
• with regard to the circuit-breaker mounting orientation:
  • switchgear with horizontally mounted circuit-breakers,
  • switchgear with vertically mounted circuit-breakers,
• with regard to the supporting structure:
  • switchgear with a separate supporting structure,
  • self-supporting switchgear (enclosures also serve as the supporting structure),
  • composite construction, special supporting frame with bay enclosures mounted on it,
• with regard to the installation location:
  • indoor switchgear,
  • outdoor switchgear.
• with regard to the type of circuit-breaker:
  • circuit-breakers with SF₆ as the arc-quenching medium,
  • vacuum circuit-breakers [9].

Individual manufacturers designed the components (elements) of switchgear so as to allow different switchgear configurations, different switching devices, different arrangements and connections (cables, overhead lines). In the case of enclosed switchgear for the highest voltages, a bay-type configuration is generally used. Such switchgear is most commonly installed indoors. It is considered that, from a configuration standpoint, the most favorable arrangement is cable routing downward, with busbars placed at the top. For large substations, a double busbar system is most commonly used, and in all such solutions the cables are routed downward.Enclosed switchgear contains the same set of apparatus as conventional switchgear, but with a different design — suited for use in a closed enclosure and for SF₆ insulation. Therefore, the primary advantage of SF₆-insulated enclosed switchgear is its significantly reduced physical size.When analyzing the designs of switchgear from various manufacturers, certain “architectural” differences in construction can be observed, but these do not have a significant impact on their operation. The use of stainless and non-magnetic steel enclosures, rolled aluminum enclosures, and cast aluminum enclosures can be noted. Manufacturers typically emphasize the superiority of their chosen enclosure type.Naturally, manufacturers did not stop at building 123 kV switchgear. Leading companies subsequently installed 245 kV, 300 kV and 525 kV switchgear. Most importantly, as the rated voltage increases, the size ratio between conventional switchgear and enclosed switchgear grows. As a result, the floor area occupied by the switchgear is reduced many times over, and the overall economic benefit of the installation becomes apparent due to reduced land costs.For enclosed switchgear at the highest voltage levels, the high operational reliability (low failure rate) is of particular importance, as risks from external factors are eliminated (pollution, lightning strikes, insulator cracking, birds, etc.). In order to give the reader some insight into the structural details of the interior of an enclosed switchgear, we select as an example a switchgear already installed in Poland — the ABB switchgear (Fig. 6.1) [38].Figure 6.1. Cross-section of a circuit-breaker bay with cable connection of the 123 kV ELK-0 (ABB) switchgear: 1 – double busbar system, 2 – circuit-breaker, 3 – current transformer, 4 – voltage transformer, 5 – cable connection, disconnector and earthing switch compartment, 6 – disconnector–earthing switch drive, 7 – control cabinet [38]Basic parameters of the presented switchgear:

• rated voltage: 72.5 – 170 kV,
• rated current: 1250 – 3150 A,
• SF₆ pressure outside the circuit-breaker (absolute): 420 kPa,
• rated breaking current of the circuit-breaker: 25/31.5/40 kA,
• SF₆ pressure in the circuit-breaker: 600 kPa.

To appreciate the miniaturization benefits of this type of switchgear, attention should be drawn to the dimensions of the bay and the minimum room required for it — as shown in Fig. 6.2 [38].Figure 6.2 presents a top and side view of a five-bay switchgear in the H configuration (ABB), containing:

• in feeder bays F1 and F4:
  • circuit-breaker,
  • disconnector–earthing switch on both sides of the circuit-breaker,
  • earthing switch on the feeder side,
  • current and voltage transformers,
• in coupler bay F3:
  • circuit-breaker with current transformer,
  • disconnector–earthing switch on both sides of the circuit-breaker,
• and in outgoing feeder bays F2 and F5:
  • disconnector–earthing switch on the busbar side,
  • earthing switch on the outgoing feeder side,
  • voltage transformer [38].

Anyone familiar with the appearance and space requirements of a five-bay switchgear (in the H configuration) in conventional design must acknowledge that the enclosed SF₆-insulated switchgear occupies many times less space.

The first circuit breakers using sulfur hexafluoride as an arc extinguishing agent were developed approximately 10 years earlier than SF₆ insulated switchgear. The results of arc extinction tests in SF₆ became the foundation for the idea of designing circuit breakers for the highest voltages. Westinghouse introduced the first circuit breaker into operation in the first half of the 1950s (at 115 kV). It was a six-break circuit breaker with capacitive voltage distribution control [26, 27].

Figure 7.1 shows in the simplest way the physical mechanism of arc extinction in the quenching chamber of an SF₆ circuit breaker. We can see that during contact opening, simultaneous gas compression occurs in a movable cylinder. Due to the closure of the critical nozzle cross-section by the fixed contact and the arc, in the initial phase of circuit breaker opening, gas is compressed in the movable cylinder (item B). When the contacts separate at a distance where arc extinction can occur, the gas pressure is so large that its strong flow occurs in the arc zone. Thus, arc extinction is based on the principle of self-compression and self-regulation of gas flow causing arc cooling. The arc column limits gas flow and causes momentary, additional pressure increase, proportional to the value of the interrupted current [16, 17].

[Figure 7.1. Process of arc extinction in the quenching chamber of an HV circuit breaker with SF₆ (according to MAGRINI GALILEO): A – chamber in closed position, B – beginning of interruption, C – discharge of compressed gas – arc extinction [16, 17]]

During interruption of operating currents (below rated current), the phenomenon of gas flow damping is negligible and arc extinction occurs immediately when the contacts separate at a distance where the dielectric strength of the gap at the moment of current zero crossing is sufficiently large [29, 30].

Interrupting short-circuit current – especially of a value close to the breaking current – is associated with blocking of SF₆ flow by the nozzle, which promotes pressure increase due to gas compression in the cylinder and its heating by the arc in the nozzle zone. Only the disappearance of the arc column near the zero crossing of the current enables gas flow, cooling of the arc column and recovery of the dielectric strength of the gap. The process proceeds very “smoothly” – arc extinction and current interruption at zero, which helps limit the so-called switching overvoltages [7].

In practice, three basic design solutions are used for the quenching chamber regarding the method of gas flow, compressed in the cylinder during contact opening, into the arc zone (Fig. 7.2) [38].

[Figure 7.2. Models of quenching systems in quenching chambers with SF₆; a – single-stream chamber with insulating nozzle, b – two-stream asymmetric chamber with insulating nozzle, c – two-stream symmetric chamber with conducting nozzles (metal, graphite) [38]]

It was noted that in the quenching chambers discussed above, there is no possibility of fully utilizing the portion of compressed gas and the energy capacity of the drive. When interrupting small currents, there is no “blocking” of the nozzle by the arc and gas flows out almost freely from the compression reservoir. Hence, the gas pressure increase is relatively small. To interrupt a circuit with high voltage recovery rate, it is necessary to ensure an appropriate pressure value. This requires a drive guaranteeing high circuit breaker opening speed. Conversely, during interruption of large short-circuit current (e.g., breaking current), the nozzle is “blocked” by the electric arc and gas flow is stopped. The drive is slowed down, because high SF₆ pressure in the compression cylinder causes great resistance to movement. With improper selection of the compression cylinder dimensions and drive energy, an unfavorable rebound (so-called bouncing) of the movable contact may occur until the moment of zero current crossing. Only in the zero current zone does the nozzle gas flow open and the drive motion is restored. Eliminating these “rebounds” in contact movement requires an appropriately powerful drive. The energy of such a drive is not effectively utilized when interrupting smaller currents. This defect was minor when using hydraulic drive, and particularly significant with spring drives.

[Figure 7.3. Diagram of self-compressing quenching chamber with thermal expansion; A – closed position, B – operation of compression chamber (V₂), C – thermal expansion action [38]]

The latest generation of SF₆ circuit breakers has a modified self-compressing chamber so that the discussed operating defects are eliminated [38].

For example, companies ABB, AEG and GEC Alsthom applied a two-stage gas compression system in their quenching chamber designs (Fig. 7.3). The compression chamber is here divided into two volumes V₁ and V₂, connected by valves. In the first phase of cylinder movement during circuit breaker opening, gas is compressed in volume V₂ and pumped through opened valves to space V₁ and further into the arc zone. Thus conditions were created for extinguishing small currents (e.g., operating currents). When the nozzle is “blocked” by a large short-circuit arc, a phenomenon of strong “thermal expansion” appears, i.e., pressure increase of gas in volume V₁ as a result of its temperature increase (heating by the arc). Valves between volumes V₁ and V₂ close, preventing reverse gas flow. Since volume V₂ continues to decrease, further SF₆ pressure increase occurs in it – which could cause drive motion slowdown. This is prevented by opening the “safety” valve in the piston and releasing gas from this volume. In this way, automatic pressure regulation in the arc extinguishing zone occurs. A circuit breaker with such a chamber can have an economical spring drive [38].

[Figure 7.4. Quenching chamber of ELK type (ABB): 1 – contact for conducting continuous current, 2 – erosion contact (conducting current during interruption), 3 – nozzle (insulating), 4 – SF₆ compression cylinder, 5 – gas space [38]]

In Fig. 7.4 we see the quenching chamber of the ELK circuit breaker (ABB), in which a two-stage current interruption process is applied. Note that the quenching system of this chamber is highly optimized – which is a result of many years of research by designers. The chamber has the mentioned above two compression sections (V₁ and V₂) utilizing the processes of self-compression of gas, thermal expansion (i.e., pressure increase of SF₆ as a result of heating by taking energy from the arc) and ablation, i.e., pressure increase of gas as a result of gasification (evaporation) of nozzle material. While in the self-compressing chamber gas pressure depends only slightly on the value of the interrupted current, in a chamber utilizing thermal expansion and ablation, a clear dependence of gas pressure in the nozzle on the value of the interrupted current is achieved [38].

Since one of the factors of gas pressure increase during arc extinction is ablation (gasification of nozzle material), a question may arise about the degree of nozzle wear in multiple interruption processes. Nozzle durability tests show that within the range assigned to the circuit breaker’s switching capability, there is no excessive nozzle wear. This results from two facts: appropriate selection of material and exposure of the nozzle to ablation only during interruption of large short-circuit currents.

It is easy to notice from the above descriptions of quenching chamber operation, why in the gas of circuit breakers, in addition to products of gas decomposition, there are compounds of these products dependent on the material of contacts and nozzles. Each subsequent current interruption involves another portion of chemical compounds as a result of arc action on gas and contact and nozzle materials. A significant part of these products is adsorbed in special filters consisting of Al₂O₃ adsorbent granulate.

Magrini Galileo company applied in its SB6 123 – 245 kV circuit breaker a single-break quenching chamber with its own drive mechanism (Fig. 7.5). All its elements (quenching chamber, insulating column and drive) are filled with SF₆ gas, which in this circuit breaker is an insulating, extinguishing and drive agent for movable contacts during closing and opening. Closing and opening of the circuit breaker is performed by a double-acting piston, directly connected to the movable contact.

The source of energy for the drive is SF₆ gas at working (higher) pressure, located in the column and quenching chamber. Energizing the opening or closing solenoid valve causes the control valve to switch. The result is opening of the main valve and gas flow from the upper part of the pole to the cylinder with the double-acting piston. In this way, the piston, under gas pressure, moves and operates the movable contact of the quenching chamber. After completion of the switching operation, gas flows to the lower expansion chamber. A micro-compressor, controlled by a pressure sensor differential relay, replenishes the gas amount in the chamber to the initial pressure. Holding the movable contact in extreme positions – open or closed – is ensured by a two-position bistable spring mechanism, independent of the presence of gas in the drive cylinder [38].

[Figure 7.5. Cross-section of the column of circuit breaker type SB6 by Magrini Galileo: A) space at working (higher) pressure, B) low-pressure expansion space, 1 – safety valve, HV connections, 3 – fixed erosion contact, 4 – nozzle, 5 – main movable contact, 6 – insulating pull rod, 7 – filter, 9 – set of closing valves, 10 – auxiliary contacts block, 11 – micro-compressor, 12 – valve for air evacuation and gas filling, 13 – molecular sieve, 14 – main fixed contact, 15 – movable erosion contact, 16 – upper insulator, 17 – lower insulator, set of opening valves, 19 – double-acting piston, flat springs of bistable mechanism, 21 – socket for control circuits, 22 – temperature compensated pressure sensor [38]]

SF₆ circuit breakers in the voltage range of 12 to 36 kV have more varied design than those for high voltages. Different companies found their own design – convenient for their own technology. However, the principle of operation of most quenching systems results from the known self-compressing systems. In the 1980s, circuit breakers with rotating arc also appeared in this voltage range, in which the effect of magnetic field on the electric arc and the possibility of arc cooling in SF₆ as a result of its rapid movement were utilized.

An example of a self-compressing circuit breaker is the HC type shown in Fig. 7.6 (ABB company). Each pole of the circuit breaker is enclosed in a separate epoxy resin housing. In order to eliminate wear of the movable seal, a different sealing method was used for the drive lever than in other circuit breaker types. This type of circuit breaker, with rated current from 630 A to 2500 A, has a breaking current of up to 25 kA at voltage up to 24 kV [38].

[Figure 7.6. Cross-section of the pole of self-compressing circuit breaker type HC (ABB company) in open state: 1 – fixed contact assembly, 2 – continuous current contacts, 3 – fixed erosion contact, 4 – movable erosion contact, 5 – insulating nozzle, 6 – quenching gas discharge space, 7 – shaft (common to 3 poles) 8 – movable seal, 9 – drive shaft, 10 – drive lever [38]]

ABB company introduced into production a medium-high voltage circuit breaker with rotating arc (Fig. 7.7) [38].

[Figure 7.7. Cross-section of the pole of rotating arc circuit breaker type HB (ABB company): 1 – connection, 2 – cylindrical coil, 3 – contact lamellae, 4 – “raceway” of rotating arc, 5 – movable contact, 6 – quenching chamber, 7 – hot gas evacuation area, 8 – drive lever, 9 – drive shaft, 10 – auxiliary compression piston [38]]

During circuit breaker opening, the electric arc ignites between erosion contacts: a fixed contact in the form of a disc and a movable tubular contact. The interrupted current then flows through a cylindrical coil, generating a magnetic field acting on the arc (as on a current-carrying conductor). As a result, a force appears directed transversely to the arc axis, causing its rotary motion. During this motion, arc cooling is so effective that during zero current crossing, current interruption occurs [38].

Medium voltage circuit breakers with chambers self-generating SF₆ pressure are also produced. Characteristic of this circuit breaker is the complete separation of the path for conducting continuous current and during interruption. During circuit breaker opening, the “isolating switch” opens first, i.e., the contact conducting continuous current. Current is commutated to the erosion contact. Then the erosion contact opens and the arc ignites. While the movable erosion contact is within the space closed by a special insulating nozzle, the burning arc heats a certain portion of gas. Its pressure increases as a result of thermal expansion and ablation. When the contact further moves out of the nozzle, gas is discharged to the expansion space and the arc is extinguished (during zero current crossing). The switching parameters of this circuit breaker, with its arc extinction technique, strongly confirm the excellent extinguishing properties of SF₆.

Sulfur hexafluoride, due to its excellent dielectric properties, finds application in other electrical power equipment (not only in switchgear compartments and disconnectors).

High-voltage busbars with SF₆ insulation are increasingly used in cases where safe energy supply is needed, even over considerable distances – resistant to the effects of contaminated atmosphere (e.g., zones of chemical exposure) (Fig. 8.1) [30].

[Figure 8.1. High-voltage busbar filled with SF₆ and SF₆ – air bushing;
1 – porcelain insulator, 2 – current path, 3 – SF₆ space, 4 – internal insulator, 5 – control screen for electric field distribution, 6 – internal SF₆ space, 7 – pressure gauge [38]]

Interestingly, following current and voltage transformers with SF₆ used in enclosed switchgear (Fig. 8.2), transformers insulated with this gas instead of oil also appeared for use in conventional switchgear.

[Figure 8.2. Transformers of enclosed switchgear: on the left current transformer; 1 – barrier insulator, 2 – bushing insulator (secondary circuits), 3 – winding body, 4 – winding, 5 – secondary circuit terminal strip; on the right voltage transformer: 1 – barrier insulator, 2 – magnetic core, 3 – primary winding, 4 – secondary winding, 5 – secondary winding terminals, 6 – connection, 7 – expulsion diaphragm [38]]

More than fifty years of experience using SF₆ in electrical power equipment has shown that no serious occupational health and safety problems arise, provided that certain precautions are taken and established procedures are followed. Numerous international studies emphasize that in all cases where operating personnel suffered injuries, safety procedures were not followed or personnel were not equipped with appropriate protective equipment. It can be concluded that working with SF₆ equipment is not dangerous, only failure to follow procedures is dangerous.

Employees working with equipment containing SF₆ should be trained in:
– information about the properties of primary SF₆, the formation of decomposition products and their effects on the human body,
– principles of safe work with SF₆ equipment,
– use of auxiliary equipment for working with equipment containing SF₆,
– use of general and personal protective equipment, and hygiene principles,
– handling contaminated gas and solid decomposition products to eliminate hazards to people and the environment,
– first aid provision.

Switchgear operators should be provided with:
– a separate room for changing clothes, storing protective clothing and personal protective equipment,
– access to washrooms with hot and cold water,
– sealed containers for storing used clothing and materials that have been in contact with SF₆ decomposition products,
– a separate room for storing personal clothing, eating meals and rest.

During work with sulfur hexafluoride, there may be five following cases with different degrees of hazard to personnel [40]:
– work with primary gas,
– SF₆ leakage during normal operation of the working device,
– work with hazards from SF₆ decomposition products: maintenance or service work, expansion of enclosed switchgear,
– emergency operation of switchgear device: e.g., internal short circuit or external fire causing housing seal failure,
– decommissioning of switchgear device, gas removal and powder contaminant removal.

Due to the specificity of the hazard and procedures, each case will be discussed separately.

Table 9.1. Reactivity and toxicity of gaseous SF₆ decomposition products [35]

| Decomposition product | Chemical stability in air | Stable reaction products | Degree of toxicity TLV | Threshold value | Odor |
|—|—|—|—|—|—|
| S₂F₂ | decomposes rapidly | S, HF, SO₂ | 0.5 | – | pungent, acidic |
| SF₂ | ibid. | ibid. | 5.0 | – | ibid. |
| SF₄ | ibid. | HF, SO₂ | 0.1 | – | ibid. |
| SOF₂ | slow decomposition | SO₂F₂, HF | 0.6÷1 | 1.0÷5 | rotten eggs |
| SOF₄ | decomposes rapidly | ibid. | 0.5 | – | like HF |
| SO₂F₂ | stable | SO₂F₂ | 5.0 | – | none |
| SO₂ | stable | SO₂ | 2.0 | 0.3÷1 | pungent |
| HF | stable | HF | 1.8÷3 | 2.0÷3 | ibid. |
| WF₆ | decomposes rapidly | WO₃, HF | 0.1 | – | like HF |
| CF₄ | stable | CF₄ | non-toxic | – | none |

When determining limiting concentration values of compounds for the situations mentioned above that may arise during operation, so-called threshold limit values (TLV) are used, i.e., the maximum permissible concentration that does not have a negative effect on people working eight hours a day throughout their working life.

Sulfur hexafluoride is supplied in pressure cylinders of various volumes in liquid state. Primary SF₆ must meet the requirements of PN-EN IEC 60576 and the supplier should provide a gas quality certificate.

When working with pure SF₆, the permissible TLV level in the room is 1000 ppm_V (SF₆) [40] – which corresponds to a concentration of 6000 mg/m³. Pure gas in the room atmosphere can appear as a result of leaks from unsealed equipment containing this gas, from cylinders with unclosed valves, or due to improper staff handling. Natural ventilation is sufficient to reduce the concentration of pure SF₆ or completely remove it from the room. In rooms where pure SF₆ may accumulate, devices with open heaters must not be used, welding is prohibited, and equipment with temperatures exceeding 200⁰C must not be used. Smoking is prohibited in rooms!

Rooms where SF₆ equipment is installed, gas containers are stored, or work is performed should be marked with:
– a sign at the entrance reading “SF₆“,
– signs inside reading “No smoking” and “No open flame”.

During work in enclosed spaces (e.g., inside a switchgear building) related to filling equipment with gas, the gas may accumulate in the air for a period of time. Without ventilation, the permissible concentration level may be exceeded. Therefore, it is recommended to develop a written procedure providing the method and procedure for periodic measurement of SF₆ concentration in air. It must be adapted to the measuring equipment used in given conditions. If measurements show that the level of 1000 ppm_V has been exceeded, the room should be ventilated until the concentration drops below the limit value.

Special care should be taken in low-lying rooms (cable ducts, basements) where gas heavier than air may accumulate. When working in these rooms, forced ventilation should be used to ensure air flow.

When work is performed in open areas, natural “ventilation” protects against gas accumulation. Special precautions in these conditions should be observed in the immediate vicinity of equipment.

Transport and storage of SF₆ cylinders by users requires compliance with the following rules:
– cylinders with SF₆ gas must be transported and stored with closed valves and screwed-on caps. Cylinders must not be dropped or overturned; they must be protected from mechanical impact (e.g., blows). Transport should be carried out on special trolleys with cylinders secured,
– gas storage should be in separate, ventilated rooms, away from other types of gases, away from heat sources, flammable and explosive materials. The room must not have heaters with open elements with temperature above 200⁰C. Gas leaks flowing into other rooms (e.g., basements) where people work must be prevented,
– sunlight exposure on cylinders with gas is prohibited. Like any compressed gas, SF₆ can cause cylinder explosion if excessively heated,
– it is impermissible to unscrew or loosen the regulator screwed onto the cylinder connection when the valve of the pressurized gas cylinder is not closed,
– releasing gas from the cylinder in a large stream is not recommended – the hand opening the valve may freeze to it,
– gas-emptied cylinders should be separated, with closed valves and protective caps screwed on,
– protective gloves must be worn during transport and when handling cylinder valves.

Gas recovered from operating equipment should be stored in separately marked cylinders – preferably in different rooms than primary gas.

Basic work with primary SF₆ is filling new equipment or equipment after repair with gas. Primary gas during filling of switchgear equipment must be transferred from containers containing gas (e.g., cylinders) under high pressure to the housings of apparatus whose rated pressure is significantly lower. Therefore, during filling, the following recommendations must be observed:
– use equipment adapted for this operation,
– tubing connections must be properly protected against mechanical damage,
– valves and regulators must always be in working order,
– pressure gauges (manometers) should be regularly calibrated,
– pipes and valves connected to them, used for gas transfer, should be connected in such a way that they can be dismantled from the filled housing without risk of gas contamination.

The vast majority of equipment constitutes so-called sealed pressure systems, which means that after filling at the factory, during the entire period of operation (from installation until decommissioning), no gas replenishment is required.

Some equipment constitutes so-called “closed pressure systems”. Equipment of this type must be filled or replenished at the installation site.

Procedures used in the gas filling (replenishment) process during equipment commissioning should be defined so that four fundamental requirements are met:
– operating personnel must not be exposed to unjustified risk,
– gas leakage to the atmosphere must be minimal,
– leakage from the housing after filling must not exceed the permissible level,
– housings after filling should contain gas of required quality (specified by the manufacturer).

The most commonly used method of filling switchgear equipment housings involves:
– evacuating air from the selected closed compartment of the switchgear using a vacuum pump to the residual pressure – specified by the apparatus manufacturer,
– checking tightness (under vacuum) according to the manufacturer’s recommendations. Such a procedure also promotes internal moisture evaporation and its evacuation during re-activation of the vacuum pump,
– slowly filling with gas to rated pressure. Equipment must not be filled above the rated pressure specified by the manufacturer (for example, the SF₆ pressure sensor may be damaged),
– checking the tightness level in the case of equipment installed or filled with gas at the installation site. The manufacturer’s instruction should specify the method of tightness checking and the equipment needed for this.

Eliminating leaks in switchgear equipment is necessary for three fundamental reasons:
– due to equipment operating conditions; switching capability and insulating ability decrease when gas density drops as a consequence of leakage,
– personnel safety. Gas leaks mean the possibility of gas accumulating in the room and increased gas concentration in the atmosphere. The hazard increases when leakage occurs from circuit breakers,
– negative impact on climate.

The most important conditions that must be met during proper gas filling (and replenishment) of equipment are:
– high vacuum before filling,
– guaranteeing filling purity and thus guaranteeing good gas quality,
– filling to the proper gas pressure (taking into account gas temperature),
– minimizing gas leakage to the atmosphere.

Gas should be released from the cylinder in a small stream. If gas flows too rapidly from the cylinder (no regulator or improper adjustment), the cylinder valve may freeze due to adiabatic expansion. To prevent the hand opening the valve from sticking, this operation should be performed wearing protective gloves.

When transferring SF₆ from liquid state, two isolating valves in the connecting line must not be closed simultaneously to trap a portion of liquid SF₆, because temperature increase may lead to explosion.

Before beginning the gas filling (replenishment) operation of SF₆ equipment, the ambient temperature should be measured and the rated pressure value at that temperature determined from the gas state diagram (e.g., Fig. 9.1).

The filling pressure value of a given device should be checked after the gas temperature has equalized with the ambient temperature – waiting approximately 1 hour.

Some instructions provide a table of SF₆ pressure values for individual densities (lines a, b, c) depending on temperature. In practice, these tables should always be used. If such a pressure correction table is not available, the diagram should be used (Fig. 9.1).

The switching capability of the apparatus and electrical strength of the gas depend, among other things, on gas density. In the case of SF₆, gas pressure at a given density depends significantly on temperature – for example, looking at line a we see that at –20⁰C we have pressure of approximately 0.34 MPa (3.4 bar) and it increases to approximately 0.54 MPa (5.3 bar) at 40⁰C. Therefore, each time gas is replenished in equipment, it is essential to check what the proper pressure value is at that temperature.

[Figure 9.1. Dependence of SF₆ pressure on temperature: A – liquefaction line (liquid-gas state line), B – gas density line, a – gas density line at filling time, b – signal density line, c – blocking density line]

Example 9.1.

The rated pressure of a circuit breaker is 500 kPa (i.e., 0.5 MPA at 20⁰C). What should the pressure be when replenishing losses at 30⁰C, and what at –30⁰C?

Looking at line “a” we find that:
– at temperature 30⁰C the pressure is 0.52 Mpa (5.2 bar),
– at temperature -30⁰C the pressure is 0.4 Mpa (4 bar) [28].

The issue discussed above regarding determining SF₆ pressure corrections does not apply to pressure sensors installed in equipment. They are designed in such a way that they are activated by changes in gas density [28].

Replenishing SF₆ in equipment where pressure drop has been detected (e.g., the first stage of the sensor has activated) is essentially work with pure gas. This is the case provided that when connecting (or disconnecting) the connecting line to the circuit breaker we do not cause leakage of an unnecessary portion of contaminated gas from it.

When filling (replenishing) SF₆ equipment, the following should be used:
– protective gloves,
– safety glasses (chemical type).

Manufacturers’ instructions should specify how often and when testing of SF₆ gas samples is necessary, provide the method and equipment, and specify the limit values for contaminants [38]. This check is intended to confirm that the gas condition ensures proper equipment operation.

Of fundamental importance is obtaining a sample representative of the gas in the equipment, therefore care must be taken to ensure that gas sampling does not pass through filters that may be installed in the equipment and to ensure sufficient gas flow to obtain a representative sample.

Testing of gas condition from electrical power equipment can be performed by two methods:
– directly at the equipment – using the so-called field technique, involving the use of portable analyzers, this is a cheaper but less accurate method,
– by laboratory method, with sample collection into an intermediate container, enabling complete testing of contaminant content.

The dimensions of the intermediate cylinder should correspond to the volume of SF₆ required for laboratory testing. It should be assumed that this volume should be from 2 to 3 liters, since such an amount is needed to perform complete gas testing, i.e.: purity testing, composition, moisture content and acidity.

Special attention should be paid to the cleanliness of the container and the sampling process. Contamination of the intermediate reservoir will be added to the contaminants already present in the monitored equipment. Therefore, cylinders for collecting samples cannot be used for substances other than SF₆. After each use, they must be thoroughly emptied and evacuated.

Sampling of gas for diagnostics from operating equipment – especially circuit breakers – is treated as work with limited hazard from SF₆ decomposition products. Necessary protective equipment must be used during these operations and effective room ventilation must be ensured.

At the current levels of equipment tightness – even in the case of installation in a room, there is no need to fear that gas concentration in air will reach a level hazardous to personnel (1000 ppm). The general conclusion, also confirmed in international studies, states that the risk to personnel health, even when leakage is significantly higher than permissible, is very small.

The relevant standards specify three types of equipment filled with SF₆ gas:
1. a controlled system, which is automatically replenished from an external source (rarely used),
2. a closed system, which is periodically replenished from an external source by operating staff,
3. a sealed system, requiring no replenishment, completely assembled and filled and tested at the factory.

Gas leaks can only be discussed in cases a and b (mainly b), since leakage in a type c system apparatus leads to equipment malfunction and the need for decommissioning.

We already know that gas leaks should be limited for the following fundamental reasons:
– due to equipment operating conditions; switching capability and insulating properties decrease when gas density falls as a consequence of leakage,
– personnel safety,
– negative impact on climate.

For these reasons, manufacturers improve sealing systems and currently the tightness of SF₆ equipment is specified by the number of years 10, 20 or 30 – corresponding to tightness of approximately 0.1%. Medium voltage equipment has gas working pressure only slightly higher than atmospheric, and here leaks are definitely limited.

The hazard to operating staff posed by SF₆ leaks depends mainly on the gas condition, its concentration in the atmosphere and the location of equipment installation.

Gas escaping from equipment in which it has not undergone thermal decomposition is not contaminated (most often in switchgear and busbars) and practically there is little risk of exceeding a dangerous state. Conversely, leaks from equipment in which the gas is contaminated (e.g., circuit breakers) pose some risk. Fortunately, SF₆ contamination is perfectly detectable by smell (hydrogen sulfide – rotten egg smell), already at the level of 1÷5 ppm_V, i.e., below the TLV factor for this situation.

Under conditions of SF₆ contamination, its concentration in the room, determined by the TLV factor, is 1.6 ppm_V for SOF₂ and 200 ppm_V for SF₆.

Concentration determined by the TLV factor is specified for prolonged work. It is therefore worth knowing that momentary exposure to SOF₂ at concentration up to 500 ppm_V does not pose a health risk. This means that manipulations can be performed under these conditions, but not longer work.

If operating staff detect a perceptible hydrogen sulfide smell, no work should be performed in the room until it is effectively ventilated. Then the leak location should be identified and, if possible, eliminated.

In principle, ventilation should be installed in switchgear buildings, although as experience shows, natural ventilation prevents gas accumulation escaping from equipment due to normal (but not emergency) leaks.

Gas may accumulate in rooms below the switchgear – in cable ducts, basements, etc. Before entering them, they should be intensively pre-ventilated.

Of course, the gas concentration values in air given above can only appear in the case of indoor equipment. Outdoor equipment poses practically no toxic hazard to operating staff during normal operation, due to gas dilution in the atmosphere. This does not mean, however, that gas leaks can be tolerated at a higher level than from equipment installed indoors. This concerns climate hazard (see chapter 5) due to the long persistence of SF₆ in the atmosphere.

In summary, it can be stated that work under conditions of normal gas leakage from SF₆ equipment poses no hazard to operating staff – even if it is leakage from apparatus in which gas decomposition occurs (e.g., circuit breakers).

Work with contaminated SF₆ takes place when work is performed in contact with gas that has been inside the casing of switchgear equipment for some time and may therefore be partially decomposed or contain contaminants. The need to work with contaminated gas arises in the following situations:
– replenishing a closed pressure system with gas where electrical discharges have occurred,
– during sampling from equipment as above,
– removing and replacing gas during maintenance, repairs or expansion of switchgear equipment,
– total or partial gas emission in an emergency situation,
– dismantling of electrical power equipment related to its decommissioning.

Work with contaminated gas can only be performed by personnel thoroughly familiar with the properties of SF₆ decomposition products, aware of health hazards and informed about necessary safety measures to be taken to minimize risk. Trained in first aid provision.

When significant quantities of SF₆ decomposition products are present in the atmosphere, this can be recognized by an unpleasant smell (hydrogen sulfide) and irritation of the upper respiratory tract. Such symptoms may appear within seconds of contact (e.g., due to leakage). In such a situation, personnel should immediately leave the room and wait preferably in fresh air, after taking appropriate measures – e.g., ventilating the room, starting ventilation and clearing the atmosphere.

When solid SF₆ decomposition products and adsorbents or vacuum cleaner bags are removed, workers must be aware that adsorbed gaseous products are contained inside them and may be released from them, therefore appropriate protection must be provided.

When the casing of equipment that has operated for some time in the power system containing SF₆ is opened, we remember that it contains powdered decomposition products (metal fluorides).

When contact with SF₆ decomposition products is unavoidable, personal protective equipment must be used!

The Regulation of the Minister of Development and Finance of December 7, 2017 on minimum technical equipment appropriate for performing activities covered by a certificate for personnel in the scope of fluorinated greenhouse gases and controlled substances specifies the following required personal protective equipment:
– protective suit covered with waterproof layer, without pockets, with fasteners at wrists and legs,
– shoe covers (PVC or neoprene) or safety boots,
– disposable gloves (nitrile or neoprene) or industrial gloves,
– industrial safety glasses (chemical type),
– half masks or protective masks equipped with dust filters type FFP2 and absorbers for acid-like compounds FFE 1P2, in accordance with the standard introducing PN-EN 14387+A1 and the standard introducing PN-EN 149, used for short-term inspection,
– protective equipment used when opening equipment with fluorinated greenhouse gas SF₆ and removing powdered decomposition products of fluorinated greenhouse gases SF₆ from its interior:
– high-performance industrial vacuum cleaner, designed for collecting non-explosive dusts hazardous to health, equipped with a filter adapted to capture particles of approximately 1 µm size and a hose ending with a non-metal nozzle, and with automatic closure of the container after it is filled,
– two-layer plastic bags for storing used vacuum cleaner bags and used disposable personal protective equipment,
– preparations for neutralizing powdered decomposition products of fluorinated greenhouse gases SF₆, containing sodium carbonate, sodium bicarbonate or slaked lime,
– plastic containers for storing hazardous waste, including bags.

Personnel working under conditions of contact with contaminated SF₆ must have access to washrooms (showers) with hot and cold water. Conditions are necessary for preparing appropriate neutralizing solutions. The area where work is performed should be marked. Information about the smoking ban is mandatory.

When opening housings in the open air, certain safety measures must be taken. It must be considered that:
– wind may cause solid decomposition products (in the form of light loose powders) to float around equipment,
– rain or high air humidity may accelerate hydrolysis of compounds which may lead to HF formation.

Powders should therefore be removed (with a special vacuum cleaner) immediately during opening of the casing. For example, when opening a circuit breaker, the lower flange bolts should first be unscrewed, the cover should be slowly raised with a lever and floating powder should be simultaneously collected. Work in the rain is highly inadvisable!

Work in rooms should be conducted with effective ventilation running so that the permissible concentration of SF₆ and its decomposition products in air is not exceeded. Concentration level should be continuously monitored. Always when opening a casing, appropriate equipment protecting the respiratory tract should be used.

In the case where gas diagnostics was not performed before opening the casing, one should expect that the gas condition depends on the type of switchgear equipment, the energy and type of electrical discharges (arc, spark discharges, partial discharges) and the concentration of contaminants in SF₆ is as follows:
– casing not containing circuit breaker quenching chambers and not connected to other circuit breaker housings (disconnectors), gaseous – concentration zero to low, solid products – total absence or small amount,
– casing containing or connected to a circuit breaker quenching chamber, gaseous – medium concentration, solid products – amount dependent on number of switching operations and current value,
– casing inside which an internal arc occurred interrupted by circuit breaker tripping (safety diaphragm did not activate – so-called expulsion valve), gaseous – difficult to determine high concentration, solid products – very large amount, composition depends on materials in the casing,
– internal arc with diaphragm activation, gaseous – mixture of air and gaseous products, may be hydrolysis which leads to large amount of acidic compounds, solid products – large amount, some may have been blown out and contaminated air.

When organizing the dismantling of SF₆ equipment, preparation should be made for gas evacuation operations from casings. Before removing used SF₆ from electrical power equipment, it is recommended to take a gas sample and perform tests aimed at determining the degree of contamination (moisture content, oxygen, acidic compounds and hydrated fluorine compounds). This is intended to properly organize further operations and decide on the further fate of the gas (reuse, filtration, disposal).

One of the fundamental issues that arise before switchgear personnel in the case of the need to evacuate SF₆ from equipment is the technology of this procedure. Currently, international recommendations state that the primary form in such cases is recycling and regeneration. In short, it comes down to recovering the gas from the equipment into cylinders, assessing its condition, and then – depending on the condition, designating it for recycling or regeneration and reuse or, as a last resort, disposal.

Despite proper use of protective equipment, there is some hazard to workers during work with contaminated SF₆, especially for non-essential persons. Therefore, the principle must be observed that: “DURING WORK WITH CONTAMINATED SF₆ IN A ROOM, ONLY PERSONS ESSENTIAL FOR PERFORMING A SPECIFIC OPERATION ARE PRESENT!”.

The effects of SF₆ decomposition products can cause various symptoms in persons exposed to them. Individual susceptibility to irritation is to some extent a matter of individuality. Always the extent of irritation will depend on exposure time and concentration of contaminants in air. SF₆ decomposition products can cause irritation of skin, eyes, mucous membranes of the respiratory tract, and with high concentration and long exposure time, pulmonary edema.

For a multi-component gas mixture, toxicology distinguishes three cases [40]:
– each component acts differently or on a different organ, the effect of each factor is considered separately,
– factors acting similarly on the same organs, their effects accumulate,
– the effect of one factor significantly exceeds the effect of others, only this factor is considered.

In the case of SF₆, for total toxicity assessment, SOF₂, its amounts and health effects are considered. However, this compound undergoes hydrolysis, therefore:
– for long exposure times, the products of this reaction should be considered, i.e., HF and SO₂,
– for short times or when strong ventilation is activated, the hydrolysis phenomenon may be omitted.

Since the accumulation of SOF₂, HF and SO₂ effects is a greater hazard, the last point shows the advisability of effective room ventilation. The threshold limit value TLV for SOF₂ is 1.6 ppm_V (5.66 mg/m³).

During discharges of low energy (e.g., partial discharges), the most toxic SF₆ decomposition product is S₂F₁₀, therefore, despite small quantities, its health effects are considered. Other compounds that may appear under these conditions are not considered. Unfortunately, the effects of this compound on the human body have not yet been fully studied.

Employees performing work with exposure to gas decomposition products must be trained in first aid. Especially in the following cases:
– skin irritation – a person with such symptoms should be immediately removed from the room where work is being performed, the suit and upper clothing should be removed and irritated areas washed with cold running water, if irritation does not subside, medical attention should be sought,
– eye irritation – a person with such symptoms should be immediately removed from the room where work is being performed, eyes should be immediately rinsed with clean water for at least 15 minutes (possibly eye wash solution) and medical attention should be sought immediately, informing about the cause of irritation, in acute cases call emergency services,
– breathing problems – a person with such symptoms should be immediately removed from the room where work is being performed, the suit and upper clothing should be removed, the person should be covered with a blanket and constantly monitored, emergency services should be called, if breathing weakens, artificial respiration should be started.

By SF₆ emissions to the atmosphere we understand its leakage during failure in the following circumstances:
– large leak caused by mechanical damage to elements or seals,
– internal short circuit with arc accompanied by safety diaphragm activation or melting of the device casing,
– external fire, which may lead to seal failure (damage to seals, diaphragm activation due to gas heating). The probability of this case occurring, while complying with fire prevention regulations, is very low.

Leakage causing significant gas flow may result in reaching the permissible limit value for contaminant concentration in air, but this will occur only when, compared to the permissible level, the leakage level is exceeded several times.

The occurrence of such a leak may be signaled by:
– installed alarm devices,
– portable detector with high sensitivity, meters installed on the equipment,
– characteristic strong unpleasant smell (hydrogen sulfide).

Before undertaking any preventive measures, it should be checked whether permissible concentrations have not been exceeded! Best by direct measurement of decomposition product concentration, or by indirect method – by measuring SF₆ concentration.

Greater hazard is posed by violent SF₆ discharge together with its decomposition products during internal short circuit. The causes of internal short circuit are:
– electrical damage to solid insulation,
– mechanical damage to insulation, causing e.g., change in field distribution,
– improper switching operation.

Internal short circuit causes a large pressure increase in the casing. The value of this pressure depends on current intensity, arc voltage and duration of the short circuit, as well as on the casing volume. The resulting pressure causes the diaphragm (expulsion valve) to activate and large quantities of contaminated gas to escape to the atmosphere. In sporadic cases, local melting of the casing may occur and gas may escape through that opening.

Such a situation will create great hazards for personnel. They should immediately leave the room because large concentrations of SF₆ decomposition products and secondary compounds are immediately created. Additionally, toxic compounds such as metal vapors, organic compounds, etc. are created.

Such a situation does not necessarily mean very great health hazard, provided that exposure time is limited to a minimum.

In indoor switchgear, personnel should, using appropriate personal protective equipment, clean the room:
– collect settled powders with a vacuum cleaner,
– then wash surfaces using weak alkaline solutions,
– activate room ventilation.

It is recommended to perform gas concentration measurements before starting repair work.

A situation similar to internal short circuit may result from a fire. The consequences will depend on the intensity, duration and extent of the fire, and consequently on the extent of damage to SF₆ equipment.

A fire in a switchgear room may increase the hazard due to SF₆ decomposition in flames, therefore pure gas escaping from equipment immediately becomes gas undergoing decomposition. During fire extinguishing, personal protective equipment should be used! In the event of calling the fire brigade to extinguish the fire, they should be informed at the moment of the call about the fact of danger from toxic chemical compounds.

After extinguishing the fire, operations described for the case of internal short circuit are begun. Measures appropriate to the situation are taken.

The final consequence of hazard from decomposition products is decommissioning of equipment filled with SF₆. In such a case, measures related to environmental protection should be applied.

The gas condition in the equipment and the amount of powders depend on the accumulated arc energy (or discharges). This depends on the location of installation in the power network and on the “switching history” (number, frequency, current value). In most cases, even in circuit breakers, the amount of powders is small. According to [40] a typical medium voltage circuit breaker after 10 years of operation contained gas contaminated by the following compounds:
– air: single ppm_V,
– CF₄: 40÷60 ppm_V,
– SOF₂: trace amount,
– SO₂F₂: trace amount.

The reasons for low contamination concentrations are as follows:
– in operation, large currents (especially limit currents) are very rarely switched,
– adsorbents installed in the equipment perform their function.

In the case of decommissioning equipment containing SF₆, optional procedures can be applied:
– work completely performed by a specialized subcontractor (contractor), this method is usually applied to small equipment, safety rules must be observed during transport,
– gas is removed by the user. Further work is conducted by the subcontractor, applied to large equipment in which gas pressure reduction or partial disassembly for transport is necessary. After delivery to a specialized company, equipment renovation or disposal is carried out in accordance with environmental protection rules,
– work conducted completely by the user, the user is fully responsible for organizing proper work technology from the point of view of personnel safety and environmental protection.

Table 9.4.1. Expected amounts of SF₆ decomposition products [40]

| Switchgear equipment | Expected degree of contamination (gas, powders) |
|—|—|
| Busbars | small, from zero to tenths of a percent; minimal amount of powders |
| Cable connections | ibid. |
| Grounding conductor | ibid. |
| Disconnector | ibid. |
| Medium voltage disconnector, medium voltage switchgear for ring network, medium voltage or high voltage circuit breaker | medium to several percent, powder products |
| Casing in which electric arc occurred | high (exceeds 5%) large amount of powders |

When undertaking dismantling, a room equipped with the following should be available:
– exhaust ventilation at floor level and supply ventilation at the top, ensuring complete air exchange in the room within one hour,
– gas recovery equipment,
– gas cylinder,
– industrial chemical type vacuum cleaner,
– plastic containers for parts and used materials (filters, cleaning materials, etc.),
– neutralizing solution,
– source of running cold and hot water,
– changing room for employees performing the work,
– signs informing about the ban on smoking and open flame.

Employees performing equipment dismantling must use appropriate personal protective equipment (chapter 9.3), especially to protect skin and eyes from powders and liquids with high concentration, and respiratory tract.

Compressed air must not be used to remove gaseous and powdered SF₆ decomposition products – it causes their spreading and scattering.

Dismantling begins with the casing. Further handling of it depends on the type of equipment and the amount of decomposition products:
– small concentration (e.g., busbars, switchgear busbars), no special procedures are required, segregate elements for reuse and scrapping,
– medium concentration (e.g., medium and high voltage switchgear without circuit breaker), casing elements should be neutralized within 1 hour, then rinsed with running water, dried and sealed in plastic bags,
– high concentration (e.g., circuit breakers, disconnectors), when slowly raising casings, powder collection with a vacuum cleaner is necessary, neutralize the casing within 1 hour, then rinse with running water, dry and seal in plastic bags.

Adsorbents and vacuum cleaner bags must be placed in appropriate containers and subjected to neutralization process within time T₂ (table 9.4.2).

Adsorbents and vacuum cleaner bags must not be removed by burning (harmful dusts, vapors and fumes are released).

Parts removed from the interior of the equipment should be immersed in neutralizing solution for 1 hour (for small concentration of decomposition products) or for time T₂ (for high contamination), then carefully cleaned, rinsed, dried and sealed in plastic film.

Clothing after completion of work should be immersed in the solution for 1 hour, then washed and dried (or disposed of).

This clothing (including shoes) and tools should be used only for such work (with decomposition products). They cannot be used for other purposes.

Tools should be washed in the solution, rinsed and dried.

For neutralization of solid SF₆ decomposition products, aqueous solutions meeting the following recommendations should be used:
– the neutralizing liquid should not cause corrosion,
– the liquid should be sufficiently alkaline so that acidic compounds do not remain unneutralized,
– it should not be excessively alkaline to avoid difficulties in its removal in accordance with regulations.

In PN-EN IEC 60480, three different compounds are given for neutralization and washing, with slightly different properties (see table below).

Table 9.4.2. Compounds and their concentrations for neutralizing SF₆ decomposition products according to PN-EN IEC 60480 [41]

| Compound | Chemical formula | Concentration kg/100 l | T₁ [h] | T₂ [h] |
|—|—|—|—|—|
| Lime | Ca(OH)₂ | Saturated | – | 24 |
| Sodium carbonate | Na₂CO₃ | 1.1 3 10* 10÷14* | – cleaning – 1 | 24 – 0.25 48 |
| Sodium bicarbonate | NaHCO₃ | 1** | – | – |

* Caution should be exercised and contact with skin and eyes should be avoided.
** Recommended for skin washing.

Neutralized powdered decomposition products (vacuum cleaner bags, collected from element surfaces) and liquids after neutralization should be disposed of in accordance with applicable local regulations, i.e., handed over to an institution authorized to collect this type of waste.

The issues of SF₆ gas recycling and regeneration are discussed in this chapter based on PN-EN IEC 60480 “Requirements for sulfur hexafluoride (SF₆) and its mixtures for reuse in electrical equipment” and Regulation (EU) No 517/2014 of the European Parliament and of the Council of 16 April 2014 on fluorinated greenhouse gases.

In the past, the most important issue was minimizing leaks from equipment. Progress in the field of tightness of electrical power equipment with SF₆ resulted in reducing leaks to a level of approximately 0.1% per year. Hermetically sealed equipment also appeared, which requires no maintenance (replenishment) during the entire operation period. Now the most important issue is preventing deliberate release of SF₆ from equipment to the atmosphere – during maintenance, servicing or disposal of equipment – and ecological management of SF₆ gas in a closed circuit, which includes gas recycling and regeneration.

The term regeneration should be understood as complete purification of SF₆ gas in order to restore its primary gas parameters, which is confirmed by laboratory tests and a gas quality certificate. This procedure must comply with standards, procedures and involve the use of appropriate equipment. The benefits resulting from this are as follows:
– reduction of equipment operating costs,
– compliance with public policy worldwide to avoid greenhouse gas emissions,
– demonstration of voluntary environmental effort,
– activities in compliance with standards.

In recent years, the need for SF₆ recycling and regeneration has been recognized, which was contributed to by the development of clear standards and regulations containing a wide range of recommendations and procedures for recovery, recycling and regeneration processes. Realistic standard recommendations for purity of regenerated SF₆ to be reused in electrical energy equipment have also been developed.

Discussion in outline of recommendations regarding recycling and regeneration procedures requires defining several terms used in the text:
– recovery – means the collection and storage of fluorinated greenhouse gases from products, including containers and equipment during maintenance or servicing or before disposal of products or equipment,
– virgin substance – means a substance that has not been previously used,
– recycling – means the reuse of recovered fluorinated greenhouse gas after performing a basic cleaning process,
– regeneration – means reprocessing of recovered fluorinated greenhouse gas to achieve operating properties corresponding to the operating properties of virgin substance, taking into account the intended use,
– destruction – means a process by which all or most of the fluorinated greenhouse gas is permanently transformed or decomposed into one or more stable substances that are not fluorinated greenhouse gases,
– regenerated SF₆ – sulfur hexafluoride that has been subjected to a regeneration process,
– reuse – use of regenerated gas to refill an electrical power equipment containing SF₆.

Many design features of SF₆ equipment contribute to successful application of gas recycling and regeneration:
– removal (adsorption) of decomposition products and moisture by internal absorbent devices keeps gas contamination at low levels, which greatly facilitates gas purification during recycling and regeneration,
– minimizing the volume and pressure of gas reduces the amount of gas to be recycled and regenerated,
– division of the equipment into sealed compartments limits the amount of gas to be recycled and regenerated, especially in the case of internal arc occurrence – when seriously contaminated, polluted SF₆ must be dealt with,
– special connections in the SF₆ gas installation and self-sealing check valves prevent cases of gas loss or its contamination with air caused by improper equipment operation.

The issue of contamination generation in sulfur hexafluoride has essentially been discussed in chapters 3 and 4. There, attention was mainly drawn to gas decomposition as a result of normal (circuit breakers) or emergency operation of equipment, and then to the hazards these contaminants pose to operating staff. Since gas condition is of fundamental importance for the recycling process, it is worth briefly discussing the issue of SF₆ contamination generation from this point of view.

SF₆ contaminations that may be generated in electrical power equipment originate from six main sources, namely:

• gas handling,
• leaks,
• contaminated surfaces: casings, structural elements, adsorbing devices,
• SF₆ decomposition products as a result of electrical discharges,
• secondary chemical reactions,
• mechanical generation of dust particles in the equipment.

Air left in the casing during vacuum execution before SF₆ filling and introduced during filling and replenishment of equipment, since it remains in pipes and valves, may be inadvertently added to SF₆. This is usually the result of operating errors or use of faulty equipment and can constitute significant contamination.

The amount of air (and even dust) introduced in this way can be reduced by:

• appropriate design of pipes and valves,
• observance of appropriate operating procedure,
• thorough evacuation of air from the casing before filling the equipment with SF₆, (e.g., below 1 millibar).

Gas contamination by leak is the result of infiltration (diffusion) of air and moisture into pressure casings from outside, because the partial pressure of air and moisture (water vapor) outside the casing is higher than inside. The main leak paths may be: casing porosity (osmosis), seals of movable elements, and “O” type sealing rings. Osmosis through metal and insulating elements is practically insignificant in practice, because the diffusion coefficients of air and water vapor in these materials are very small. Greater significance is had by “paths” through seals.

Moisture and air are adsorbed on the internal surfaces of casings and on element surfaces before their assembly, especially as a residue after their cleaning. Polymeric materials contain moisture in their interior and prove to be the most important source of moisture in the gas. Adsorption devices that are not properly operated (activated) may contain both moisture and air, and absorb SF₆ decomposition products, which in turn may be released during the evacuation (vacuum) process or at elevated temperature. The amounts of substances that have been adsorbed are difficult to estimate, because they depend on the use of specific materials, production methods, quality control, and on assembly and filling, maintenance and operating procedures.

SF₆ contamination as a result of electrical discharges was discussed in considerable detail in chapter 4. Therefore, now only a few sentences summarizing this issue. We already know that SF₆ undergoes partial decomposition through electrical discharges, which can be grouped into four main types:

• incomplete corona discharges,
• spark discharges,
• arcs during current interruption,
• internal arcs.

The first of these, occurring only in the case of defects and insulation faults, introduce relatively small amounts of contaminants. The second type of discharges appears with a large number of insulation faults or during switching operations. A similar type of decomposition products is then produced as in corona discharges, but their amount and composition are different. For example, in disconnector compartments they are very small, because these units rarely operate and interrupt only small capacitive currents. Higher amounts can only be accumulated with serious insulation defects causing constant spark discharge generation, and when the compartment with such a fault is not equipped with an absorber [13].

Significant amounts of contamination can be expected in apparatus in which, by the principle of operation, a switching arc occurs. In circuit breakers, the arc during interruption of large current causes erosion of contact materials and nozzles as a result of the action of hot arc plasma.

The main cause of decomposition, SF₆ breakdown, is the reaction of these products of material erosion with fragments of thermally dissociated SF₆ and other trace gases such as air and water vapor. The most important of these reactions are described by the following summary formulas:

Cu + SF₆ → CuF₂ + SF₄

W + 3SF₆ → WF₆ + 3SF₄

CF₂ + SF₆ → CF₄ + SF₄

The first two reactions are related to material from erosion contacts, for the production of which copper (Cu) and tungsten (W) are usually used. The last reaction results from erosion of PTFE (CF₂ polymer), which is used on nozzles. CuF₂ is a solid substance (powder). Two gases SF₄ and WF₆ are highly reactive, but in significant quantities are removed from the gas volume through:

• adsorption followed by transformation through chemical reaction,
• secondary chemical reactions in the gas, mainly hydrolysis with residual moisture in the gas,
• secondary reactions on casing surfaces, mainly hydrolysis with moisture absorbed from the surface and reactions with metal oxides/hydroxides and silicon-containing substances such as quartz filters and polymeric insulators [37].

In switching apparatus, adsorption in filters (for this purpose) is the dominant mechanism for removing decomposition products. The two remaining processes are relatively insignificant from a quantitative point of view. Thanks to adsorbents, practically only in rare cases, when very large currents are interrupted frequently, may significant contamination concentration occur in a short time. After several hours or days, this condition improves, determined by the period and effectiveness of adsorption. Internal arcs are the result of solid insulation breakdown or improper interruption by switching apparatus and occur extremely rarely [37].

Table 10.1.1. SF₆ contaminations [21]

*cannot be quantitatively determined

In these cases, the arc most often ignites between metal elements that are not arc-resistant, e.g., made of aluminum, copper and steel. Materials undergo very large arc erosion. The concentration of products contaminating SF₆ can in such cases reach a high level (up to several percent of gas volume).

Mechanical generation of dust particles is mainly from contacts. In a properly designed connector, particles of worn metal should fall in an area where they have no effect on the insulation condition. However, if they fall into the area of high-voltage electric field, e.g., on the surface of an insulating partition, they can cause surface flashover of the insulator and eventually lead to arc discharge. In the recycling process, these particles must always be effectively removed.

The effects of the discussed SF₆ contaminations can be summarized in the following points:

• threat to human health and the natural environment,
• material corrosion,
• deterioration of insulating strength of the contact gap,
• deterioration of surface insulation strength,
• deterioration of switching capability of apparatus,
• change in heat dissipation.

Table 10.1.1. presents, according to [21], the complete picture of the most important contaminations, their sources and destructive effects.

According to the definition contained in Regulation (EU) No 517/2014 of the European Parliament and of the Council, recycling of fluorinated greenhouse gas is performing a basic cleaning process, while regeneration is a complete process consisting of restoring the original properties of a given substance. In practice, the basic differences between the recycling process and the regeneration process are presented in Table 10.2.1.

Table 10.2.1. Recycling versus regeneration

Pumps and filters (Table 10.2.1), used to carry out the SF₆ recycling process, are available on the market and have been used for a long time. They are accessible, purchasable and mobile. Currently their costs are decreasing, their mode of operation is improving, their size is reducing to portable equipment suitable for handling smaller quantities of gas. With their help it is also possible to process gas heavily contaminated with decomposition products. The quality of SF₆ obtained as a result of recycling can be checked using market-available equipment measuring SF₆ purity, moisture content and some decomposition products and impurities. It should be remembered that gas after the recycling process can be used only in the same equipment from which it was recovered.

Table 10.2.2. Types of filters used for SF₆ recycling and regeneration

The regeneration process, in addition to mobile filters, requires the use of cryogenic technology, which is currently the only known method to remove non-reactive gases such as air and CF₄ from SF₆ gas. Performing cryogenic regeneration of SF₆ is currently not possible at the location of installed equipment, it is necessary to transport the gas to a specialized company and perform regeneration by personnel qualified in this field. Gas quality after the regeneration process is checked in an analytical laboratory. Regenerated SF₆ can be reused in any electrical equipment. This is an expression of ecological awareness and conducting socially responsible management of SF₆ gas in a closed loop.

Re-filling of equipment with gas after the recycling or regeneration process should take place after air has been removed from the electrical power equipment using a vacuum pump (in accordance with the instructions of the manufacturer of the given equipment). Then, the equipment should be refilled with gas from a cylinder or tank, using appropriate compressors and hoses. Separate use of hoses for recovery and refilling is intended to avoid additional gas contamination.

The purity requirements that SF₆ gas must meet after the regeneration and recycling process in order to be reused are defined in PN-EN IEC 60480 “Requirements for sulfur hexafluoride (SF₆) and its mixtures for reuse in electrical equipment”. The values are expressed (Table 10.3.1) in volume percentages (%_V, ppm_V), as these units have become standard in relation to electrical power equipment insulated with SF₆. These purity requirements can be related to three different critical levels, namely:

• maximum contamination levels in virgin SF₆,
• maximum contamination levels in SF₆ after recycling and regeneration,
• maximum tolerated contamination levels in electrical power equipment.

The contamination levels of virgin SF₆ are given in the standard PN-EN IEC 60376 “Requirements for technical sulfur hexafluoride (SF₆) and supplementary gases for its mixtures used in electrical equipment”. The contamination levels of SF₆ after recycling or regeneration are specified in the standard PN-EN IEC 60480 “Requirements for sulfur hexafluoride (SF₆) and its mixtures for reuse in electrical equipment”. The maximum tolerated contamination levels in equipment are those above which the functioning of electrical power equipment insulated with SF₆ may begin to deteriorate, or above which health hazards must be considered.

Contaminations can be measured using inexpensive, portable measuring equipment. Current measuring instruments in use can be equipped with sensors measuring SF₆ purity, moisture, SO₂ content, HF, CO and H₂S.

It should be emphasized that SF₆ gas parameters can also be measured with much greater accuracy in an analytical laboratory using technologies such as gas chromatography, ion chromatography, infrared absorption, gravimetry and photometry. These are not possible to use at the equipment installation location, as they are relatively expensive and require highly qualified operation. Gas parameters after regeneration must be confirmed by testing in an analytical laboratory [14].

Required purities that have been determined for regenerated gas that is to be reused are specified in PN-EN IEC 60480 and are provided in Table 10.3.1.

Table 10.3.1. Required parameters for regenerated SF₆ according to PN-EN IEC 60480 [41]

ppm_v = parts per million by volume ppm_w = parts per million by weight

The required parameters for regenerated SF₆ are less restrictive than in the case of virgin SF₆ (Table 10.3.2), however in the case of moisture and mineral oil content the permissible values are the same. Regenerated SF₆ can however and is recommended to meet the parameters of the virgin gas standard.

Table 10.3.2. Requirements for technical SF₆ according to PN-EN IEC 60376 [40]

ppm_v = parts per million by volume ppm_w = parts per million by weight

Basic issues of SF₆ reuse

From both an ecological and economic point of view, it is desirable to maintain contamination in SF₆ at a low level so that the gas can be regenerated multiple times.

The same portion of SF₆ should be used during product testing, installation, maintenance and repairs. When the limit levels specified in Table 10.3.1 are already reached in the equipment, the gas should be withdrawn from service. After regeneration, it should be transferred for use in newly installed equipment. Thus, it should be in continuous use. Such continuous use of gas is possible by ensuring a high level of its quality so that it can perform its functions repeatedly. This can only be achieved through proper gas handling and regeneration, confirmed by laboratory testing.

Regenerated gas before being reintroduced into electrical equipment must be subjected to quality checking to ensure that contaminations do not exceed the values specified in Table 10.3.1, and preferably do not exceed the values specified for virgin gas, presented in Table 10.3.2. Four levels of contamination must be checked, namely:

• SF₆ purity,
• total level of non-reactive gaseous contaminants (air and CF₄),
• moisture content (H₂O),
• total level of acidity.

Additionally, PN-EN IEC 60480 indicates that the content of potential contaminants such as H₂O and CO does not need to be tested due to lack of currently sufficient testing and data. Mineral oil content also does not need to be monitored because only oil-free equipment for SF₆ gas management is currently available on the market, therefore contamination of the gas with this substance is impossible.

There are four basic methods for performing quality control of gas with which an electrical power equipment is filled:

• continuous monitoring of all contamination levels in the gas reservoir or in the gas stream in the pipe and alarm if one of them exceeds the specified limit of purity requirements,
• periodic monitoring of contamination levels in the storage reservoir using portable or permanently installed sensors,
• gas checking after refilling – verification of purity requirements after refilling the electrical power equipment with gas. However, checking after filling involves some risk that excessive contamination, if it occurs, may only be revealed when it causes certain damage to the electrical power equipment. Gas containing too much moisture may excessively wear adsorption devices, and moisture may condense on the internal surface of the casing,
• gas sampling and laboratory analysis – this is the best and most reliable method for detecting contamination levels, however the disadvantage of this method is significant time delay, as well as the need to precisely collect the required amount of gas in order to achieve a representative sample.

Regulations concerning SF₆ transport

Regenerated sulfur hexafluoride can be stored at the regeneration site or can also be transported to other locations for reuse. This requires appropriate regulations concerning storage and transport of contaminated and regenerated gas.

The classification of SF₆ gas transport method depends on its history – on whether it is virgin gas, recovered from equipment or after the regeneration or recycling process [6].

Virgin SF₆ – containers for storage and transport of virgin gas must comply with national legislation concerning pressure vessels (ADR 2.2).

Regenerated SF₆ or recycled and suitable for reuse in electrical equipment – for this gas category, contamination levels must comply with purity requirements for reuse in electrical equipment (PN-EN IEC 60480). It can be stored and transported like virgin SF₆, but cylinders should be labeled: “Regenerated/recycled SF₆ intended for reuse in electrical equipment”. Proposed transport category as for virgin SF₆.

Recovered SF₆ – recovered SF₆ is treated as hazardous waste (waste code 16 05 04*) and transported as such. It must be considered toxic and assigned to one of the hazardous groups defined in the regulations. The cylinder must be labeled: “Recovered SF₆, transported solely for the purpose of analysis, destruction, regeneration or recycling” (ADR 2.3 and 8).

Final disposal of SF₆

In cases where SF₆ cannot be subjected to regeneration or recycling, it can be destroyed through thermal processes.

According to the definition contained in Regulation (EU) No 517/2014 of the European Parliament and of the Council, by disposal we understand a process by which all or most of the fluorinated greenhouse gas is permanently transformed or decomposed into one or more stable substances that are not fluorinated greenhouse gases.

SF₆ heated to a temperature above 1000°C begins to dissociate, the gas molecules decompose into sulfur and fluorine atoms, which then undergo ionization. Therefore, SF₆ gas can be destroyed when thermal processes occur at a temperature above 1000°C.

Contracts concerning SF₆ handling

Careful reading of the chapter suggests general recommendations for manufacturers and users of electrical power equipment:

• deliberate release of SF₆ to the atmosphere should be avoided,
• SF₆ must be subjected to recycling and regeneration (closed loop economy).

Manufacturers are expected to implement the following postulates:

• manufacturers should inform about the possibility of applying recycling and regeneration of SF₆ and other materials used in equipment production,
• manufacturers should specify the conditions under which gas reuse is possible,
• manufacturers should encourage, where possible, reuse of gas and provide users with appropriate instructions,
• leak monitoring devices with very high sensitivity should be used,
• documentation should be maintained concerning gas sales, supplies from gas producers, shipments to customers, and gas returning to the factory from customers and specialized recycling companies.

Recommendations for users of electrical power equipment can be summarized in the following points:

• users should enter into contracts with specialized companies performing gas regeneration,
• electrical power equipment with SF₆ should be operated and maintained in accordance with manufacturers’ instructions and environmental protection principles,
• electrical power equipment with SF₆ should be repaired if gas leakage increases above permissible values,
• documentation should be maintained of all work related to gas (including the quantities of SF₆ used in individual jobs).

1. BALL; “Modern cable terminations in metalclad switchgear”, IEE Conference on metalclad switchgear. Conf. Publ. 83,
2. BATTAGLIA; “L’hexafluorure de soufre, nouveau moyen d’extinction de l’arc dans les disjoncteurs a haute tension”, Bull. SFE, ser 8-e. T. III. 1962, nr 34,
3. BERG, C. WORKS; “Effect of Space Charge on Electric Breakdown os Sulfur Hexafluoride in Nonuniform Fields”, AIEE Trans. PAS, t. 77, cz. III, 1958,
4. J. BRUNT, J.T. HERRON; “Fundamental Processes of SF₆ Decomposition and Corona Discharges”, IEE Trans. On Electr. Insul. 1990 nr 1,
5. CLAY i inni; “Human Health Risk Assesment Process for the Use of Sulfur Hexafluoride in the Electrical Utility Industry”, VII Int. Symp. Gaseous Dielectrics, USA 1994,
6. FROST, R. LIEBERMAN; “Composition and Transport Properties of SF₆ and Their Use in a Simplified Enthalpy Flow Arc Model”, Proc. IEEE, t. 59,1971, nr 4,
7. JANKOWICZ, A. PIECHOCKI, W. PROGA; “Doświadczenia z eksploatacji rozdzielnicy i wyłącznika napowietrznego wolnostojącego WN w izolacji SF₆”; Wiadomości Elektrotechniczne, nr 13-14, 1985,
8. JANKOWICZ; “Gaz SF₆ i produkty jego rozkładu w rozdzielnicach osłoniętych z izolacją gazową”, Wiadomości Elektrotechniczne, nr 7, 1992,
9. JANKOWICZ, A. PIECHOCKI, W. PONIECKI i inni, “Aparatura rozdzielcza w Instytucie Elektrotechniki”, Biuletyn Informacyjny I.El. 1/1995,
10. KAWAMURA i inni; “Operating experience of gas insulated Switchgear (GIS) and its influence of the future substation design”; Materiały CIGRE 23-01 1982 Session,
11. KNOTHE; “Rozdzielnice wysokonapięciowe izolowane”, WNT Warszawa 1976,
12. LAMPE, H. SŁOWIKOWSKA, A. PIECHOCKI, W. PONIECKI, J. SŁOWIKOWSKI; “Pogorszenie własności dielektrycznych tworzyw izolacyjnych pod wpływem łuku w sześciofluorku siarki”, Przegląd Elektrotechniczny, nr 5 1981,
13. LATOUR-SŁOWIKOWSKA, S. JANKOWICZ, A. PIECHOCKI, J. LAMPE, J. SŁOWIKOWSKI; “On the possibillity of diagostics of GCB and GIS by gas chromatografy”, Gaseus Dielectrics V Pergamon Press,
14. LINGAL i inni; “An Investigation of the Arc-Quenching Behaviour of Sulfur Hexafluoride”, AIEE Trans PAS, t. 72, cz. III, 1953,
15. LEEDS i inni; “The Use of SF₆ for High-Power Arc Quenching”, AIEE Trans. PAS, t. 76. Cz. III, Dec. 1957,
16. MAKSYMIUK; “Aktualne problemy budowy i eksploatacji wyłączników wysokonapięciowych”, Przegląd Elektrotechniczny, nr 3, 1998,
17. MAURY i inni; “Mise en service et premiers resultats d’exploitatio de postes blindes a 225 kV isoles au SF₆”, CIGRE, 1970, ref 23-06,
18. MAUTHE i inni; “SF₆ and the global Atmospfere”, CIGRE 23.10. – Projekt Dokumentu,
19. MAUTHE i inni; “Handling on SF₆ and its decomposition products in gas insulated Switchgear”, Grupa Robocza 23-03, CIGRE, Elektra nr 136 i 137, 1991,
20. MAUTHE i inni; “SF₆ and the global atmosphere”, Elektra nr 164, 1996,
21. MAUTHE i inni; “SF₆ recycling Guide, Re-use of SF₆ gas in electrical power equipment and final disposal”, Electra, nr 173, 08, 1997,
22. PIECHOCKI, W. PONIECKI, A. RABAJCZYK; “Wyłącznik wysokonapięciowy typu WGN 1”, Prace Instytutu Elektrotechniki, Zeszyt 105 1978,
23. PIECHOCKI, W. PONIECKI; “Wpływ napędu na przebiegi gazodynamiczne w wyłącznikach z SF₆”, VI Sympozjum Podstaw Teorii Wyładowań Elektrycznych w Gazach, Wilga 1982,
24. PIECHOCKI, “Krajowy wyłącznik wysokonapięciowe z sześciofluorkiem siarki?”, Wiadomości Elektrotechniczne, nr 5-6 1986,
25. PIECHOCKI, “Wybrane zagadnienia procesu wyłączania prądu przez wyłącznik z SF₆”, Prace Instytutu Elektrotechniki, Zeszyt 141 1986,
26. PIECHOCKI, “Calkulation of the dimensions of the extinguishing chamber of a SF₆ Circuit-Breaker”, Prace Instytutu Elektrotechniki, Zeszyt 143 1986,
27. PIECHOCKI, W. PONIECKI; “Krajowe prace konstruktorskie i badawcze w dziedzinie wysokonapięciowych wyłączników z SF₆”, Wiadomości Elektrotechniczne, nr 7-8 1988,
28. PIECHOCKI, W. PONIECKI; “Metody obliczania parametrów gazu w zbiorniku sprężania wyłącznika samosprężnego z SF₆”, Przegląd Elektrotechniczny, nr 4 1989,
29. PIECHOCKI, W. PONIECKI; “Influence of the drive on the swirching of process in the self compressing SF₆ circuit breaker”, Sixth International Conference Switching Arc Phenomena. Łódź 1989,
30. PIECHOCKI; “Wyłączniki wysokonapięciowe z sześciofluorkiem siarki a ekologia”, Wiadomości Elektrotechniczne, nr 8 1995,
31. RIEDER; “Schwefelhexafluorid als Schaltmedium”, Elektrotechnik Maschinen-bau, t. 87, 1970, nr 1,
32. HM RYAN, GR JONES; “SF₆ switchgear”; 1989: Peter Peregrinus Ltd; London,
33. SALLESKY: “Die Anwendung von Schwefelhexafluorid in Hochspannungs-schaltgeraten”, Electrie, t.21,1967, nr 10,
34. TROGER i inni; “The state of international development and experience with SF₆ gas insulated high voltage Switchgear”, Materiały CIGRE 23-01 1982 Session,
35. CIGRE Oprac.zbior.; “Diagnostic Techniques to Defect Abnormal Operating Conditions in Gas Insulated Substations”. Raport 23-08 na sesję 1984,
36. CIGRE; Study Committet 13, “SWITCHGEAR”; Tutorial; september 1995, Florianopolis – Santa Catarina – Brazil,
37. PIECHOCKI, “Minimalizacja zagrożenia toksycznymi produktami rozpadu sześciofluorkiem siarki powstałymi w wyłącznikach wysokonapięciowych” – projekt wykonany w ramach XIII edycji szkoły CP programu Polsko-Norweskiego NOT – NIF: 1994: praca nie publikowana, dostępna w Bibliotece NOT – Warszawa, nr proj. 13.166,
38. KATALOGI i INSTRUKCJE APARATÓW z SF₆ różnych firm (ABB, ABB SACE, BBC, AEG, GEC ALSTHOM, MAGRINI GALILEO, SIEMENS itd.),
39. WYSOCKI; “SF₆ w elektrotechnice – mity i rzeczywistość”, Wiadomości Elektrotechniczne, nr 12, 1998
40. PN-EN IEC 60376 Wymagania dotyczące technicznego heksafluorku siarki (SF₆) i gazów uzupełniających do jego mieszanin stosowanych w urządzeniach elektrycznych,
41. PN-EN IEC 60480 Wymagania dotyczące heksafluorku siarki (SF₆) i jego mieszanin do ponownego zastosowania w urządzeniach elektrycznych.