SF6 Decomposition in Gas-Insulated Equipment-Tix.pdf

IESEE Trarnsactions o fl ectrical Insulatiorn Vc;s EI--l No.5, cJctober l?B6

SF6 DECOMPOSITION IN GAS-INSULATED EQUIPMENT

F. Y. Chu

Ontario Hydro Research Division

Toronto, Canada

ABSTRACT

The increasing application of SF6 as an insulating gas has

led to many studies on SF6 decomposition in gas-insulated

equipment. In the presence- of an electric arc, spark or

corona, SF6 decomposes to a wide variety of chemically active

products which possess completely different properties from

SF6. The accumulation of these decomposition products in the

equipment has caused concerns regarding personnel safety and

material compatibility problems. This paper reviews previous

research in SF6 decomposition relating to the operation of gas-

insulated switchgears, gas-insulated transmission lines, and

electrostatic accelerators. Results on the qualitative and

quantitative determination of the by-products and their forma-

tion rates in various modes of electrical discharges are sum-

marized. The mechanisms leading to the formation of transient

and stable products are described. In particular, the influence

of discharge energies and impurities on the formation of SOF2

and S02F2, the two dominant stable by-products, is discussed.

The effects of the by-products on personnel safety and equip-

ment dielectric integrity are presented. The application of

SF6 gas analysis as a tool for diagnosing the internal con-

dition of gas-insulated equipment is assessed. Based on the

results of many phenomenological observations, future research

activities are suggested to address the issues of safety, com-

patibility and equipment aging. More fundamental studies on

electron, ion, and neutral reaction rates in an SF6 discharge

are required to gain a better understanding of the decompositon

mechanisms and the influence of the products on equipment

operation.

1. INTRODUCTION

The increased applications of SF6 in gas-insulated

switchgear (GIS), gas-insulated transmission lines

(GITL), electrostatic accelerators, x-ray equipment and

pulse power apparatus [9,20,50,110,176] have led to

growing interest in the mechanisms of SF6 decomposition

and the properties of the decomposition products. In

the presence of an electric arc, spark or corona, SF6

decomposes into lower fluorides of sulphur which in turn

react with the electrodes or gas impurities to form many

chemically active products. Although SF6 is chemically

inert and environmentally acceptable, the decomposition

products of SF6 are known to be toxic and corrosive

[23,30,45,65,83,85,165]. The motivation behind the

studies on SF6 decomposition is the concern for person-

nel safety and material compatibility. The studies may

also lead to improved equipment diagnostic techniques.

From a health and safety point of view, the toxicity

of SF6 decomposition products must be clearly identified

and the quantities generated under all possible circum-

stances must be determined so that procedures can be

established to protect personnel during maintenance and

clean-up of faulted equipment [21,146]. The require-

ments for long-term equipment reliability impose severe

constraints on the material compatibility with the SF6

environment. The effects of SF6 decomposition products

on materials used in gas-insulated equipment have to be

examined carefully to ensure that no degradation of the

materials' insulating, physical and chemical properties

would occur, leading to premature aging. In the develop-

ment of SF6 circuit breakers, the influence of decomposi-

tion products on the dielectric recovery after current

zero is critical to the breaker's function. Much of the

earlier work in SF6 decomposition focused on the thermal

phenomena during high-current interruption in a hot SF6

gas gap [73,108,125,127]. The insulating properties of

the SF6 dissociation products in the predischarge stage

are reported to be critical in influencing the formation

of leader channels in the discharge process [139,140,150].

On the more positive side, the identification of SF6 de-

composition products in electrical equipment offers a

means to diagnose the equipment's internal condition.

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Diagnostics by SF6 gas analysis is very similar to the

well known technique of gas-in-oil analysis for detect-

ing abnormal conditions in transformers.

process: power arc, spark and corona or partial dis-

charge. Table 1 summarizes the types and character-

istics of discharges in gas-insulated equipment. Under

In order to address adequately the issues outlined

above, an in-depth knowledge of the SF6 environment,

decomposition mechanisms, decomposition rates, proper-

ties of the decomposition products and the interaction

of the by-products with the equipment are required.

The objective of this paper is to review the major in-

vestigations relating to SF6 decomposition which have

been carried out in the past 35 years. Due to the

diversified nature of the work, which crosses many dis-

ciplines such as chemistry, physics, electrical engi-

neering, material science, and industrial hygiene, it

is impossible to give a comprehensive review covering

topics ranging from the fundamental aspects of SF6

decomposition to practical issues. Instead, the empha-

sis of this review is to extract information from the

many phenomenological observations and to outline the

possible factors influencing the formation of decom-

position products.

Much of the earlier work on SF6 decomposition focused

on the qualitative analysis of the by-products formed

under conditions easily simulated in the laboratory

with a gas cell [30,64,78,165,178]. The limitations in

those early days included the lack of sophisticated

analytical techniques to determine unambiguously the

types of species formed. As newer techniques were

developed and researchers gained a better understanding

of the decomposition environment, research emphasis

shifted to the quantitative analysis of the by-products

and attempts were made to estimate the amounts formed

in a realistic environment such as encountered in oper-

ating GIS [14,129,179,193]. One of the objectives in

the earlier works was to determine the amount of ab-

sorbent material required inside the GIS to remove the

decomposition products. Boudene's work in the mid-

seventies signified a major step forward in understand-

ing the complicated issue of health and safety and SF6

decomposition [23]. As the development of SF6 circuit

breakers proceeded at a rapid pace, many studies were

carried out to determine the equilibrium concentration

of the decomposition products in a high temperature arc

plasma [18,52,72,73,108,123,126]. The influence of

ions as well as neutrals in the plasma was recognized,

and theoretical formulations of the equilibrium con-

centration of the by-products were developed. However,

the circuit breaker work mainly focused on the recom-

bination process without devoting too much attention to

the stable neutral species. With the increase in con-

cern about safety, the formation and effects of the

stable neutral species started to receive more atten-

tion, and sophisticated techniques to trace the forma-

tion of these species were developed. Investigations

were carried out to address the various modes of forma-

tion in high current arc, spark and corona [11,44,45,

100,116,147,159-161, 166, 180,184,187,188]. The effects

of impurities on the formation rates were systematically

investigated. In recent years, the equipment reliabil-

ity problems and the diagnostics issues have also re-

ceived considerable attention [12,35,47,104,105,116,130,

141,171,172,180].

TABLE 1. Discharge In SF6 InsuZated Eqadpment

Types of Disdharge

Discharge CharactrLstics

Powver Arc

(Current interruption, in-

service fault)

3-100 kA, 50-150 ms

-10'fi- 10' J

Spark

(Capacitive discharge, fault

during testing, disconnector

switching)

High instantaneous cur-

rent, As duration

10' - 10' J

Partial Discharge or Corona

(Stress enhancement, elec-

trically floating component,

free conducting particles)

10- 10' PC

10- - 1O- J er pulse

10' 10' Hz

normal operating conditions, SF6 can be decomposed in a

power arc during current interruptions in circuit break-

ers. The energy dissipated in the discharge depends on

the arc current which ranges from several kA to 100 kA.

The energy can be expressed as:

fr(t)I(t)dt

(1)

where I is the arc current, U is the arc supporting

voltage in the range of several hundred V and t is the

duration of arcing [16,46]. Typical values of t are in

the range of 50 to 150 ms. The energy dissipated is in

the order of 105 to 107 J and the arc temperature at the

axis can reach 20000 K [5,142,164]. In SF6 circuit

breakers, the electrode material is usually a Cu-W alloy

which is highly arc resistant and the hot plasma gas

flows through a Teflon® nozzle. Hot SF6 (T>2000C) is

known to interact with metals, especially copper [36a,

93].

Another type of power arc is generated during short

circuits inside a gas compartment. The characteristics

of such fault arcs are very similar to those for arcs in

circuit breakers except for the absence of the cooling

process from the flow gas, and the electrodes are typi-

cally aluminum instead of copper alloys. The occurrences

of fault arcs in gas-insulated equipment are less fre-

quent than in circuit breaker operations, but under ab-

normal circumstances such as an enclosure burnthrough

or a pressure relief disc operation [41], the decomposi-

tion products generated can be discharged into the am-

bient environment creating hazardous situations.

Spark is the term used to describe capacitive dis-

charges across a gas gap of very short (usually ps)

duration. The energy level involved in the discharge

in GIS is in the order of 10-1 to 102 J per spark. The

spark channel is much narrower than the arc channel de-

scribed in previous paragraphs and the temperature dis-

tribution is highly localized. In electrical power

equipment, flashovers during equipment testing by a

resonant test device, where all the energy is that

stored in the equipment's capacitance and inductance,

can be classified as a spark. During a disconnector

operation in SF6 insulated switches, arcing can occur

across contacts as they separate or approach one another.

The breakdown magnitudes, durations, energies, and fre-

quencies ef occurrence of this type of arcing (usually

2, THE SF6 OPERATING ENVIRONMENT

2.1 Conditions for SF6 Decomposition

In gas-insulated equipment, decomposition of SF6 by

an electrical discharge is the most common mode of dis-

sociation. The discharges can be broadly divided into

three types according to the energy dissipated in the

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C1h--u: de_:3foo6i-! ,_ - n

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called restrikes or prestrikes) have been investigated

extensively [19,138]. Except for fast-acting switches,

normal arcing time in a disconnector operation lasts

from 0.5 to 1.5 s depending on the disconnector's de-

sign. Within this operating period, there are a few

hundred restrikes across the contacts with each re-

strike lasting tens of ps. Because of the capacitive

nature of the discharge, each strike can be classified

as a spark. In a typical disconnector operation, up

to 500 J of energy can be dissipated in the discharge

process [45]. The quantities of SF6 decomposition

generated depend on the number of disconnector

operations.

expressed in gram per joule. In corona discharge, the

energy dissipated is sometimes difficult to obtain. In

many cases, the unit of charge transport (C) is used

instead of the energy unit. The concept of mole per

coulomb or ,umol/C will be used to express the formation

rate of decomposition products in corona or partial dis-

charge.

2.3 Impurities

The decomposition of SF6 is greatly influenced by gas

impurities. In industrial grade SF6 the typical im-

purities are CF4, N2, 02, and H20. Table 2 shows the

new gas specifications, the American Society for Testing

The third type of discharge discussed in this review

is corona or partial discharge. Under abnormal operat-

ing conditions, partial discharge or corona discharge

can occur in SF6 insulated equipment due to electrically

floating components, stress enhancements, metallic

particle discharges and voids inside solid insulators

[111,181]. Many electrostatic accelerators employ a

series of continuous corona discharges over the length

of the accelerator to establish a uniform gradient

between the HV terminal and the ground [9,96,143,156,

182]. Prolonged partial discharge or corona gradually

decomposes the SF6 leading to the accumulation of high

levels of decomposition products inside the compartment.

The magnitudes of the partial dischage are normally ex-

pressed in pC per pulse and the energy dissipated in a

partial discharge pulse varies from pJ to mJ depending

on the discharge level and configuration. Since

partial discharge can be a continuous process, the

levels of SF6 decomposition products formed in a com-

partment are time dependent. With pulse repetition

rates in the 100 Hz range, the total energy dissipated

over a period of time can approach 10 kJ, resulting in

significant accumulation of arc by-products in the

compartment.

TABLE 2. Impricis in SF6

Spec 161

2472-S1i21 Operating GIS (10)

Typical Values

Purity (wt %)

99.9

99.8

Air (ppmv)

2500

1,00010,000

H,O (ppmv)

100500

CP, (ppmV)

800

640

100500

Hydrolyzable

0.3

Fluoride (ppmw)

Oil (ppmw)

N/A

Max. SOF, concentrations for operating circuit breakers

Max. SOF, concentrations for operating bus ducts

and Materials (ASTM) standards, and typical concentra-

tions found in operating GIS in North American utilities

[2,6,100,101]. It is not surprising to find that in

reality the impurity concentrations in operating GIS are

much higher than the specifications. This is partly due

to the introduction of impurities during filling and

partly due to the desorption of moisture into the dry

SF6 after filling [48,57]. A survey of major North

American utilities indicated that the average air con-

centrations in SF6 compartments are 5000 ppmv and the

average moisture content is about 500 ppmv [133,175].

In addition to impurities which exist in the gaseous

phase, there are adsorbed and dissolved impurities, main-

ly H20, in electrodes and solid insulators. During an

electrical discharge, moisture and oxygen will be driven

out from the host matrix to react with the primary SF6

decomposition products. In practical GIS environments,

the presence of impurities in the 100 to 1000 ppm range

is unavoidable.

In addition to the above three major mechanisms, SF6

can be decomposed thermally without the presence of an

electrical discharge [36a,93]. Poor contact resistance

in high-current contacting points is the frequent cause

of overheating in power equipment which eventually re-

sults in thermal decomposition of the SF6 insulating gas

[65,94,181]. Optical radiation can dissociate SF6 mole-

cules, and in an arc, the ultra-violet radiation may de-

compose SF6 located away from the arc zone. The use of

x-ray radiography to diagnose GIS and the use of ioniza-

tion-type gas density monitors [155] may lead to SF6

decomposition by the ionizating radiation [86]. When

SF6 is used as an insulating gas in rf or microwave

power cables, it can be dissociated in a rf discharge.

Recently SF6 has been used in the production of fluo-

rine atoms in plasma etching of silicon or other semi-

conductor materials [3,149].

2.2 Energy Units and Formation Rates

3. DECOMPOSITION OF SF6

3.1.1 SF6 Decomposition in a Power Are

This Section reviews the phenomenological observations

of SF6 decomposition in a power arc while Sections 3.2

and 3.3 deal with decomposition in spark and corona con-

ditions respectively. The first reported observation of

decomposition of SF6 was made by Schumb et al. [165] in

a 20 kV corona discharge and in an ac arc. A portion of

the arced gas was absorbed by potash lye, while the non-

absorbed portion had oxidizing properties. The authors

concluded, on the basis of the chemical reactions, that

the products were lower fluorides of sulfur. No further

attempts to analyze the products were made. Camilli,

Gordon and Plump [30] reported the observation of SF2

Throughout this review, the term "energy per unit vol-

ume" will be employed in describing the formation rates

of many SF6 decomposition products. The discharge

energy is calculated according to the nature of the dis-

charge and is expressed in J or kJ. The volume of SF6

at standard temperature and pressure (STP) conditions

is used to obtain the value of energy per unit volume.

In most analytical techniques, the quantity of the de-

composition products is expressed in microliter per

liter (pl/1) or parts per million by volume (ppmv). In

order to adopt a uniform unit for comparison purposes,

the concept of mole will be used. Taking a mole as

22.4 1 at STP, the unit of mole per joule (mol/J) or

10-9 mole per joule (nmol/J) will be adopted in this re-

view. The unit for solid arcing by-products is

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and SF4 by IR spectroscopy in SF6 which had been expos-

ed to a 5 kA arc discharge between copper electrodes

separated by 1.3 cm. Traces of C02, H2S, and HF were

also identified, but no S2FJO was found. A more thor-

ough investigation was carried out by Edelson using an

IR technique to study the formation of SF6 arc by-

products in a 60 Hz discharge [64]. SF2 was identified

as the main decomposition product, along with SOF2 and

SiF4.

The formation of SF4 in a power arc was further con-

firmed by Becher and Massonne [14] who carried out SF6

arcing experiments in a geometry similar to Edelson's.

A 220 V ac breaking arc was established with a striking

frequency of 1 Hz. The arc current was estimated be-

tween 5 to 10 A with a duration of 0.23 s. The concen-

tration of the by-products in the cell was monitored in-

situ by IR spectroscopy. The results showed that SF4

and SOF2 were formed preferably in arcs, whereas the

compounds of SOF4 and S02F2 were prevalent in the spark

discharges. S2Fl and S2FlOO were never detected during

SF6 decomposition in arcs while quantities in the range

of 100 ppmv of S2FlO were detected in spark experiments.

Tokuyama et al [179] studied the influence of 15 elec-

trode materials on the amount of decomposition in SF6.

Current interruptions in 3 atm SF6 at 800 A and 127 V

produced up to 10% decomposition. Experiments were

carried out with full-sized circuit breakers. Empirical

formulae for the amount of decomposition as a function

of the arc current and duration were derived. For Cu and

Cu-W electrodes the volume of the by-products is given

(2)

Leeds, Browne and Strom [118] studied the formation

of SF6 arc by-products in a commercial size 132 kV cir-

cuit breaker and proved the effectiveness of activated

alumina in removing the by-products. A few kg of acti-

vated alumina was proven to be sufficient to remove all

the by-products generated during 50 current interrup-

tions at 5000 A. The drop in pressure due to absorption

was about 2%. No test was made to determine the iden-

tity of the products.

Gerasimov and Sidorkina [78] carried out experiments

with gas mixtures containing SF6 and air in a 500 W

arc. IR analysis confirmed the formation of SOF4 as

well as S02F2 as the main breakdown products. The

authors' observations were quite different from many

other investigations which confirmed SOF2 as the main

decomposition product. Manion, Philosophos and Robin-

son [129] studied the stability of some electronegative

gases in low-energy discharges and arcs with copper

electrodes. In an 80 A arc, the formation rates of SF4

and SOF2 were determined quantitatively by mass spec-

troscopy as a function of arc duration. The SF6 decom-

position rates and the dielectric strength of the gap

are shown in Fig. 1. The dielectric withstand of the

v = 0. 043I2; t (with Cu-W)

v = 0. 02018t (with Cu)

where v is the absolute volume of the decomposed gas in

liters, lis the interruption current in kA rms and t is

the duration in s. The gas, decomposed in the presence

of aluminum and zinc electrodes, contained 100 times the

amount of arc by-products as gas decomposed in the pres-

ence of silver or copper electrodes. The primary com-

position of the gases identified in circuit breaker in-

terruptions by mass spectrometer were SF4, SOF2.

Boudene et al. [23] carried out a comprehensive study

of SF6 arc by-products covering formation mechanisms,

detection techniques and toxicities of the stable arcing

by-products. In experiments with arc parameters of 500

A, 60 V and 40 to 80 ms, the formation rates of various

arc by-products as a function of the input arc energy

per unit gas volume were determined. The arc by-product

concentrations were measured by gas chromatographic (GC)

and infrared techniques. A linear relationship was ob-

tained on the production rate of SOF2 and total SF6 de-

composition as a function of arc energy. The formation

of SOF2 in Cu-W electrodeswas determined to be 90x10-9

mol/J while the formation of S02F2 was estimated to be

about 150x10-9 mol/J. Fig. 2 shows the quantities of

SOF2 and S02F2 generated as a function of arc energy per

unit volume for cutene electrodes. The influence of

electrode materials, moisture, and oxygen on the forma-

tion of SF6 arc by-products was also investigated.

Other gaseous by-products including SOF4, S02, HF, and

WF6 were observed but only at trace quantities as com-

pared to SOF2.

The influence of moisture in the decomposition process

was investigated by Hirooka et al. [93] who used the ' F

nuclear magnetic resonance (NMR) technique to observe

the formation of arc by-products in a 9.2 kV, 30 kA arc.

When the test vessel was dried before the arcing experi-

ment, lower fluorides of sulphur such as SF4, S2F2, SOF2

were detected. However, when the vessel was not dried

before the experiments, the major products were WF6,

S0F2, S02F2, CF4, and SF4. The compound Si(CH3)2F2 was

identified in the wet vessel experiment which the authors

claimed was formed from the reactive arc by-products and

the silicone vacuum grease.

(3)

500 1000 1500 2000 2500

ARC TIME - ARC CURRENT PRODUCT (COULOMBS)

Fig. 1: SF6 decomposition, decomposition products

and dielectric strength as a function of arc time-

arc current products. Conditions: arc current

80 A, atmospheric pressure, arc gap 3.2 mm. t129]

test gap remained unchanged even after 40% of the SF6

was decomposed into SF4 and SOF2 after arcing. The

authors also observed the interaction between SF4 and

the pyrex wall of the 0.6 1 sample cell forming SiF4

and SOF2.

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compound S02F2 existed in far greater quantities than

SOF2. However, S02F2 was completely absent in arc dis-

charges. No SOF4 was detected in the GC/IR or GC/MS

analysis in either type of discharge. The by-product

formation rate was found to be proportional to I 2t where

I is the current and t is the time.

Baker et al. [11] performed high-current arcing ex-

periments in the 30 to 50 kA range to determine the

formation of gaseous and solid arcing by-products in

conditions simulating a power arc fault in gas-insulated

equipment. In a 15 kA arc for a duration of 133 ms, the

level of SOF2 can reach 2% by volume in a 120 1 chamber.

The authors reported a linear relationship between the

amount of decomposition products formed and the arc

energy. Of the seven samples analyzed by GC and GC/MS,

only one sample contained any detectable SO2F2 and its

concentration was 67 times lower than SOF2. The authors

observed up to 6000 ppmv of CF4 in a 41 kA experiment

involving polymeric spacers.

Tominaga et al. [181] examined the amount of decompos-

ed gas in an SF6 arc for aluminum, copper and steel

electrodes. The current ranged from 1 to 50 kA which is

typical of short-circuit currents in gas-insulated sub-

stations. In all cases of the high-current arc experi-

ments, the major by-product was SF4 which subsequently

was reported to convert to SOF2. The decomposition

product formation rate at 30 kA was almost 10 times that

of the rate reported by Boudene at 500 A. The author

suggested that the formation rate discrepancies may be

due to the differences in current between the two experi-

ments.

>0J}

> _

0 E

vq -

CL ,

E o w

-o

10 20

Energie dissipee par Litre de S F6 ( kJ/ t )

Dissipated energy by Liter of SF (kJ/L)

Fig. 2: Formation of SOF2, 502F2, SOFa4 under the

effect of arcs between,electrodes of cutene and

cunitene as a function of total energy dissipated

per liter of SF6. 02 content, 4000 ppmv; H20

content 200 ppmv. Curve 1: SOF2, Cutene, Curve

2: SOF2, cunitene, Curve 3: S02F2, Curve 4:

SOF4 [23].

A series of experiments simulating fault arc conditions

were reported by Chu et al. [43,44,45]. The major by-

products generated in experiments with aluminum elec-

trodes and copper electrodes was SOF2. The formation

rates were in the range of 300 to 450x10-9 mol/J.

Latour-Slowikowska et al. [117] examined the influence

of gas contaminants such as moisture in SF6 arc dis-

charges. Experiments with 1 kA arc current generated

SOF2 and S02 as the major by-products. Even at high

moisture contents of 2400 ppmv, the major species de-

tected was still SOF2 with SO2 being the minor compound.

No S02F2 or SOF4 was detected.

Bartakova [13] performed several experiments with arc,

corona, and dc spark discharges. Analysis of the by-

products was made with the GC technique. The major

products identified in the high-current discharge were

SF4 and SOF2-

Ruegsegger et al. [153] employed a molecular beam set-

up to detect the decomposition species from a high-

pressure arc discharge in SF6. A 15 kA arc was generated

between graphite electrodes at 1 atm pressure. The in-

situ mass spectrometer detected SF4 and SOF2 in the time-

of-flight mass detection technique. The recovery time

of SF6 after the discharge was estimated to be in the

order of ms. The experimental geometry did not allow

enough time for the SF4 to react with moisture outside

the arc zone to form SOF2. Therefore, SOF2 could be

formed in the neighborhood of the arc as a result of the

reactions between moisture in the electrodes and SF4.

3.1.2 Discussion of Power Arc Results

Table 3 summarizes the gaseous decomposition products

identified after high-current arcing had taken place in

SF6 for various electrode materials. The trend is quite

clear that lower fluorides of sulphur, SF2 and SF4, were

identified in experiments using IR techniques where in-

situ determination of the by-products was possible. This

observation indicates that these species are the primary

products in the decomposition process. With only two

exceptions [30,78], SOF2 was identified in every investi-

gation. This species also dominates in quantity in all

high-current arc experiments. The formation rate of this

Kulseta et al. [109] measured the formation of SOF2

in high current arcs (3.7 to 38 kA) for durations rang-

ing from 60 to 200 ms. The electrode materials used

were aluminum and copper. The formation rate of alumi-

num electrodes were determined to be between 90 to

500x10-9 mol/J. The formation rate of by-products with

aluminum electrodes was 3 to 5 times higher than the

rate with copper electrodes. The exothermic reactions

between aluminum and SF6 at high temperature is respon-

sible for the higher by-product formation rates with

aluminum. Significant quantities of CF4 were found in

experiments in which the arc burned near epoxy spacers.

Grasselt et al. [80] carried out high and low current

arc experiments in SF6. The arc by-products were sepa-

rated by gas chromatograph and the species were identi-

fied by means of IR spectroscopy for S02F2 and mass

spectrometry for CF4 and SOF2. The authors observed

that in low-current discharges such as a spark, the

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