A111D5 15bE04

Safety Considerations for Handling

SF 6 Used in Electrical Equipment

David A. Leep and Richard J. Van Brunt

Electrosystems Division

Center for Electronic and Electrical Engineering

National Bureau of Standards

U.S. Department of Commerce

Washington, DC 20234

June 1980

Sponsored by:

department of Energy

th Street & Pennsylvania Ave., N.W.

ishington, DC 20234

NATIONAL BUKBATJ 07 STANDABBS

LIBRARY

NBSIR 80-2063

SAFETY CONSIDERATIONS FOR HANDLING

SF 6 USED IN ELECTRICAL EQUIPMENT

JUL 1 1 1980

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David A. Leep and Richard J. Van Brunt

Electrosystems Division

Center for Electronic and Electrical Engineering

National Bureau of Standards • U.S. Department of Commerce

Washington, DC 20234

June 1 980

Sponsored by:

Department of Energy

12th Street & Pennsylvania Ave., N.W.

Washington, DC 20234

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

TABLE OF CONTENTS

I. Introduction . 1

II. Pure SFg . . . 2 III. Contaminated or OecomDosed SF

g

3

IV. Recommendations for Safe Handling of SF^ . t 4

V. ApDendix . . . 6

VI. References . . 7

I

i

Safety Considerations for Handling SFg

Used in Electrical Eauipment

Prepared by

David A. Leep

and

Ri chard J. Van Brunt

' Applied Electrical Measurements

Electrosystems Division

National Bureau of Standards

This report has been prepared for the benefit of those within the National

Bureau of Standards who must use caseous sulfur hexafluoride for electrical

insulation. The information presented herein has been gathered from the

National Institute of Occupational Safety and Health's Registry of Toxic

Effects of Chemical Substances and from other sources believed to be reliable,

but this information is not the result of any critical data evaluation or

testing activity of NBS. The publication, reprinting, reproduction , or

open-1 iterature listing of this report either in whole or in part, is not

authorized unless permission is obtained from the Office of the Director,

National Bureau of Standards, Washington, D.C. 20234.

I. INTRODUCTION

The purpose of this report is to introduce laboratory workers at the National Bureau

of Standards to the hazards associated with use of sulfur hexafluoride (SFg)

in electrical equipment and to recommend precautions that should be taken to

insure safety in handling of this gas. This report is not concerned with other

uses of SF^ or its mixtures with other gases.

Compressed gaseous SF^ is a common electrical insulant in various t.ynes of

high voltage electrical apparatus such as circuit breakers, oower transformers

,

capacitors and bushings. Occasionally it is necessary to vent or otherwise

expel this gas from equipment in order to perform routine maintenance or repairs.

There also exists the possibility that an SF^- containing vessel will accidentally

develop leaks resulting in release of significant quantities of nas into the

laboratory environment. One should thus be aware of the possible dangers of

exposing oneself to SF^, particularly if the eauipment involved is known to have

experienced an electrical failure which might have caused decomposition of a

significant portion of the gas.

Under normal atmospheric conditions at room temperature, SF^ is an inert,

chemically stable gas and is not by itself considered to be toxic. However,

some of its decomposition products which might, for example, result from

» electrical arcing in the gas are known to be highly reactive and toxic. It

is not the purpose of this report to Drovide detailed information about the

toxicity of all possible SF^ decomposition products. Some information concernina

the more common gaseous by-products is given in the Appendix. More information

can be obtained from the references cited.
- 2

-

II. PURE SF

6

Sulfur hexafluoride in the oure state is not considered dangerous to life

unless it accumulates to such a degree that it displaces necessary oxygen

for a sufficient length of time to interfere with normal respiratory functions.

This could occur only in situations where SF^ is released into an enclosure o**

depression having poor ventilation. Because it is five times more dense than

air SFg will, if leaked into a low area of little circulation, tend to settle

upon displacing air. The danger of asphyxiation from SFg which has displaced

air is enhanced by the fact that this gas is odorless and colorless.

Laboratory tests"' have shown that animals exhibit no ill effects upon prolonged

breathing of an atmosphere containing 20% 0^ and 80% SF^, i.e. , an atmosphere

in which naturally occurring M

2

is replaced by SF

p

. Nevertheless, the

p Occupational' Safety and Health Administration (OSHA) regulations' limit

occupational exposure to SF^ in air to a time-weighted average (TWA) of 1000 ppm

by volume.

Not only is SF^ nontoxic, but it is also chemically inert and does not

decompose under normal atmospheric conditions. Moreover, it is chemically

stable with respect to most common laboratory materials even at red heat.

Although SFg is nonflammable in air, it will react readily if heated or sparked

with H

2 or H

2 S, but since these gases, if present, are considerably more hazardous

than SF

fi

, it is unlikely that a situation would exist where these gases would

accidentally mix in the laboratory environment. Mixtures of SFg and for example

are potentially explosive and require special precautions in handling which

are not covered here.
-3-

III. CONTAMINATED OR DECOMPOSED SF C6

It is known that if SFg (particularly SFg contaminated by air, moisture or

hydrocarbon containing materials like oil) is thermally decomposed, a variety

of toxic by-products can be produced depending on the environment in which the

gas exists. This can happen, for example, in an electrical arc or when the

gas has been subjected to continuous corona discharges. The toxic by-products

that represent the greatest immediate hazards are, of course, those that exist in

the gaseous state under normal conditions, and can, therefore, be released into

the atmosphere of the laboratory in the event of a rupture or intentional opening

of an SFg-containing vessel. The nocuous gaseous by-products of chief concern

are"

1 ’^ -7

the ox.yfluorides SOF^, SO

2 F2

, SO

F^

and HF and SC^. All of these with

the exception of have strong odors and thus provide adequate warning of

their presence. It should be noted, however, that SC^F^ is much less toxic than

the other gases and, when formed from SFg under arcs, is always accompanied by

the others. Tests indicate that of the oxyfluorides resulting from decomposition

of SFg in an arc, the most prevalent is usually S0F ?

, which has a prominent rottenegg odor.

Toxic, lower valence, sulfur fluorides such as SF^, S^, and S 9 F^

Q as well as

free fluorine, although often formed in initial decomposition of SFg

7

, tend to

react among themselves or with moisture to form SFg, oxyfl uorides , SO^ and HF.

Of these SF^ is usually generated in greatest abundance and could escape into

the laboratory at a sufficient rate to be harmful. This gas also has a disagreeable

odor which permits its presence to be easily sensed. The Appendix contains

toxicity information obtained from Ref's 1-4 on some of the gaseous decomposition

products mentioned above.
-4-

In addition to being toxic some of these gases such as SO^ 9 HF and SF^ are

known to be very reactive or corrosive and may, for example, readily attack

insulating materials. This can lead to production of other toxic gaseous

or solid compounds. If tungsten or silicon are present in the apparatus one

can expect production of gaseous VJF g and SiF d. Presence of carbon or

hydrocarbons like PTFE (polytetrafluorethylene) can yield CF^, CO or CO^, although

of these only CO is considered harmful. If arcing occurs along or near

polyethylene or epoxy or phenolic resins, one can also expect formation of

5 CS 9 , COS, or COF^ . Most metal fluorides which might be formed in electrical

apparatus, such as those of aluminum or copper are solids under normal conditions

and are thus unlikely to represent an immediate hazard. However, when cleaning

or repairing SF

g -containing equipment, care should be taken to remove oowders

and other solid deposits that might be formed so as to avoid possible inhalation

of these.

IV. RECOMMENDATIONS FOR SAFE HANDLING OF SF

g

Although SF

g is not considered a toxic substance there are conditions under

which improper handling of this gas can be hazardous to one's health. Based

on the information given above we recommend that the following orecautions be

taken in working with SF

g

:

1. Avoid unnecessary exposure to this gas at concentrations greater than

specified by OSHA regulations.

2. Vessels containing SF

g should never be vented directly into an

enclosure or room with poor ventilation.

3. A hose or other arrangement should be used. in venting SF

g to carry

the gas either outside of the buildina or into a venting system or

fume hood.
-5-

4. If possible, vessels containing pressurized SF^ should be

equipped with pressure gauges that should be checked regularly to

determine possible existence of leaks. This is important not only

because leaking gas may be hazardous but also because insulating

quality of gas decreases with decreasing pressure, thus increasing

the risk,of electrical breakdown.

5. If an SFg containing vessel is known to be leaking, it should if

at all possible be placed in a well ventilated area preferably outside

the building, and one should be alert for obnoxious odors (pungent,

like F^S or S0

?

) which would indicate the presence of toxic decomposition

products, primarily SO F^

•

6. In the extreme case of an electrical fault leading to rupture

of an SFg-containing vessel, one should leave the room immediately

and report this as an emergency. Re-entry should not be attempted

without proper respiratory equipment as lonn as obnoxious odors persist.

In cleaning up damaged equipment containing contaminated SFg resulting

from electrical failure it is essential that all decomposed gas be

removed preferably by dumping gas into a fume hood or gas cart with

a pump or blower. The dust-like metallic fluorides should be removed

using a well filtered vacuum cleaner, and finally the vessel should be

wiped clean with rags. Those performing this task should protect not

only their lungs but also their skin leaving no parts of their body

exposed, e.g., they should have sleeves and pants legs taped, wear

gloves, etc.

7. It is possible to filter out toxic S0F

?

, SO^F^, S0Fd

, SF^, S0

? and

HF from SF^ by flowing the gas through activated charcoal or alumina

filters. The effectiveness of such filtering depends on the selection

and conditioning of the filters, details of which are not included
- 6 -

here. Such filters may be useful with SFg-insulated equipment in

which arcs are to be expected during normal operation, e.a., spark

gaps or circuit breakers. Most commercially available equipment

of this type are already provided with activated alumina absorbers

to help remove decomposition products.

It should be noted that these recommendations are believed to conform to current

practices recommended by industrial users and manufacturers of SF^ for

electrical insulation.

V. APPENDIX

The toxicity data summarized below are reproduced from Ref. 4, in which

references to the primary data sources may be found. Brief definitions of

the standard abbreviations for inhalation toxicity of gases in air are aiven

below.

LC^q = lethal concentration reported to kill 50% of test population

LCLo = lowest concentration reported to have caused death of humans

or test animals.

TLV = threshold limit value, the maximum deemed tolerable for

daily 8-hr. exposure

TWA = time-weighted average

ppm = parts per million by volume

SFg : OSHA standard TWA = 1000 ppm

TLV = 1000 ppm

SOF^ : LCLo (mouse) = 260 ppm/1 hr

LCLo (rabbit) = 1000 ppm/1 hr

3 NIOSH-recommended standard TWA =2.5 mg/m
-7-

SO

2 F2

: LCLo (mouse) = 1200 ppm/1 hr

LCLo (rabbit) = 5000 ppm/1 hr

LCjq (rat) = 3020 ppm/ 1 hr

OSHA standard TWA = 5 ppm

TLV = 5 ppm

SF^ : LCLo (rat) - 19 ppm/4 hr

TLV = 0.1 ppm

S

2 F1 Q

: LCLo (rat) = 1780 ppm/1 hr

LCLo (mouse) = 130 ma/m / 1 0 min

OSHA standard TWA = 250 pg/m^

TLV = 0.025 ppm

S0Fd : [No Quantitative information available, but believed on the basis

of information given in Ref. 1 to be similar in toxicity to SF^.l

SO

2

: LCLo (mouse) = 6000 ppm/5 hr

LCLo (frog) = 1 ppm/15 min

OSHA standard TWA = 5 ppm

TLV = 5 ppm

One could qualitatively describe the odors of these gases as follows:

S0F o like rotten eggs; SF^, S0F,,, SO

2 acrid; and SFg, SO2 F2

, S

2

F.JQ odorless.

VI. REFERENCES

1. C. Boudene, J. Cluet, G. Keib and G. Wind, "Identification and Stifdy of

Some Properties of Compounds Resulting from Decomposition of SFg Under

the Effect of Electrical Arcing in Circuit - Breakers," Revue Generale

De l' Electricite (RGE) - Numero Special, pp. 45-78 (1974).
- 8 -

2. Federal Register (Washington), Vol . 37, n. 22139 (1972).

3. Matheson Gas Data Book (Matheson Company, East Rutherford, NJ).

4. 1978 Registry of Toxic Effects of Chemical Substances , National Institute

for Occupational Safety and Health. U.S. Department of Health, Education

and Welfare (NIOSH), Publication No. 79-100; U.S. G.P.O. Stock No.

017-033-00346-7. A continually updated version of this reference is

"RTECS," a computer-searchabl e data base accessible through the

National Library of Medicine and through the NIH/EPA Chemical Information

System.

5. J. Castonquay, A. Theoret and R. Gilbert, "Arc Degradation of SF^. in the

Presence of Polymeric Insulating Materials," Conference Record of 1978,

IEEE International Symposium on Electrical Insulation, Philadelphia,

pp. 162-166 (1978).

6. K. Hirooka, H. Kuwahara, M. Noshiro and Y. Jitsuairi, "Decomposition

Products of SFg Gas by High-Current Arc and Their Reaction Mechanism,"

Elec. Eng. in Japan, Vol. 95, No. 6 (1975).

7. S. Tominaga, H. Kuwahara, and K. Hirooka, "Influence of Arc-Decomposed

Gas on Cast Epoxy Insulators for Gas Insulated Switchgear," IEEE Trans.

Power Apparatus and Systems, Vol. PAS-98, No. 6, pp. 2107-2114 (1979).
,

I