10
SECTION 6 - MAINTENANCE OF SF
6
GAS
CIRCUIT BREAKERS
6.1. PROPERTIES OF SF
6
(SULFUR
HEXAFLUORIDE) GAS. -
a. Toxicity.- SF
6
is odorless, colorless,
tasteless, and nontoxic in its pure
state. It can, however, exclude oxy-
gen and cause suffocation. If the
normal oxygen content of air is re-
duced from 21 percent to less than 13
percent, suffocation can occur without
warning. Therefore, circuit breaker
tanks should be purged out after
opening.
state. It can, however, exclude oxy-
gen and cause suffocation. If the
normal oxygen content of air is re-
duced from 21 percent to less than 13
percent, suffocation can occur without
warning. Therefore, circuit breaker
tanks should be purged out after
opening.
b. Toxicity of arc products.- Toxic
decomposition products are formed
when SF
decomposition products are formed
when SF
6
gas is subjected to an elec-
tric arc. The decomposition products
are metal fluorides and form a white
or tan powder. Toxic gases are also
formed which have the characteristic
odor of rotten eggs. Do not breathe
the vapors remaining in a circuit
breaker where arcing or corona dis-
charges have occurred in the gas.
Evacuate the faulted SF
are metal fluorides and form a white
or tan powder. Toxic gases are also
formed which have the characteristic
odor of rotten eggs. Do not breathe
the vapors remaining in a circuit
breaker where arcing or corona dis-
charges have occurred in the gas.
Evacuate the faulted SF
6
gas from
the circuit breaker and flush with fresh
air before working on the circuit
breaker.
air before working on the circuit
breaker.
c. Physical properties.- SF
6
is one of
the heaviest known gases with a den-
sity about five times the density of air
under similar conditions. SF
sity about five times the density of air
under similar conditions. SF
6
shows
little change in vapor pressure over a
wide temperature range and is a soft
gas in that it is more compressible
dynamically than air. The heat trans-
fer coefficient of SF
wide temperature range and is a soft
gas in that it is more compressible
dynamically than air. The heat trans-
fer coefficient of SF
6
is greater than
air and its cooling characteristics by
convection are about 1.6 times air.
convection are about 1.6 times air.
d. Dielectric strength.-
SF
6
has a di-
electric strength about three times
that of air at one atmosphere
that of air at one atmosphere
pressure for a given electrode
spacing. The dielectric strength
increases with increasing pressure;
and at three atmospheres, the
dielectric strength is roughly
equivalent to transformer oil. The
heaters for
spacing. The dielectric strength
increases with increasing pressure;
and at three atmospheres, the
dielectric strength is roughly
equivalent to transformer oil. The
heaters for
SF
6
in circuit breakers are
required to keep the gas from
liquefying because, as the gas
liquifies, the pressure drops, lowering
the dielectric strength. The exact
dielectric strength, as compared to
air, varies with electrical configuration,
electrode spacing, and electrode
configuration.
liquefying because, as the gas
liquifies, the pressure drops, lowering
the dielectric strength. The exact
dielectric strength, as compared to
air, varies with electrical configuration,
electrode spacing, and electrode
configuration.
e. Arc quenching.- SF
6
is approxi-
mately 100 times more effective than
air in quenching spurious arcing.
air in quenching spurious arcing.
SF
6
also has a high thermal heat capacity
that can absorb the energy of the arc
without much of a temperature rise.
that can absorb the energy of the arc
without much of a temperature rise.
f. Electrical arc breakdown.- Because
of the arc-quenching ability of SF
of the arc-quenching ability of SF
6
,
corona and arcing in
SF
6
does not
occur until way past the voltage level
of onset of corona and arcing in air.
SF
of onset of corona and arcing in air.
SF
6
will slowly decompose when ex-
posed to continuous corona.
All SF
6
breakdown or arc products are
toxic. Normal circuit breaker operation
produces small quantities of arc
products during current interruption
which normally recombine to
produces small quantities of arc
products during current interruption
which normally recombine to
SF
6
.
Arc
products which do not recombine, or
which combine with any oxygen or
moisture present, are normally re-
moved by the molecular sieve filter
material within the circuit breaker.
which combine with any oxygen or
moisture present, are normally re-
moved by the molecular sieve filter
material within the circuit breaker.
6.2. HANDLING NONFAULTED SF
6
.
The procedures for handling nonfaulted
SF
6
are
well covered in manufacturers'
(FIST 3-16 1/92)