Tech Quiz #88
SF6 Circuit Breakers
Jim White
Shermco Industries
1. What is the primary advantage of sulfur hexafluoride gas
as an insulator?
a) costs a lot less than mineral oil
b) if released into the atmosphere it causes no harm
c) it is not affected by moisture
d) its insulating strength increases with pressure
2. List four risk categories of SF6 gas contamination:
5. What is meant by a “dead tank breaker?”
a) The breaker operating mechanism is deenergized.
b) The contacts are arranged in such a manner that they
extend through the porcelain housing.
c) They are safe to rack in or out while under load; no
PPE is needed.
d) The external tank is grounded making it “dead.”
a) ____________________________________.
c) ____________________________________.
d) ____________________________________.
3. What is added to SF6 gas for cold climates?
a) hydrogen (H2 ) and/or freon-14 (CF4 )
b) acetylene (C2H2 ) and nitrogen (N2).
c) nitrogen (N2) and/or freon-14 (CF4 )
d) None of the above
4. When arcing occurs in SF6 , what is one hazardous
byproduct?
a) metal fluorides.
b) sulfur dioxide.
c) hexadecimals.
d) None. SF6 is inert and doesn’t react to arcing.
Jim White is the Training director for Shermco Industries and the
principal Shermco representative on the NFPA 70B committee. Jim is
the alternate NETA representative on the NFPA 70E committee and
serves as the NETA representative on the IEEE/NFPA Arc-Flash
Hazard Work Group (RTPC) Ad Hoc Committee. He served as the
Chairman of the 2008 IEEE Electrical Safety Workshop. Jim is a NETA
Certified Level IV Electrical Testing Technician and a member of the
NETA Safety Committee.
www.netaworld.org Answers:
1. (d) SF6 has a number of advantages, so the primary one is
open to opinion. The dielectric strength of SF6 increases with
pressure so smaller breakers can be used at higher voltages. The
breakers have a smaller footprint, so they use less real estate
which can be very expensive. They are very reliable and require
less maintenance than air blast or oil breakers and can operate
at higher voltages effectively.
2. (a) New gas – in cylinders from the gas manufacturer
(b) Non-arced – has been used or handled, but contains no
arcing byproducts
(c) Normally arced – corrosive byproducts less than 200 ppm
(d) Heavily arced – contains arcing byproducts greater than
one percent by volume
Editor’s Note: Jim has referenced CIGRÉ Task Force
B3.02.01 SF6 Recycling Guide for this question.
3. Nitrogen and freon-14. They are added to keep SF6 gasified
at lower temperatures. SF6 has a tendency to liquefy and pool
at colder temperatures. This reduces the internal pressure, which
reduces the dielectric strength.
4. (a) Metal fluorides. As SF6 decomposes during arcing metal
fluorides are created, which are corrosive. When servicing SF6
breakers that have been faulted, full body protection and oxygenfed respirators may be needed to prevent the fluorides from
contacting the skin or being inhaled. Any moisture on or in the
body will create hydrofluoric acid.
5. (d) Dead tank breakers have grounded tanks. Live tank
breakers use the insulating column to house the mechanism and
contact assemblies, so they are energized at system voltage (live).
b) ____________________________________.
Summer 2009 NETA WORLD
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