CIRCUIT BREAKERS
Circuit breakers basically consists of two main parts, the interrupting chambers
and the operating mechanism. The interrupting chambers normally do not require routine
preventive maintenance other than cleaning but operating mechanism do require proper
upkeep.
(a)Interrupting Chamber
Circuit breaker interrupting chamber is an enclosed unit mostly filled with oil or
SF6 gas. Lower voltage Circuit breakers have vacuum interrupting chambers also. There
is stress on the contacts during fault current interruption and damages may happen in
arcing contacts or main contacts. The breaker interrupting chamber is recommended to be
opened only based on condition monitoring tests or as per advice of the manufacturers.
(b)Operating Mechanisms
Normally circuit breakers have pneumatic, hydraulic and spring operating
mechanisms. As operating force is required for closing and tripping of circuit breakers,
there can be combination of these mechanisms in one circuit breaker. Since operating mechanisms have a number of moving parts, they need more maintenance such as greasing, lubrication, cleaning, setting of limit switches, etc. Compressors/ oil pumps/ spring charging motors also require maintenance. Other maintenance on particular operating mechanism such as air compressor maintenance, nitrogen priming pressure checking in hydraulic mechanism, checking of over travel, checking of gaps in operating plunger of close/ trip coils etc. are to be carried out as the case may be and as specified by the manufacturers.
(c)SF 6 Gas
Most of the higher voltage circuit breakers adopt SF6 in interrupting chamber. The
density of SF6 gas is about five times that of air and heat dissipation is also much more
than air. At atmospheric pressure, dielectric strength of SF6 gas is about 2.4 times that of
air and at about 3 kg/cm2 it is same as that of oil. As SF6 is Green House gas, it needs to
be handled carefully and should not be let to the atmosphere.
(d)Emptying and Re-filling of Gas
The breaker is evacuated by means of the gas treatment equipment that purifies
and also compresses the gas for storage, so that it can be reapplied. For economic and
ecologic reasons, SF6 contained in electrical equipments, should not be vented into
atmosphere. Prior to the gas removal, the quality of the SF6 gas should be tested.
Operational contamination should be absorbed with suitable filter unit provided in
the gas handling plant. Such filters/ sieves should already be installed into the SF6 gas
maintenance/handling unit. When SF6 is suctioned from a gas compartment, the gas is
passed automatically through filters, which dry and purify the gas.
(e)Evacuation of SF6 Gas Circuit Breakers
After maintenance/overhaul of the circuit breaker, it should be evacuated by
vacuum PUI1l] before filling in the SF6 gas so that SF6 gas does not mix with ambient
air and also humidity and dust particles are removed from the Breaker. With vacuum
pump, a final vacuum must be reached less than 5 mbar