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Posts Tagged ‘power breakers’

Circuit Breaker Arc Chute Missing – Remove the Cover

April 4th, 2011 Comments off

MIDWEST has its own internal blogosphere that especially hums away when something unusual happens, like when one of the Engineers or Engineering Technician discovers something important or rare or both with the electrical equipment they are working on. Here is an example of some internal buzz that also proved the

MA31000 Square D Circuit Breakers For Sale

MA31000 Square D Circuit Breakers For Sale

importance of removing the covers from electrical power circuit breakers, whether Square D circuit breakers, Cutler Hammer circuit breakers or GE General Electric circuit breakers. The need is not related to the manufacturer. In this example, the Engineering Technician was reconditioning and testing a 1000 amp Square D circuit breaker. When he removed the cover of the circuit breaker, he was amazed to find the center pole arc chute was gone. Completely missing. Fortunately the arcing and main contacts appeared undamaged. There was some arcing marks on the arcing contacts, but nothing serious. One could do high current tests, contact resistance tests, and insulation resistance tests and never catch this defect. But the first time this breaker tried to interrupt a large fault current, it could have failed, with the resulting arcing causing an eventual phase to phase arcing fault on the line side of a large circuit breaker. This arcing fault might not be large enough to trip the main breaker immediately. If for some reason the panel board was a MLO, main lugs only, the protective device might be the main breaker or fused switch on the line side of an upstream transformer. This is a worst case environment, but it does happen. It is especially dangerous because the arcing fault may last several minutes before the main protective device operates. And more scary is the possibility of someone being injured by the fault. This is an incipient failure. Also an insidious failure. It is another example of the reason to properly recondition replacement power circuit breakers. Testing alone is not enough. 

How Circuit Breaker Arc Chutes Work

June 16th, 2010 1 comment

Buy EHD3070 Westinghouse Molded Case Circuit Breaker

Buy EHD3070 Westinghouse Molded Case Circuit Breaker

The arc chutes for molded case circuit breakers and old power breakers look simple enough, but they actually are a complex design and perform extremely important functions.  For excitement, read the patent application for an old circuit breaker arc chute, if you can stay awake.  MIDWEST sometimes is asked to explain what an arc chute does. We always go for the short version.  Basically an old or new circuit breaker arc chute stretches the arcing that takes place when a circuit breaker opens, such that the arc is too long for the voltage to keep it going.  Arc chutes have arc dividers in the form of flat segments stacked one above the other, with an air gap between them.  When the arc occurs, it is expelled into the arc chute and into the arc dividers, such that it wraps back and further between the arc dividers. The wrapping back and forth around the arc dividers effectively stretches the length of the arc until it is just too long for the voltage to keep it going.  When this happens, the arcing stops.  The arc has been extinguished.  When the circuit breaker opens, the main current carrying contacts open first and a different set of contacts, the arcing contacts, open second, such that the arcing contacts endure limited damage from the arcing, until the arc chutes interrupt the arc. So the combination of the arc chutes and the arcing contacts protect the main contacts from arcing damage when the circuit breaker opens and when it closes.  When a circuit breaker is closed, the arcing contacts close first, again taking on the arc such that the main contacts are protected from arcing damage when closed.  This is especially important when the circuit breaker interrupts a high current fault and there is a real blast in the arc chutes.  Each phase, ie pole, of a circuit breaker has a separate arc chute. This is pretty much how the arc chutes of obsolete, old, and new replacement circuit breakers operate.  So the arc chutes perform an extremely important function.