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How Do You Test a Circuit Breaker with Ground Fault Protection

June 2nd, 2011 2 comments

Over the years MIDWEST has been asked many times how we test circuit breakers that have ground fault protection. High current test sets inject single phase current through one pole, ie phase, of the circuit breaker and the test is timed to see if the breaker trips open within the manufacturer’s specified time, based on the TCC, time current curve.  Whether a GE General Electric circuit breaker, or Cutler Hammer or Square D circuit breaker, molded case circuit breaker or air circuit breaker, the same theory applies to the test procedure. Some electronic overcurrent devices on circuit breakers have a feature allowing you to turn off or defect the ground fault protective function. The manufacturer’s specification sheets should explain this. But, if there is no way to turn off the ground fault protective function on a Westinghouse circuit breaker, for example, a specific test procedure must be followed or the circuit breaker will trip open on ground fault function long before you can put enough current through the breaker to properly test the long time or short time function. Maybe the ground fault pickup range is 100 to 1200 amps and the time delay range is 0.1 to 1.0 seconds.  But your 1600 amp Siemens breaker should be long time tested at 300% or 4800 amps and it will take the breaker 22 seconds to trip at that current level. The procedure is to inject current through one phase, current transformer, and then connect the test set up such that the current returns through a second phase, current transformer, in the opposite direction. The currents will cancel out such that the ground fault pickup sees zero current.  Be sure to test in all three possible combinations. Then each phase is tested for ground fault pickup and delay by just injecting current through that phase. These tests are more time consuming for many molded case circuit breakers.  Always check the manufacturer’s literature if you are not sure how to test a specific circuit breaker. The test requirements may differ between a Federal Pacific circuit breaker and a Westinghouse circuit breaker, for example. But they may also differ between types of circuit breakers by the same manufacturer.  And, of course, always be safe.