Posts Tagged ‘air circuit breaker’

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.

Circuit Breaker Will Not Reclose

April 16th, 2010 Comments off

An all too frequent scenario experienced by customers working in power distribution involves an air circuit breaker which after having tripped open fails to close again. When racked-in and the breaker charged it may trip free when the close button is engaged. 


Running down the list of possible factors, there are several key culprits which come to mind. Topping the list is the breaker racking-in interlock mechanism.  This interlock system is designed to ensure the breaker is fully in the racked-in position, the contact fingers fully engaged with the bus contact stabs, before closing.  What often happens is all too typical. The close tolerances designed in the racking-in mechanism can get out of tolerance through use if you so much as breathe on it wrong, or so it may seem. Even out of the gate as new equipment, they can be quite finicky birds requiring some TLC to keep them working smoothly.


The fix can be as simple as removing the front cover of the circuit breaker and lubricating the racking mechanism allowing its resets and mechanicals to function properly. The mechanism’s dogs can get stiff preventing the mechanism from seating properly.


In other cases we look for evidence the breaker may be over-racked in the racked-in position, throwing the interlock system out of alignment.


The circuit breaker proper may be out of alignment in its cell. This is very bad and runs the risk the load side connections (the finger clusters) may not be seated properly on the bus stabs. This is a nightmarish scenario if somehow the breaker manages to close and becomes energized.


When part of a good maintenance program, an air circuit breaker can be expected to delivery good reliable service for many years to come.