Checking a miniature circuit breaker


Thread Starter


I want to know how to check MCB (miniature circuit breaker) practically whether it is working or not. and suppose the rating of MCB is C2 (i.e 2A 240v) AC siemens make. so when we are giving 2.9A DC CURRENT it is not being tripped. and if its rating is 2A then on what rating/reading MCB should trip?

Please have a word on this thanks in advance.

Bob Peterson

Take a close look at the trip curve. It can take minutes to hours for a breaker of this size to trip with a small overload like that. The breaker is there to protect the conductor insulation and it takes quite some time for the wire to heat up enough to hurt the insulation any.
Look at the breaker trip curve. It's not supposed to trip instantaneously, especially at 145% of its rating. Maybe you didn't continue the test long enough.

Also, I believe some AC breakers will not work on DC due to their trip mechanism, so make sure yours will, or use AC for your test.

Finally, I understand that repeated tripping weakens a breaker, causing it to trip at lower current levels. If that is the case, then frequent testing is obviously to be avoided.
look at your trip characteristics in the Siemens literature, the 2 amps is for the normal current rating for continuous use,

trip conditions are much higher usually to protect the wiring (shorts) and could be in 10-20 ampere range.