Using the MS-2A for 3-phase testing: Circuit breakers may have three phases that are mechanically linked, so that if one of the phase trip elements trips, then all of the phases trip. However, each phase has its own individual trip element. So if we can get the breaker to trip using a three phase supply, then we don't know which phase tripped it, or even if all three trip mechanisms worked. That is why we need to do single phase trip testing so that we can verify that each phase mechanism trips, but it trips according to specified characteristics. However, what this also means is that if there is a ground fault trip feature, to the circuit breakers, then it must be disabled to check the other phase trip elements.
