A 400A breaker was experiencing nuisance trips. Various technicians had worked on the problem. From the notes, it seems that all of the loads were determined to be okay. The notes don’t say how the loads were checked or if current leakage testing was done. The plant engineer’s DMM didn’t show any spikes, so he decided to replace the breaker. This didn’t solve the problem.
How do you solve it?
While the DMM is an essential tool, it’s not designed to capture all power events or reveal all of the various power issues that could cause a breaker to nuisance trip. Put a power analyzer on this circuit, so you have a complete picture of what’s going on. You need to see high-voltage transients (especially very short-term ones that the DMM can’t see), power factor (complete history), harmonic distortion, very short-term voltage loss events, and other issues the plant engineer’s DMM didn’t see.
It may just be that this breaker is the wrong type for this application. Once you have your power analyzer information, you can also determine whether that’s the case.
Conduct current leakage testing and insulation resistance tests, and look for bonding deficiencies that may be producing transient spikes.