During the last maintenance shutdown, your electrical testing firm identified an old 800A breaker as needing to be replaced. Among other things, it would not trip while under test.
The breaker supplies 480V to several branch circuit transformers. None of the loads are critical, so it was possible to schedule a mini-shutdown to replace the breaker.
There were no nuisance trips with the old breaker, but that is not the case with this one. Production is upset because you “replaced a good breaker with a bad one”. An electrician hung an inductive current clamp on the load side of the new breaker and read only 635A. What other tests should you perform, and what issues should you investigate?
First, find out if this is the correct type of breaker. Is it an instantaneous trip breaker when the application calls for an inverse time breaker? Are large motors starting across the line?
What about the settings on the breaker? It may be just one setting that is causing the problem. For example, you can adjust Continuous Amps to as low as 20% of the breaker’s rating. If you set the Continuous Amps for 80%, then the breaker would trip at 640A. The electrician measured 635A, so not much load change is needed for a trip. You could look at the setting of the old breaker, but was it correct? Determine the calculated load by the same process used in the NEC, Annex D3(a).
It would be good to use that clamp with a DMM that has a min/max recording feature, so you can see how high the actual load goes over a 24-hr period.
