Electronic Trip Breakers

[ventilator]

Does anyone here have any experience with failures of electronic trip units for circuit breakers? I have a funny situation where long story short it looks like the instantaneous sensing on a cutler hammer RMS310 trip unit isn't functioning properly but the 'thermal' sensing is. It will hold and run motors but when we try to start a FVNR starter it trips out. The HMCP for the motor holds (250a hmcp) but the 800a switchboard breaker trips out.

Can part of an electronic trip go bad?

 

[Ship Wreck PE]

Has this motor and trip unit been in service very long or is it a new installation? Where do you have trip settings?

 

[ventilator]

I know the motor is prob close to 10years and I'm guessing the breaker is too but the plant could have changed it out at some point and I'd never know, we're just contractors.

The TCCs for the 800a and the HMCP are coordinated with no overlap. STPU for the 800 is 3200a with a fixed 14000a instantaneous.

Add to that the fact the 2000a switchboard main is actually set lower in the instantaneous region than the 800 (for arc flash purposes) and it hasn't tripped yet.

So somehow I have a motor that instantly trips out an 800a breaker even though it has higher settings than the HMCP and 2000a main in the instantaneous region.

The other motors powered by the 800a are on drives so there is no inrush.

The motor having a problem makes more sense than the breaker but this all started as soon as we changed it from a VFD to an across the line starter. So the breaker in question had all drive loads (no inrush so no instantaneous) and now it has a 150HP inrush on it.

 

[Matt Bellott]

I have a utility background mostly load flow and short circuit. So this may be way off. Have the CT ratios been checked? From a settings standpoint it shouldnt trip it sounds like. Lemme know what you find out its always interesting how these relays have a mind of their own.

 

[ventilator]

Turned out the trip unit was bad. Plant had an extra one and swapped it out, started up first try and has been running ever since. Best guess is since all of the loading on the breaker had been on VFDs for the last couple of years at some point the instantaneous trip went out but had no way of knowing b/c of the VFDs.

I never could get an answer from manufacturer about the trip only being bad on the inst.