Unexplained tripping of circuit breaker 1

[BK32]

Hi All,

I need some help in troubleshooting an unexplained tripping of an MCCB. This breaker is installed in the main distribution board on an 440V, 60Hz, 3 ph, 3 wire system.

The breaker is a cutler hammer (eaton) AQB-L250 type breaker with a 175A plug. The instantaneous trip setting is Lo (these type of breakers have either Hi or Lo settings which correspond to a numerical pickup depending on the plug rating installed).

This circuit breaker feeds the control panel for an AC chiller plant. The control panel contains DOL motor starters for two (2) off 63kW compressors, two (2) off 5kW chilled water circulating pumps and two (2) off VFD driven 5.5kW sea water circulating pumps.

The AC Chiller plant had been operating with one compressor, one chilled water circulating pump and one seawater circulating pump online. This had been the case for a number of hours. Then a fault occurred somewhere else on the ships power distribution network which resulted in a black out scenario (I.e all supplies to the ship were tripped. Note at the time the ship was alongside and being fed from shore supply connection). In any event, this was not a result of any faults within the AC chiller plant.

Based on information from the contractor conducting the stw and commissioning of the system, in the 24 hrs or so following restoration of power this AQB-L250 circuit breaker tripped approximately 5-6 times. The current draw recorded on the circuit was approx.100A on 2 phases and 70A on the other phase.

They stated that on each occasion all pumps had been started and were running normally. It was also noted that the protection devices provided in each of the starter circuits did not trip.

Given the cumulative load was significantly less than the breaker thermal pickup of 175A, I do not see that the breaker could have legitimately tripped on a thermal overload event. Also, I do not believe that the breaker is fitted with either a shunt or uv trip.

Could anyone provide some other advice and what to look at? At this stage it is a bit of a mystery to me. The only obvious conclusion appears to be that the trip unit on the breaker is failing (nuisance tripping).

Thanks in advance for your help.

 

[unclebob]

Any shunt trip used with this breaker?

 

[BK32]

Hi all, thanks for all your help.

We ended up changing out the breaker with a spare and the new breaker was fine, no nuisance tripping. We are getting the suspect breaker inspected and tested again in the workshop.

We did test for phase to earth faults as each piece of equipment came online and it all was healthy. We also measured running current throughout the process and did I not see the imbalance that had previously been reported. Interestingly, the imbalance was not occurring when we were testing with the suspect breaker either.

We will continue to monitor the circuit just in case it trips off again (not expecting it to.)

Thanks again to everyone for your advice I did learn a few things just from the posts on here.

 

[krisys]

Trouble shooting the MCCB is a problem. If you find that it is tripping in service when the system is healthy, better replace and have a comfortable sleep. If the MCCB is new, can be sent to vendor's works, if it is worth taking so much pain, otherwise forget it.

 

[Skogsgurra]

In a case like this, "not knowing" is OK. You don't risk anything and your equipment is back in service. Not much time wasted and your boss is happy. You know what curiosity did to the cat, don't you?

Gunnar Englund

http://www.gke.org

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