Motor inrush, lockrotor and ground fault

[wolfie1a]

I have a new chiller installation that will trip a SQD LX style breaker on ground fault. I does not do it every time and multiple megger readings indicates there are no problems with the equipment. I'm questioning the ground fault time delay (0.1). The way I understand how the GF works on this style breaker is a vector summing of all 3 phases so they cancel each other out. I'm thinkng that increasing my time delay will take care of the problem, but I would like to understand what is happening during the inrush period of the motor. Will there be momentary phase imbalances that could be detected as GF?

 

[Zogzog]

Your assumptions are correct but be careful increasing the GFD, you might impact your system coordination and cause bigger problems.

How about posting you test results that were taken.

Last time your breaker was tested? These trip units fail often enough to consider that and when they do they usually fail to the minimum pickups.

Have you taken any readings of the currents during startup?

 

[wolfie1a]

Already looked at the coordination and I can increase it to 0.2 with out any problems.

I don't have the test results for the breaker handy. I know its a new breaker and was tested when it was installed.

Attached are some starts using a fluke. When checked the motor with our clamp on fluke the starign current looked like it was right a 6X.

 

[Zogzog]

I was wondering how the 3 phases compared during start up. And I was asking about your Megger test results.

 

[wolfie1a]

Meggar 11.0 meg on all three phases.

 

[rbulsara]

Along with time delay, you may want to look at the GF Pick Up current setting. It itself may be too low.