Megging procedures

[Bassclefjef]

I have a 1200 amp GFP breaker feeding two chillers. Having a nuisance problems with high current shut down after a tree fell on a power distribution lines. I hired an electrical contractor to meg parallel underground feeders. They said A phase and B phase meg together and to ground. The 1200 GFP breaker never trips. The company did not disconnect load side feeders from 1200amp GFP breaker when tested. My question is, "Wouldn't it be necessary to disconnect the conductors from the load side of breaker due to the GFP sensors giving a false reading not to mention possible damage to the GFP sensors?" Voltage is steady on all 3phase when chiller is running. Amperage just starts climbing until chiller shuts itself down after about 15 min of runtime.

 

[zog]

Yes, they need to be isolated. Call a testing company, Shermco is a good one in your area.

 

[Bassclefjef]

Thanks Zog, appreciate your reply.

 

[just the cowboy]

maybe not if breaker is open

maybe not if breaker is open

If the breaker is open and no metering on it they would not have to "disconnect" the load if they are testing the line side. But they should of given you values of the readings. If it has ground fault proctectors on it they should trip with bad megger readings

 

[Bassclefjef]

Thanks for you reply. Actually they measured the load side of the 1200amp open GFP breaker to test the conductors feeding the chillers. The parallel feeds were taken off of the taps at the junction box but not the 1200 amp GFP breaker which has GF sensing on load side of breaker. The switchgear could not be shut down at that time to remove conductors. Let me clarify, the chillers are fed UG via Switchgear.

 

[big john]

The GFP in the breaker will not be affected by a megger as this is measured by CTs that will be insulated for the system voltage. However, if the breaker has potential taps for power monitoring or undervoltage or anything like that, this stuff absolutely can be damaged by megging it, so they need to be careful. In their defense this usually doesn't come off the load side.

If they're reading a hard fault between two phases that are otherwise running fine and have high L-G measurements, then they're actually reading through a connected load resistance. My guess would be a CPT or PT in the switchgear itself as that's often the cause.

 

[zog]

My guess would be a CPT or PT in the switchgear itself as that's often the cause.

My guess as well, that's what I meant by isolated, not necessarily removing the cables. Should have been more clear.