Nuisance Tripping on Ground Fault

[JatTiw]

I need help to solve a puzzle where some of the feeder breakers for healthy circuit were tripped cause of fault in one feeder, connected to same bus.
Problem started with failing of few capacitors of capacitor bank and caused the tripping of capacitor bank feeder breaker on 3-phase instantaneous over current only. But few of other feeder breakers (which are on same bus) were also tripped instantaneous ground fault current (50GS). The feeder breakers tripped on Ground fault has core balance type configuration for sensing the GF current.
The system is low resistance grounding system (set at max of 400Amp) & Ground fault sensing of each feeder breaker is set at 25 Amp. The charging current for each feeder cable is less than 1 amp.
The GF CT is 50/5Amp & relays are GE make (model # PJC11AV1A)
All feeder cables are PILC type with shielding and shield is grounded at source side & passes thru the GF CT.

 

[jghrist]

You may have inadvertent grounds on the neutrals of other feeders, causing some of the ground fault return current to flow through the neutrals of the other feeders.

 

[davidbeach]

What is on the feeder circuits? Could there be any wye-delta transformers, grounded motors, cap banks? You might have had capacitors on the other circuits discharge into the fault and that discharge seen as ground fault by the feeder relays.

 

[PWR]

The last five words of your post describing how the shield on the nuisance tripping feeders..."passes through the GF CT" is likely the explanation for your problem. The shield should not pass through the CT. Electrically that is usually accomplished by bringing the grounding connection to the shield "back down" through the CT to cancel out the path. Your problem seems to be that a portion of the ground fault current on the failed feeder is returning on paths to the source that include the shields of other feeders.

 

[KJvR]

Do you have any surge arrestors on the feeder circuits?

 

[JatTiw]

All these feeders has Delta/Wye transformers (with low resistance grounding) connected to it and has no capacitor banks, non-linear loads (ASDs etc) or surge arrestors.

 

[waross]

I'd check the shields on the circuits that tripped to see if the shields are grounded in the field. If any of the capacitors failed to ground, the ground current would divide and follow all available paths back to the main panel. If any shields were grounded in the field they may have been part of the return path.
yours

 

[patrick7]

you could use directional ground relays on these feeders and only allow tripping in the forward direction.