Ground Fault Protection Question

[polarseltzer]

Hello,
First off I apologize if the answer to this question is obvious but I am not an electrical engineer and have been having a discussion on this situation with my plant electrician. Recently corporate people had a study done on the facility I work at and produced some time current curves. I am getting educated on this stuff. The electrician and I are a little confused on a trip device on the main breaker that has ground fault. There are two opinions here:
1. The ground fault will trip on a line to ground fault that exceeds the trip point and the downstream breaker will not trip.
2. The downstream circuit would trip prior to the main breaker tripping on ground fault.

Please see attached drawing. For instance, a ground fault current of 700 amps on the ASD40 AIR COMP circuit will trip the ground fault circuit on the MDP MB instead of looking like a fault on the breaker and trip the air compressor breaker at like 20 seconds.
The other situation is what happens when ground fault is less than the breaker setting of 640 amps? will the circuit breaker where the ground fault is trip on single phase overload?

Thank you

 

[vandal06]

I'm not familiar with the fault currents available in your plant, I'm sure the engineer who produced the study (in Easypower, it looks like) ran those out.

A ground fault of 700 A on the ASD40 AIR COMP would operate the breaker in the instantaneous band. It's hard to see but it's hidden behind the HVP-2 circuit curve (the aquamarine curve). The Main Breaker ground fault curve will operate with a definite time characteristic and should not operate before 0.07 seconds for the fault currents it is expected to trip for. With low-voltage trip coordination the operating times have been built in to the curves, such that as long as you don't have any curves "touch" they are properly coordinated at that fault current level, which for all circuits but the two air dryers we have met. Note that ground faults can still be detected by phase units, so that's why the engineer showed the expected coordination between the ground element of the main and the downstream phase units.

Case #2 is the expected case here for fault currents exceeding the pick up point of 640A. It's a little fuzzy how things will operate though when looking at the air dryer thermal magnetic trip units, as there is a good deal of overlap there, but the thermal-mags would appear to operate instantaneously. Someone could probably clarify this better than me; I typically do MV and HV coordination studies and rarely touch 480V trip devices.

 

[polarseltzer]

Thank you vandal06. So if I understand you properly, for any ground fault above 640 amps, the circuit breakers will trip prior to the main breaker. Would this be similiar to having 3 fuses and one fuse will blow due to fault on that wire? In other words, the circuit breaker will trip all three legs for a fault on one wire?
But if it is 1200 amps on the HVP-2 circuit (aqua marine), the main breaker will trip before the HVP-2 breaker due to the ground fault setting on main breaker?

 

[vandal06]

For a ground fault above 640A downstream of the feeders those feeders should operate first.

Yes, all three poles will open for a fault even if it only involves one phase to ground, phase-to-phase, etc. Reason being is you don't want to single-phase any 3-phase loads downstream, and judging from the feeder descriptions I'd guess there are 3-phase motors being served off the feeders.

Another question I forgot to ask, but is there even a ground source for the MDP bus?

 

[polarseltzer]

Thank you. I asked the electrician and he said there is a ground as the secondary service is 480 and 277 volts. He looked at me funny like it was obvious. I'm learning!

 

[waross]

Here's a note from the Canadian Electrical Code that may be helpful.
It is recognized that ground fault protection may be desired for circuits other than those described in this Rule.

Ground fault protective equipment at the supply will make it necessary to review the overall system for proper
coordination with other overcurrent protection. Additional ground fault protective equipment may be needed
on feeders and branch circuits where maximum continuity of electrical supply to the remainder of the system is
required.

Bill
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"Why not the best?"
Jimmy Carter