Ground fault protection coordination.

[binwork91]

Per NEC 215.10
Ground-Fault Protection of Equipment. Each
feeder disconnect rated 1000 amperes or more and installed
on solidly grounded wye electrical systems of more than
150 volts to ground, but not exceeding 600 volts phase-tophase,
shall be provided with ground-fault protection of
equipment in accordance with the provisions of 230.95.

I face a situation, 480V system, my 1600A main breaker has LSIG trip, other downstream breakers does not have ground fault protection. if i turn off ground fault protection, I can have selective coordination with all downstream breaker. If I turn on the ground fault protection, then i do not have selective coordination.
However, per code, I need to have ground-fault protection.

Question, is there any solution?

 

[ron]

Most engineers doing selective coordination studies to not lay the GF curve on the same drawing with phase curves. They are stand alone.

If only the 1600A breaker has GF (limited to 1200A), then it will always be selective.

 

[topgone]

Per NEC 215.10
Ground-Fault Protection of Equipment. Each
feeder disconnect rated 1000 amperes or more and installed
on solidly grounded wye electrical systems of more than
150 volts to ground, but not exceeding 600 volts phase-tophase,
shall be provided with ground-fault protection of
equipment in accordance with the provisions of 230.95.

I face a situation, 480V system, my 1600A main breaker has LSIG trip, other downstream breakers does not have ground fault protection. if i turn off ground fault protection, I can have selective coordination with all downstream breaker. If I turn on the ground fault protection, then i do not have selective coordination.
However, per code, I need to have ground-fault protection.

Question, is there any solution?

You can coordinate the tripping times instead. The problem with GF protection is that they are definite time trips. If you can, give a 350 ms minimum separation between devices, you can achieve effective coordination.

 

[jim dungar]

Most engineers doing selective coordination studies to not lay the GF curve on the same drawing with phase curves. They are stand alone.

If only the 1600A breaker has GF (limited to 1200A), then it will always be selective.

I always plotted downstream breakers on my GF curves. The last thing I wanted was a 30A general purpose circuit causing the main device tripping due to miscoordination. I had too many 'what happened' calls that were traced back to ballast changes on lighting circuits resulting in ground faults with the store or insurance company going dark.
Coordination with 70A circuits was not usually possible, but these size circuits were rarely 'worked' hot.

 

[binwork91]

Most engineers doing selective coordination studies to not lay the GF curve on the same drawing with phase curves. They are stand alone.

If only the 1600A breaker has GF (limited to 1200A), then it will always be selective.

If there is a ground fault, then it is possible the 1600A breaker trip first.
Then it is not orderly shutdown.

 

[binwork91]

You can coordinate the tripping times instead. The problem with GF protection is that they are definite time trips. If you can, give a 350 ms minimum separation between devices, you can achieve effective coordination.

It is hard to coordination with ground fault protection.
For example. see picture.

Combine ground fault curve with phase curve. they are not coordinate.

 

[ron]

How much L-G fault current do you have calculated at that bus.

You can bring the GF Delay down closer to instantaneous and hope that a L-G fault will activate and clear the fault before the phase protection sees it.

 

[binwork91]

How much L-G fault current do you have calculated at that bus.

Is it matter? i mean if TCC overlap, then they don't coordinate. I think software does not show the part of TCC curve that is large than fault.

You can bring the GF Delay down closer to instantaneous and hope that a L-G fault will activate and clear the fault before the phase protection sees it.

I don't get it. If the ground fault (GF) delay is closer to instantaneous, then this breaker will trip faster, reducing the chance of coordination.

However, if the GF pick is larger and the delay is longer, there is a higher chance they will coordinate.

 

[jim dungar]

The GF function will not react to phase-phase faults or overloads so showing that lack of coordination is immaterial.

With almost all coordination studies there will be some overlap that must be evaluated and a benefits risk analysis performed.

 

[binwork91]

The GF function will not react to phase-phase faults or overloads so showing that lack of coordination is immaterial.

With almost all coordination studies there will be some overlap that must be evaluated and a benefits risk analysis performed.

For fuses or breakers, I think their TCC are the same for phase-phase and phase-ground faults.

When there is ground fault protection, we also need to consider coordination for phase-ground faults because we want an orderly shutdown; therefore, they should coordinate if required.

Yes, I agree that most coordination studies will have some overlap. However, some codes have different requirements. For example, NYC only considers coordination after 0.1 seconds.

 

[topgone]

My comments:

• You need to coordinate BL-2 with BL-25
• You need to coordinate BL-2 with BL-26
• You don’t need to coordinate both BL-25 and BL-26
• Ground current sensing is different from phase current sensing. Internal toroid in a CB with GFP element only senses ground fault current, the amount of ground current (current not returning/ passing thru the phase/ neutral wires) while the phase overcurrent are monitored by CT on each conductor.
• You need to coordinate each phase overcurrent protections with other phase overcurrent protections and GF protection of each devices needing coordination with other GF protection.
• If the downstream breakers are not provided with GF protection (as in your case, downstream CBs are rated below 1000A), only the Main breaker trips on any ground fault downstream of the main circuit breaker. You don’t have anything to coordinate with.
• From your TCCs, the phase overcurrent protections coordinate, that is BL-2 coordinates with BL-25 and BL-2 with BL-26. Overlap on TCCs of BL-25 and BL-26 does no harm to your system.
• Any phase fault between 5kA and 60kA is not clearly coordinated during the first cycle of the fault (all phase protections TCCs overlap). Above 16 ms, every TCC coordinates.

 

[binwork91]

• Ground current sensing is different from phase current sensing. Internal toroid in a CB with GFP element only senses ground fault current, the amount of ground current (current not returning/ passing thru the phase/ neutral wires) while the phase overcurrent are monitored by CT on each conductor.
• You need to coordinate each phase overcurrent protections with other phase overcurrent protections and GF protection of each devices needing coordination with other GF protection.
• If the downstream breakers are not provided with GF protection (as in your case, downstream CBs are rated below 1000A), only the Main breaker trips on any ground fault downstream of the main circuit breaker. You don’t have anything to coordinate with.

With phase ground fault, the fault current still pass OCPD, that is why breaker trip or fuse melt.

I believe the TCC curve of OCPD is not just for phase-phase fault, also for phase-ground fault. If i am wrong, please let me know.

If the BL-2 and BL-25 required to selective coordinate, then it needs orderly shut down. If phase-ground fault happen at down stream of BL-25. Then BL-2 will trip first, because the ground fault protection.

If i am wrong, please let me know.

 

[jim dungar]

• If the downstream breakers are not provided with GF protection (as in your case, downstream CBs are rated below 1000A), only the Main breaker trips on any ground fault downstream of the main circuit breaker. You don’t have anything to coordinate with.

If you have a downstream protective device without a GF function, doesn't it's phase protection curve still need to coordinate with the upstream GF function?
Say you have a wire pinched phase to ground on a 30A branch breaker. Wouldn't you consider it poor coordination if the building main tripped?

 

[topgone]

If you have a downstream protective device without a GF function, doesn't it's phase protection curve still need to coordinate with the upstream GF function?
Say you have a wire pinched phase to ground on a 30A branch breaker. Wouldn't you consider it poor coordination if the building main tripped?

It depends on how you set the GF protection sensitivity. Mains are not set to trip on puny currents.

 

[jim dungar]

It depends on how you set the GF protection sensitivity. Mains are not set to trip on puny currents.

How would you know where to set the main without comparing the branch non-GF and the main GF curves? Every manufacturer. I have come across, ship the GF function set to the minimum.

 

[JoeStillman]

This is why we have article 517.17 for hospitals.