Ground Fault Coordination

[timm333]

I have question regarding the ground fault coordination as per NEC/IEEE. Should the downstream ground fault relay be coordinated only with the upstream ground fault protection, or should the downstream ground fault relay also be coordinated with the upstream overcurrent/short-circuit/overload protection as well? Thanks

 

[timm333]

I meant to say that should the feeder ground fault relay be coordinated only with the upstream main ground fault relay, or should the feeder ground fault relay be also coordinated with upstream main overcurrent relay?

 

[paulengr]

Coordination is desirable in all cases. It is mandatory in some jurisdictions but not always achievable. For example fuses over breakers are always troublesome due to very fast trip times at high fault currents on fuses.

With small circuit breakers on solidly grounded systems standard overcurrent protection can do double duty and coordinates well. As sizes increase generally speaking ground faults become much smaller than phase overcurrent and so phase overcurrent ceases to provide adequate protection. For instance if your ground fault path has an impedance of 1 ohm on a 480/277 system the theoretical maximum ground fault is 277 A. For example a 100 A MCCB on a B or C curve might not trip for a minute on ground fault, while we can agree 100 A isn’t very large, and a 400 A breaker will never trip.1 ohm is considered pretty low in many grounding standards. Hence the reason NEC pushes for ground fault protection in the first place.

If you run high resistance grounds by design ground faults are well under 100 A and typically 10-30 A. At this range a 15 A breaker probably won’t trip. Ground fault relaying is mandatory. So coordination might be a consideration with small breakers but most of the time you can ignore phase/ground fault coordination,

Coordination on ground fault relaying between relays is nearly always done using time delays. On high resistance for instance a motor starter might be set to 0-1 second, a feeder at 2-3 seconds, and a main at 4 seconds. In low resistance systems the resistor is often sized for 10 seconds so this is also adequate. In solidly grounded systems though like phase overcurrent you want to aim for 350 ms between levels using the standard from the old electro mechanical days and coordinate even faster if practical to do so (think arc flash) to minimize hazards and damage. Many ground fault relays are also strictly definite time (not time-current curves) so timing is much simpler than having to worry about tap/time dial/curve shape.



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[ATSman]

Zone Interlocking

Zone Interlocking

GFR Zone Interlocking, although more costly, may be a feature you may want to consider to enhance the coordination.

 

[jim dungar]

I meant to say that should the feeder ground fault relay be coordinated only with the upstream main ground fault relay, or should the feeder ground fault relay be also coordinated with upstream main overcurrent relay?

The NEC does not differentiate between phase overcurrents and ground faults when it discusses selective coordination.

 

[WA_Sparky]

Coordinating Protective Devices

Coordinating Protective Devices

I meant to say that should the feeder ground fault relay be coordinated only with the upstream main ground fault relay, or should the feeder ground fault relay be also coordinated with upstream main overcurrent relay?

Feel free to correct me if I'm wrong.

Are your Ground Fault relays for breakers over 1200A?

I recently ran a coordination study for a large hospital. Per adopted code we needed to have Ground Fault protection on all breakers over 1000A(not sure when this was adopted but 230.95) During my study I had to coordinate all the breakers from MDP (3000A) down to 70A (starting at 0.1sec). It's really difficult coordinating small breakers due to the lack of electronic trip sensors below 60A I believe. If you have a power system analysis software (skm,easypower) you can visually see the coordination curves. I remember my LSIG (long,short, instantaneous, ground fault) breakers didn't have the flexibility/range as the smaller breakers to coordinate in the long term region which helped for coordination.

For me, as long as I used the same manufacturer (Eaton,SQ D) my curves were easy to coordinate using electronic trip units. I can't say I remember reading it in NFPA code but our Eaton rep explained it to me that all regions on your TCC curves needed to be coordinated (starting at 0.1 sec, unless life safety then it's 0.01 sec) except very small breakers that simply can't achieve coordination.

 

[paulengr]

In a hospital Code makes coordination mandatory. I don’t recall a time limit on it. In most environments it’s good practice but only where practical.

Electronic trips are slowly moving down in terms of frame sizes. 15 years ago the GE Spectra RMS at 400 A was much cheaper than the thermal magnetic equivalent. Below about 250 A when they are available the prices tend to get crazy high.



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