High Impedance Grounding Systems

[robva]

A high impedance grounding system, recently was requested as a solution to reducing high fault levels. The fault levels are in the 100K plus range, creating issues of concern on transfer switches, downstream panels, circuitbreakers etc.

Some givens in the program - fuse combinations are not permitted, series rated circuit breakers are forbidden, distance from power source to 20 amp circuit breakers are insufficient to lower the cost. Voltage is 480, Utility Transformer is 2500KVA, multiple gen sets are in play to provide tier 2 power redundancy

A solution offered for consideration is using a high impedance ground system, increasing the impedance on the system to reduce incoming fault. Yet, to achieve this result, the neutral conductor from is disabled, code disallows line to neutral connections, resulting in a main switchboard with out a neutral.

The main switchboard contains three phase breakers to UPS equipment, and mechanical loads. A small 480 to 208/120 volt wye transformer would be attaached for general "dirty" power receptacles and lights.

Yet, while on the initial read, the concept appears to solve some problems, I am fearful that it will create other issues that I cannot at the moment discern. Beyond the somewhat obvious issues of not having a neutral within the main switchboard and someone accidentally trying to attach a single phase load.

Any ideas and thoughts are sincerely appreciated.

RobVA

 

[haskindm]

Look at 250.36. Does your installation meet ALL of those requirements? I see that this system may be installed because continuity of power is required. Why is it required? Is there a safety issue? I don't see anything here about allowing a High Impedance ground to limit fault current because the properly rated devices are expensive. A long conversation with the AHJ should be had before proceeding too far with this idea.

 

[zog]

100kA from a 2500kVA transformer? That sounds way too high, depending on %Z that should be in the 40kA range.

 

[kingpb]

100kA from a 2500kVA transformer? That sounds way too high, depending on %Z that should be in the 40kA range.

Not sure what your basis for this statement is, the Z% would need to be 3%, which is very realistic for a utility transformer.

If you need to decrease fault current, then increase the impedance between the transformer and the switchgear, or have the utility replace the transformer with one that has a higher impedance.

You could also put a 1:1 ration transformer inbetween. e.g. 480 - 480V with the impedance you need, that way you can also use the taps to maintain better voltage control.

 

[winnie]

A high impedance grounded system will not reduce the available line to line fault current.

For what it's worth, I believe that impedance grounded systems do reduce fault energy for most faults, and should be permitted even in situations where continuity of power is not required. For this use, I envision a system that is operationally similar to a conventional solidly grounded system, in which a ground fault leads to a circuit breaker opening; ground fault protection on all circuits would be required to achieve this since the impedance would limit ground fault currents. Such an installation is not envisioned nor permitted under the NEC.

-Jon

 

[jim dungar]

Beyond the somewhat obvious issues of not having a neutral within the main switchboard and someone accidentally trying to attach a single phase load.

Someone possibly adding an incorrect L-N should not be the determing factor in your power system design. If your system requires the complexity of multple gen sets then you shouldn't allow un-qualified people to work in your facility. Remember qualified means "... has skills and knowlege related to... [the] installation..."

But, as Jon pointed out a high resistance ground does not not reduce your 3-phase bolted fault current. If you want to keep your existing equipment, you need to increase your "phase impedance" by adding transformers or inductors, by changing existing transformers to higher impedance units, or by increasing your conductor lengths.

 

[robva]

ThankYou

ThankYou

I appreciate your input! And, it parallels some information from othe very reliable sources. The fault level is additive when the generators are online in a parallel configuration. Once the third generator activates, we ramped up above the 100k mark. The utility Company fault level was 55kaic

And, as this is a critical facility, we would have very qualified individuals working on the building.

Thanks again!

R

 

[zog]

"Not sure what your basis for this statement is, the Z% would need to be 3%, which is very realistic for a utility transformer"

I was thinking more like 6% for a distribution transformer.

 

[jim dungar]

I appreciate your input! And, it parallels some information from othe very reliable sources. The fault level is additive when the generators are online in a parallel configuration. Once the third generator activates, we ramped up above the 100k mark. The utility Company fault level was 55kaic.

This is a problem most people do not consider. The effect of paralleling generators with and without the utility very often causes the available fault current to exceed the existing equipment's ratings especially transfer switches. I recently looked at a data center whose three paralleled generators produced more fault current than any of the individual utility transformers.

 

[zog]

"The utility Company fault level was 55kaic."

That sounds more like it. I didnt realize you were operating your generators in parell, in that case, yep, you have a problem.

You either have to reduce the fault current or get breakers that are rated for those levels. There are several options for LVCB's with 200kA interuption ratings that can be retrofilled in your existing switchgear.