Parallel Generator System Grounding

[natfuelbill]

Where is a grounding connection to be made for parallel generators feeding a paralleling switchboard;
- at generators, or
- at switchboard, or,
- at both?

What Code references are used for this situation?

Are the generators individually "another system" as referred to Article 100 for "separately derived system"?

There is no utility "service" involved.

The system voltage is 480/277V, 3 phase, 4 wire, wye.

The generator and switchboard operate as prime power, and are islanding.

 

[kingpb]

I like to reference IEEE-446 (Orange Book) for these types of answers.

 

[natfuelbill]

Thanks, I reviewed Figure 7-13 (IEEE Std 446-1995). Although the standard covers emergency and standby systems, this figure shows system grounding occurring in the paralleling switchboard.

EGSA "Electrical Generating Systems Association" in paragraph 29.5.2 also discusses system grounding occurring in the paralleling switchboard.

Are there any situations where it is correct to ground the grounded conductor in both the generators and paralleling switchgear?

 

[steve66]

Thanks, I reviewed Figure 7-13 (IEEE Std 446-1995). Although the standard covers emergency and standby systems, this figure shows system grounding occurring in the paralleling switchboard.

EGSA "Electrical Generating Systems Association" in paragraph 29.5.2 also discusses system grounding occurring in the paralleling switchboard.

Are there any situations where it is correct to ground the grounded conductor in both the generators and paralleling switchgear?

In general, no. This would place the grounding conductors in parallel with the neutral conductors, which would cause current to flow on the grounding conductor.

Steve

 

[natfuelbill]

Thank you Steve.

You wrote "In general, no." Are there then any specific situations when it is correct, required or allowed?

I can only think of a situation where the AHJ would require or allow grounding the grounded conductor in both the generators and paralleling switchgear.

 

[steve66]

Thank you Steve.

You wrote "In general, no." Are there then any specific situations when it is correct, required or allowed?

I can only think of a situation where the AHJ would require or allow grounding the grounded conductor in both the generators and paralleling switchgear.

None that I can think of, but as soon as I say "always", someone will find an exception to prove me wrong.