Generator grounding True or False

[nerspellsner]

From what I understand about generator grounding, my conclusion is that if a permanently mounted generator is feeding a transfer switch that does not switch the neutral, the generator does not require any type of ground or bond to the service (street) side or the transfer switch. It does not need a ground rod installed next to it to ground its enclosure, unless the manufacturer requires it. It does not need a grounding electrode conductor installed from the service disconnect (street) side to the generator. It does not need an equipment grounding conductor installed from the service (street)side to the generator. Are my statements true or false? Thanks.

 

[suemarkp]

I believe you need an equipment gorunding conductor between the generator frame and the transfer switch. This is then bonded/grounded to the service neutral by the normal utility service equipment and this in turn grounds/bonds the generator (at least as long as it is wired to the transfer switch).

 

[brian john]

True, true, true and FALSE

 

[NoVA Comms Power]

Hopefully, this picture's worth a thousand words:

Jim

 

[celtic]

In the March issue of EC&M:

There is an article:
Ground-Fault Current: Problems and Solutions

Mar 1, 2007 12:00 PM, Edited by John DeDad, senior director, editorial and EC&M development
With separately derived systems, you must adhere to special rules on neutral-to-ground connections.

Within this article, there are two by-lines (in the Hardcopy only, in the on-line version it kind of just flows into the story)

• When You Should Ground and Switch the Gen-Set Neutral
• When You Should Not Ground the Gen-Set Neutral

I don't know if this will help you, but it won't hurt you.

 

[NoVA Comms Power]

Good article; thanks Celtic.

Nerspellsner:

I've marked-up the previous Mike Holt diagram to illustrate how an improper bond of a non-SDS genset's neutral conductor at the genset (indicated in green) would result in parallel neutrals and objectional current running thru the grounding system.

In a non-SDS application, the generator's neutral is isolated from the ground at the genset (similar to a sub-panel) ... although the genset's neutral conductor is still "grounded" by the Main Bonding Jumper at the service.

 

[nerspellsner]

Thank you for your help with this topic. Your answers leave me with two more questions. I will use table 250.122 to size the equipment grounding conductor to the generator. The generator is 60000 watts at 120/240 volts. The generator has a 300 amp breaker. I will use #4 Cu or #2 Al for the EGC. Is this correct? The manufactuer wants a ground rod installed next to the generator to ground the enclosure of the generator. This seems to present a problem because the service will be grounded at two seperate points because of the EGC bonding the two earth grounding locations together. Will this cause any problems or is this grounding configuration acceptable in this situation?

 

[NoVA Comms Power]

... The manufactuer wants a ground rod installed next to the generator to ground the enclosure of the generator. This seems to present a problem because the service will be grounded at two seperate points because of the EGC bonding the two earth grounding locations together. Will this cause any problems or is this grounding configuration acceptable in this situation? ...

First ... your neutral is still only grounded (bonded) at ONE location ... at the service. There might be multiple grounding electodes installed as part of the grounding electrode system ... but the neutral is still only "grounded/bonded" at one point so as to prevent parallel neutral paths.

What the manufacturer is apparently requiring you to install is a supplementary grounding electrode (See 250.54). It shouldn't cause any problems ... although it's of dubious value because as you've correctly pointed out, the genset frame will already be fully grounded via your correctly-sized EGC. Note that the supplementary electrode is exempt from the #6 bonding jumper requirement usually associated when tying-together multiple grounding electrodes.

While it's already been posted, I'll repeat for emphasis ... what's IMPRORTANT is to ensure that the non-SDS genset's neutral is isolated from the genset's frame ... so that the service is still the only neutral-to-ground bond point in the entire system.