Generator Grounding

[davehawks]

I am involved in a grounding review of a 460 VAC, 3-Phase, 420 Hz generator that is Wye connected with the A, B, C, and N conductors available. The generator is directly routed to a step-up XFMR with a delta primary.

In looking at 250.20, my initial conclusion is that code does not require this generator output to be grounded since:
- Max. voltage to ground on the ungrounded conductors exceeds 150 Volts.
- Neutral is not used as a circuit conductor.
- System is not delta connected.

Just curious why this application is not required to be grounded, as the 150 Volt limit seems a little subjective to me.....

 

[dereckbc]

Re: Generator Grounding

It is required to be grounded. Re-read 250.20 Is the neutral going to be switched?

[ March 18, 2004, 10:47 AM: Message edited by: dereckbc ]

 

[davehawks]

Re: Generator Grounding

No neutral switching, it is a SDS.

The impetus for the question is that someone has a brilliant idea to remove the solidly grounded connection to the generator neutral conductor.

Code wise, I find no reason why this cannot be removed as the system is not required to be grounded. I just see no reason why it should be removed.

 

[dereckbc]

Re: Generator Grounding

250.30

 

[bennie]

Re: Generator Grounding

The generator star point should be connected to the frame. The frame should be connected to the earth for static discharge purposes.

The three phase conductors should be isolated from any grounded objects, to reduce chances of a fault.

Usually, the transformer primary is connected directly to the generator windings, with no switches in the circuit.

 

[dereckbc]

Re: Generator Grounding

davehawks, after re-reading your post (I was too hasty the first time) I concur it is probable not required to be grounded but is permitted.

 

[rasmithircgov.com]

Re: Generator Grounding

If the neutral is switched then it is not a seperatly derived system. You must run an equipment ground from generator to switch and from service equipment to switch. 250.20(D)

 

[roger]

Re: Generator Grounding

rasmithircgov.com,

If the neutral is switched then it is not a seperatly derived system.

if the neutral is switched it is an SDS.

Roger

 

[dereckbc]

Re: Generator Grounding

Roger not always. If you bond the neutral at the generator, and do not run it from the generator, use a 3-pole transfer switch, it is still SDS. I have a lot of them configured this way.

 

[bennie]

Re: Generator Grounding

This is a 420 hertz generator, it will not be electrically connected to a ground conductor (MGN) in another system.

It will be a stand alone, separate system.

 

[roger]

Re: Generator Grounding

Hi Dereck, maybe I'm missing something, but by definition if the neutral is solidly connected to the generator and the utility, it can't be SDS even if it is not used. Like I said, maybe I'm missing something in this thread and if so I appolgize.

Roger

 

[dereckbc]

Re: Generator Grounding

Roger, no appology needed. Have you got access to the "Orange Book". If so go to chapter 7 and look at some of the figures like 79 and 80, I think you will see what I am talking about.

 

[roger]

Re: Generator Grounding

Thanks Dereck, I do have access (not immediate) to the "Orange Book", I will check out the figures.

Roger

 

[Ed MacLaren]

Re: Generator Grounding

The impetus for the question is that someone has a brilliant idea to remove the solidly grounded connection to the generator neutral conductor.

Perhaps you could explain to that "someone" the many disadvantages of an ungrounded system.

"Continuity of service" is often the reason given for operating an ungrounded system.

A resistance grounded system provides the continuity of service, without the disadvantages of the ungrounded system.

Ed

 

[rasmithircgov.com]

Re: Generator Grounding

My bad long day. I meant to say is swiched.I apoligize.

 

[bennie]

Re: Generator Grounding

If this is a 420 Hertz system, neutral switched from what?