Generator Neutral

[andrew grant]

Sorry to bring up this subject again. I surveyed the past topics and rapidly became brain dead. I'm not the sharpest tack in the box.

I have a CNG facility which gets its power from the Utility, though a transformer down to a 480V switchboard and on out. I have brought the neutral from the transformer to the main breaker section, bonded it to the ground bus and to the GEC. There are no neutral loads in the plant.

This facility wants to bring in portable generator, when the main service is down, to power up about half of the load. This is to be done through another breaker, key-interlocked with the main breaker. There is no transfer switch. This might be what the NEC calls an "Optional Stand-By System" Art. 702. Not sure about that.

My question is what to do about the generator neutral? I'm thinking I need to bring it in and attach it to the same place my main feeder neutral is connected - mimicking the same connection as the main service.

I just don't know if this is correct. What do you guys think? Thanks.

 

[dereckbc]

Re: Generator Neutral

You have two choices:
1. If you have access to the building ground electrode system you can run a GEC from the generator neutral to the ground electrode. Bond the generator frame to the neutral, then only run phase conductors from the generator. Use this method if there is any GFP involved.

2. Make sure there is no N-G bond in generator, and run phase, neutral, and EGC from generator. This is the most common practice for portable generators where no GFP is involved.

 

[ron]

Re: Generator Neutral

Assuming that the transfer from utility to either the existing standby gen or portable gen will be three pole only (not switching the neutral), I would not consider the gens as seperately derived sources, hence not bond the n-g at either gen.
250.34(C) covers it for the portable gen, a code ref for the permanant gen I will have to find.
For both gens I would bring the grounding conductor and the grounded conductor (neutral) to the main gear and terminate. Since there is a n-g bond for the utility, it becomes wacky.

[ May 21, 2003, 07:49 PM: Message edited by: ron ]

 

[kevin]

Re: Generator Neutral

It should be permissible to disconnect the generator's neutral-to-frame connection (if it exists) and run ONLY three phase conductors to the building and operate as a 480 volt ungrounded delta system.

 

[bennie]

Re: Generator Neutral

The generator windings will be wye connected not delta.

 

[templdl]

Re: Generator Neutral

Yes, but even though it is wye connected the neutral doesn't have to be run if there are no line to neutral loads, just the lines. As the others have indicated the generator probably has been set up as a stand alone device, that is a separately derived system where the neutral must be isolated from the frame and EGC.

 

[don_resqcapt19]

Re: Generator Neutral

templdl,

As the others have indicated the generator probably has been set up as a stand alone device, that is a separately derived system where the neutral must be isolated from the frame and EGC.

If the generator is not SDS the neutral must be isolated. If the generator is SDS, then there must be a neutral to ground bond at the generator or at the first OCPD.
don

 

[templdl]

Re: Generator Neutral

Don, In rereading my reply I see how I could have been misunderstood.
"As the others have indicated the generator probably has been set up as a stand alone device, that is a separately derived system where the neutral must be isolated from the frame and EGC."
I would have been better stated as "As the others have indicated the generator probably has been set up as a stand alone device, that is a separately derived system where the neutral has been bonded to the frame and EGC. And, as I had understood the application as described by Andrew to be, the bonding jumper must be removed insulating and isolating the neutral."
Thanks,
Dave

 

[brian john]

Re: Generator Neutral

A determination needs to me made regarding the generator XO/neutral/grounded conductor, if XO is bonded to the generator frame this MUST BE LIFTED/REMOVED. Then 5 wires must be rum 3 phases, the neutral and an equipment grounding conductor. If there is a four-pole circuit breaker involved then the generator XO will need to be bonded to the Grounding Electrode for the service.

 

[bennie]

Re: Generator Neutral

Why must the star point(neutral) be disconnected from the frame or stator?

All the main generators at Bonneville hydro plant have star point ground at the unit.

All main generators at the Mojave Generating Plant have star point ground at the units.

What makes a generator supplying a separately derived system any different?

 

[brian john]

Re: Generator Neutral

First off Bennie you are crossing the old apple and orange, you are talking about (I assume) a multiple generator paralleling system. We are talking about a utility fed system, with a generator for a standby source (either temporary or permanent). The main service is grounded. The generator will be grounded at this point (the main service) when connected. If the temporary generator has 5-wire connection (as a permanent unit would) the neutral/grounded conductor will be in parallel with the equipment grounding conductor. A NO NO.

 

[hornetd]

Re: Generator Neutral

Originally posted by bennie:
Why must the star point(neutral) be disconnected from the frame or stator?

All the main generators at Bonneville hydro plant have star point ground at the unit.

All main generators at the Mojave Generating Plant have star point ground at the units.

What makes a generator supplying a separately derived system any different?

Bennie

You disagree with the NEC definition of an Separately Derived System and everyone here is aware of that. But in this case I believe that we can all agree that a system with a common neutral between the two sources is not in any sence a separately derived system.

If we leave the star point of the generator winding bonded to the generator frame we have a grounding connection to a "grounded circuit conductor on the load side of the service disconnecting means" during operation from utility power.

250.24 Grounding Service-Supplied Alternating-Current Systems.
(A) System Grounding Connections. A premises wiring system supplied by a grounded ac service shall have a grounding electrode conductor connected to the grounded service conductor, at each service, in accordance with 250.24(A)(1) through (A)(5).
(5) Load-Side Grounding Connections. A grounding connection shall not be made to any grounded circuit conductor on the load side of the service disconnecting means except as otherwise permitted in this article.

The OP was fairly clear that the transfer mechanism will consist of another breaker, key-interlocked with the main breaker. I think that it is fairly safe to assume that these breakers do not open the grounded conductor that is connected to the secondary winding of the utility transformer.
--
Tom

 

[bennie]

Re: Generator Neutral

Tom: I agree with the NEC definition of a Separately derived system. The NEC is correct.

I strongly disagree with the interpretation. The interpretation is not correct.

A generator or transformer, properly connected, do not require any special connections to be the source for a separately derived system.

 

[bennie]

Re: Generator Neutral

Correction: A transformer does have to be supplied by an ungrounded source, to qualify as a source for a separately derived system.

Refer to handbooks authored by James McPartland.

The authors who replaced Mr. McPartland added the equipment ground conductor, on the primary side, which is an extension of the utility MGN. This connection excludes a transformer from being the source of a separately derived system.

 

[don_resqcapt19]

Re: Generator Neutral

Bennie,
It is my opinion, that the figures in those handbooks left out the primary EGC just for clarity, because he was illustrating the secondary grounding connections. There is no way that the figures, showing a transformer without a primary EGC conductor are safe.
Don

 

[bennie]

Re: Generator Neutral

Don: You have to go back to the 1947 Handbook by Arthur L. Abbott, Section 2514 Alternating-current Systems without Exterior Connections.

This section was changed to 250.26 in 1956. Separately Derived Systems. The only comment for the title change was, "edited for technical correctness"

The schematics of a system without exterior connections is of a transformer with an ungrounded primary source. This definition has been corrupted by interpretation and adding the equipment ground on the primary.

The equipment ground was not left out by McPartland, it is not supposed to exist.

No grounded transformer is a separately derived system. It is a component of the premises wiring, and should be grounded accordingly.

 

[bennie]

Re: Generator Neutral

Don: We no longer have premises wiring that can be defined as a separately derived system.

The grounding requirements were included in the code when farm houses were operating from a generator, and being ungrounded. This grounding requirement began in th 1930s during the days of the REA.

Separately derived systems does not appear in the Services Article, only in the grounding article.

The intent was to alert workmen that special care must be observed when connecting the utility power to a "separately derived system".