A 'used' 3 phase, 4 wire, 120/208, 75 KW genset and a 3 pole ATS were donated to our local Fire House.

My question....if the gen is to be connected to provide standby power, do you need a 3 pole, or 4 pole ATS??

Thanks,
John

It is a design decision, if the generator XO is bonded to the frame you need a 4 pole switch, if the XO is not bonded you need a 3 pole switch.

You can bond it or float it in the field most times.

When GFP protection is involved 4 pole switches are on the drawings.

I would go with the 4 pole xfer. A 120/208 generator system without a Xo bond could backfeed the line neutral looking for a return path to ground. This may be a safety issue if someone was working on the main service, with the generator running and open the neutral ahead of the bond to ground. The above post was VERY WISE about the problems with a GFI and a second ground. I was on a service call last year for that kind of problem. GFI trip as soon as the load exceeded the GFI trip settings. This took awhile to find due to size of the building.

dfe,
Can you explain how this could happen?
Don

Hello resqcapt19

I will try to explain...as you know the main service neutral ("grounded conductor" as the nec like's to call it) is bonded to ground at the main service. This is the only place the code lets you do this, unless of course you have a separately derived system such as a generator. Now if this generator is using this neutral without a X0 bond at the generator (and a 3p xfer ) any leaks to ground or faults to ground will return to the neutral at the main service bonding jumper ("backfeed" in electrician talk). This is were it could be a safety issue, if someone was working on this neutral and open it ( this is something some people only do once.. they don't get a 2nd chance). A 4p xfer switch with its own X0 bond at its neutral, would be isolated from the main service neutral and could not backfeed when the generator stated.

Hope this helps... Maybe someone smarter then me may explain it better.

DFE welcome to ECN.

This 'backfeed', can you describe the whole circuit of this current?

Just a tip, Don is pretty sharp.

Bob

This may help.

Graphic By Ed MacLaren.


Bill if this graphic is a problem let me know and I will pull it.

Bob

A couple of questions, if I may.

1st, what do you mean the nuetral current is looking for a path to ground?
2nd, In my opinion, anybody opening the nuetral has a death wish...


And, as Bob said, welcome to ECN.

Thanks Iwire

I'm sure that there are many sharp people here, I'm here to learn as much from them as I can, and maybe share a little as I go, this is how I think it works from what I see so far.

I made the changes you requested to the picture you posted but I could not paste it. Maybe I did something wrong or maybe it was something I didn't do ?.

Lets try this. Lets say we have a small leakage to ground on the load in your picture ( a old motor maybe ), lets say its leaking to A phase. This leakage would follow the ground trying to return to its source . It would follow the ground path back to the main service to the bonding jumper, then on to the neutral, follow this back until it was cancel out by B phase at the generator. This system would work fine until someone open the neutral at the main service, ( maybe a main GFI test or a service upgrade ). When they turn main power off the generator would come on ( ATS in the 1st post ). There is no X0 bond at the ATS, it would be using the main service X0 bond for its ground path. Someone then opens this path and we hope they just get a shock... But that depends on how they open it and how much leakage we had.

Thanks Ryan

On the 1st question I used the wrong words (terms) should have said the ground current is trying to complete the circuit thru the neutral via the bonding jumper at the main service.

2nd I agree with you 100% ( maybe more ). In this circuit the bond is at the main service so any faults to ground must travel back to the main service bonding jumper then follow that neutral back to the generator to complete this circuit. If I may quote you "anybody opening the nuetral has a death wish..." they may get there wish granted. However they may not be aware that this is also the bonding path for the generator. A 4p ats would be safer, the neutral to the main service would be isolated the generator would have its own bond to neutral.

Modified image posted for dfe

dfe take the generator and the ATS out of the picture.

Now put the same fault on a circuit.

The result is the same, removing the bonding jumper or neutral at the service will be dangerous.

By using a Neutral switching ATS switch all that will be accomplished is moving this potential problem from the normal power bonding jumper to the bonding jumper that will be installed at the generator.

I see one location as safe / dangerous as the other.

Just My Opinion, Bob

Thanks Bob for posting that "Graphic" make things easy to follow, "A picture is worth a 1000 words" I think we verify that??

You are correct one location would be as dangerous as the other if you open the ground or neutral. However I would think that the electrician would be aware of the generator running and would not open this under a load at the genaerator or xfer switch.( "2nd, In my opinion, anybody opening the nuetral has a death wish..." words of wisdom from Ryan_J )
With a 4 pole ats he could upgrade the main service and not be in danger from the leakage current on the ground and neutral ( bonded at generator X0). On the other hand he may not be aware of this danger if a 3p ats is used and is using the main service bond. Many times the customer will not want the generator(s) disable while we are working ( computer rooms, medical refrigeration, food storage ).

Maybe if a 3p ats is used a nice sign at the main service may warn about this danger and protect the installer in a injury lawsuit but I don't know. What do you think??

The original question appeared to be a compliance issue. I believe IWIRE is correct. The generator may be connected to the premisis wiring with either a 3 or 4 pole transfer switch.

If a 3-P switch is installed the NEC does not allow the generator neutral to be bonded at the gen or transfer sw.(XO). Additionally the non-current carrying metal parts (frame etc.)must be grounded per section 6 (VI) of article 250 by a method listed in 250.118.
If a 4-P sw. is used the generator is a seperately derived system and grounding and bonding are required per. 250.20 and 250.30 to mention a few.

Either way all eq. grounding eventually ends up at the premisesis supply main bonding jumper. (Generally)

A ground fault with the 3-P sw. may raise the voltage to ground through the Eq. Grounding Conductor.
A ground fault with the 4-P sw. may seek the main Bond through a connection at the transfer sw as well as the Neutral at the gen. Depends how the Code is applied.

No doubt an open neutral is trouble.

Back feed (so to say) at the gen during maintenance addressed in section 445.18.

I liked the idea about additional signage at the MDP.