standby genset grounding and bonding

[murli]

Hi

I am planning to install a standby GEN SET of 60 KW at a pumping facility. The utility feeds the electrical panelboard. The utility has 3phase, 3wire system, 480v. They do not plan to upgrade this to 3phase/4wire at this time. If I use a 3 POLE TRANSFER SWITCH, and DO NOT ground the GEN SET neutral, and leave it at 3PHASE, 3WIRE system, WILL I VIOLATE THE CODE? The load is just motor loads.
thanks
murli

 

[Guest]

Re: standby genset grounding and bonding

Here is some suggested reading. Post back with questions:
NEC 250.30(A)(2), 250.24(A)(5), 702.8(A)(B), 702.6.

 

[bennie]

Re: standby genset grounding and bonding

How about more specific details about this 3 wire system?

 

[murli]

Re: standby genset grounding and bonding

Hello

The 3 wire loads are just 3 ph motors, a control transformer that takes in 2 hots. The code indirectly mentions that the GENERATOR SET may NOT be grounded. DO YOU AGREE WITH THAT ? IF I CONNECT a 3 POLE TS AND LEAVE THE GEN NEUTRAL UNGROUNDED would that be OK ?

THANKS
MURLI

 

[Ed MacLaren]

Re: standby genset grounding and bonding

I can't give you a definite answer with regard to NEC compliance, perhaps others will help.

Generally, if a supply system (your generator) has a neutral point (wye connected system) it is required to be grounded.

What is there on the 480 volt ungrounded service now for ground fault indication?

I would cover all the bases by running a neutral conductor from the generator to the transfer switch, and connecting an acceptable grounding electrode, and the load feeder EGCs, (equipment grounding conductors) to it at that point.

Similar to this sketch.

Ed

[ September 21, 2003, 09:01 AM: Message edited by: Ed MacLaren ]

 

[russ]

Re: standby genset grounding and bonding

I'm guessing at some of this, because the picture has not come into focus yet.

There is no grounded conductor in the utility service. The generator neutral is most likely already bonded to the generator frame right out of the factory. If this is an all metal conduit system, it is tied to the Service ground via the metal pipe system. It would seem to be grounded and bonded already

Russ

 

[russ]

Re: standby genset grounding and bonding

I see no reason it can't be done according to Ed's drawing either, all though the transfer switch ground may be redundant.

 

[mclain]

Re: standby genset grounding and bonding

If the generator neutral weren?t grounded at the generator I wouldn?t ground it. If you are going to float the neutral (ungrounded system) you will be required to have a ground fault indications system. We operate all our generators this way. But we aren?t connected to a service we generate all of our power.

 

[bennie]

Re: standby genset grounding and bonding

A generator is not grounded like a motor. A generator essentially requires an opposite approach. An equipment ground conductor from the transfer switch to the generator frame is only a long neutral to frame connection. Make the interconnection at the generator, not a long distance away.

A generator supplying a separately derived system, can be bonded at both the source and the first switch.

There is no overcurrent device in the generator windings. The control system will detect a winding to core fault and dump the exciter.

Phase voltage, on the faulted phase will go to zero, and set up a generator failure and shut down, mode of operation.

 

[dereckbc]

Re: standby genset grounding and bonding

Murli, I have designed many a sites with Delta 3-phase 3-wire services with a back-up generator. Since your generator neutral is bonded to the frame it is configured as SDS regardles or not if you run the neutral out from the generators and only use a 3-pole ATS. This requires you to connect the Xo of the generator to a ground electrode.

Since you have a delta service there are a few things you need to be aware of.

Under normal operation on the utility if you were to have a phase to ground fault nothing will happen except you GFD circuits will detect the fault and alert you as you would expect.

However if you are on the generator for any reason and have a phase to ground fault the generator will trip off-line because the neutral is bonded.

There is one more item that needs strict attention. That is how you GFD and shunt trip relays are configured. Lets say its that time of week/month/year when you are testing the generator. The utility power is OK and you start the generator and switch the load to the generator, and to your suprise there is a phase-to-ground fault on the conductors from the generator to the ATS. If the fault current divides and splits, with one path goping to the N-G bond on the generator and the other path through you ground fault trip relay in the main, you will trip both the generator and main shunt trip leaving you quite and dark.

 

[bennie]

Re: standby genset grounding and bonding

Dereck: I know there is something I'm missing when ever I read about the Main service switch tripping.

My question is; so what? close it.


This is post number 2100, where shall I quit?

[ September 21, 2003, 02:31 PM: Message edited by: bennie ]

 

[don_resqcapt19]

Re: standby genset grounding and bonding

Bennie,

A generator supplying a separately derived system, can be bonded at both the source and the first switch.

What about 250.30(A)(1)??
Don

 

[Guest]

Re: standby genset grounding and bonding

For a permanent install you need to remove the bonding jumper if the transfer switch does not switch the neutral too 250.24(A)(5)

I hope this is helpful in addition to all the other good advice here.

../Wayne C.

[ September 21, 2003, 04:02 PM: Message edited by: awwt ]

 

[bennie]

Re: standby genset grounding and bonding

A bonding conductor shall be permitted at both the source and service, if there is no parallel neutral path.

This is often the case when PVC is used, there is no parallel path.

The bond should be at the generator or transformer to make the shortest path for fault current.

 

[bennie]

Re: standby genset grounding and bonding

Wayne: What does the section quoted have to do with this issue?

 

[roger]

Re: standby genset grounding and bonding

Thank you Bennie.

Roger

 

[murli]

Re: standby genset grounding and bonding

Hello

Thanks for the suggestions. I read the code book a few times. 250-30 says that voltage to ground must not exceed above 150V.That means that all 208/120 Volts systems neutrals have to be grounded. Since my voltage to ground is 277v, this may not apply to me. The code then says that for 3 phase 4 wire system, if the neutral is NOT going to carry load, it does not have to be grounded. From the code book, it seems that I may be able to connect the GENSET to 3 POLE TR SWITCH as a 3 WIRE system with equipment grounds, and just connect ground indicators. Or I can use a 4 POLE TR SWITCH, and ground the neutral at the GEN end if I am running the GENSET neutral through the 4 th pole. The supply utility has a 3 phase 3 WIRE system. Could anyone agree with me on my CODE assessment ?
thanks to all
murli

 

[roger]

Re: standby genset grounding and bonding

Murli, please read 250-30 again and find where it limits voltage to ground to 150 volts.

Although I like a grounded service, if there is no need for a neutral per your previous post, you are correct that you could simply use ground indicators if all applicable requirements are met.

Roger

[ September 21, 2003, 07:22 PM: Message edited by: roger ]

 

[don_resqcapt19]

Re: standby genset grounding and bonding

Bennie,
I agree if there are no parallel paths you can bond at both the generator and the OCPD per Exception #1 to 250.30(A)(1).
Don

 

[russ]

Re: standby genset grounding and bonding

bennie:

Why quit, I figure you're just warming up.

2100 WOW!

Russ