GFP and SDS

[egr]

I know this is a touchy subject, but I am trying to clarify the issue at least in my mind...
It is my understanding that in a 480/277 Wye system provided by the utility company, NEC 250.24(C) requires the neutral conductor to be brought to the service disconnect even if there are no L-N loads to be served. It is also my understanding that NEC 250.24(A)(2) requires a bonding connection both at the transformer and the Main Service enclosure. If the Neutral conductor is not carried any further than the Main Service Disconnect for all practical purposes the system is a 3 phase, 3 wire right? If this is the case and if the system has an optional standby generator, the generator can be grounded at the generator location and the GEC connected to the utility power GEC and still be considerd an SDS right?
And along the same lines if this service is required to have GFP, this protection is not compromised by the generator grounding...?
And as last question if the generator is protected by a 1200 A breaker does it need to have GFP...? I am guessing it doe based on NEC 215.10, but I would like to hear any dissenting apinions

 

[bob]

I know this is a touchy subject, but I am trying to clarify the issue at least in my mind...
It is my understanding that in a 480/277 Wye system provided by the utility company, NEC 250.24(C) requires the neutral conductor to be brought to the service disconnect even if there are no L-N loads to be served.

That requirement is necessary so that there is a ground path back to the transformer. Without the neutral the only path would be earth.

It is also my understanding that NEC 250.24(A)(2) requires a bonding connection both at the transformer and the Main Service enclosure. If the Neutral conductor is not carried any further than the Main Service Disconnect for all practical purposes the system is a 3 phase, 3 wire right?

True but you still have to provide an EGC with the phase conductors.

If this is the case and if the system has an optional standby generator, the generator can be grounded at the generator location and the GEC connected to the utility power GEC and still be considered an SDS right?

In most cases there is no reason to provide a SDS with the generator. If you
chose to do so you must bond the neutral and grounding conductor and ground rod. If you do not provide a SDS then you remove the neutral to frame bond and use the system EGC to bond the generator frame.

And along the same lines if this service is required to have GFP, this protection is not compromised by the generator grounding...?
And as last question if the generator is protected by a 1200 A breaker does it need to have GFP...? I am guessing it does based on NEC 215.10, but I would like to hear any dissenting opinions

This discussion provides some good information.
http://www.mikeholt.com/cgi-bin/codeforum/ultimatebb.cgi?ubb=get_topic;f=3;t=000661;p=

 

[coulter]

bob -
That was an excellent discussion.

egr-
My work is all industrial. There is generally an excellent ground grid that connects to the xfmr and the served structures. So if you bond at the xfmr and again at the secondary main, there is a parallel neutral path - but, as has been discussed in other threads, the NEC requires it.

I've only found two ways to defeat this legally induced, NEC driven, error.

1. Buy your own transformer. The service is on the primary. All downstream is from an SDS. And you can use an impedance grounded secondary if you wish - the utility doesn't care. You can also usually negotiate a better rate since you are taking primary metering, the utility doesn't have to front the money for the xfmr, and the maintenance is on you.

2. If the downside makes it not worth owning the transformer, the next best is to get the AHJ to agree to allowing no bond at the first disconnect. The service is 5wire. Most AHJ's I have worked with are plenty bright and can see the legal NEC method is not a great idea.

carl

 

[egr]

Thank you both for the responses...
As I had mentioned before, I am having a difficult time why in a 3 phase, 3 W system (downstream of the Main Service Disconnect) bonding the neutral at the service and at the generator would defeat the GFP, specially if the ground current is measured on the line side of the MSD...?

Regards

 

[benaround]

egr,

You don't have a 3wire system, you have a 4wire system. A 3ph 3wire

system has no grounded conductor, it is an ungrounded system, should a

phase conductor short to a 3wire system nothing will happen, if on a 4wire

system like you have there will be sparks everywhere- thus GFP.

 

[egr]

I have attached a picture from one of the well known gen-sets manufacturers to illustrate my confusion. In addition I have attached a similar picture from the IEEE 446 "Orange Book"
The bottom line seems to be... if you had a system with no L-N loads, and the neutral grounded at the main service disconnect, located remotely from the ATS, and a generator with the neutral grounded at the generator location, which also would be remotely located from the ATS. There would be not need to run a neutral conductor from the generator to the ATS, and another one from the MSD to the ATS...
Your thouhts??

 

[benaround]

egr,

IMO I would say that would work, except for the ground rod at the gen.. I

would let the GES be used for both. Without knowing how far apart all the

components are from each other, by earthing both ends, which all tie

together as EGC in the ATS, an unequal potential might cause a lightning

event to jump around the systems that are bonded. I hope that I understand

this situation myself. Maybe one of the Forum's wiser members will chime in

if they feel I'm in error.