Question was posed to me on how to handle the common conductor of a 480v. generator being used as a backup for a utility supply. The load is 480v. only with no need for a "neutral". There are no 277v. loads. The generator is being used through a transfer switch and the question is- "Do I need to bond the common conductor to the frame of the generator and/or do I have to break this conductor with the transfer switch". My answer was that I would bond this common conductor to the frame and not break this conductor with the transfer switch. Reason - this conductor is not a current carring conductor and we are only providing equipment grounding for lightning, surges etc. If we float the common conductor we will have an ungrounded SDS and need ground detectors. Need advice on this one.

[This message has been edited by George Little (edited 03-14-2006).]

George
I Feel 250.24 5(B) applies.Then you look at the 3 or 4 pole transfer switch to decide Grounding.
Also look at 250.24 5 FPN

George
If you do not transfer the neutral(break the neutral) you run the existing system neutral to the generator neutral bushing. You use the existing system EGC and bond it to the frame of the generator. You have the option to transfer the neutral or not. I would not.
It make the installation simpler.

[This message has been edited by Bob (edited 03-14-2006).]

[This message has been edited by Bob (edited 03-14-2006).]

The only issue is objectionable current on the grounding conductor. If there is, and never will be, any 277v loads I don't see a problem with George's take on this.
Bob's solution is more like what Mr Soares suggests.

George
I feel if you have a Neutral present in the Transfer switch from the existing service then a Neutral from the Generator Is required.(to maintain same type of grounding system) If its a 3 pole transfer switch then you ground(Earth Ground) at the utilty service only. Also Bond only at the utility service. If you have a 4 pole transfer switch then your dealing with a Separately Derived sytem . Then Grounding and bonding is required at both places(Utility, & Generator)See pages 180-181 Soares Book on Grounding 7th Edition.Also view McGraw-Hill's
National Electrical Code Handbook 24th EditionPages 364,365,&366,367.

[This message has been edited by Yoopersup (edited 03-14-2006).]

No neutral anyplace either at the Utility or the Generator. That's my question and my point I guess. The 480v. Utility service does mot have any neutral load nor does the generator.

George
Then its either an Ungrounded Delta or a Grounded Delta(one Phase to ground). Grounding would be the same as usual but no neutral involved.

How are you getting past the idea that the grounded conductor shall be brought to the service disconnect enclosure and that is where the MBJ lives?

On a True Delta (ungrounded)there is no Grounded conductor, On a Conor grounded Delta one of the Phase conductors is the Grounded conductor. Look it up in Soares it explains it well there, Most Electricals never heard or ever seen either.The Key is theres No Neutral (as most know it at either place the Utility feed Or the Generator!)

I believe that it is an error to _both_ not transfer the neutral and bond the neutral to ground at the generator.

If you do not transfer the neutral, then the neutral of the generator is connected to ground via the main bonding jumper of the primary system. Thus even though there is no bond at the generator itself, you would not leave an inadvertent ungrounded system.

If you bond at the generator and don't transfer switch the neutral, then you have a situation where the _unbroken_ neutral is bonded to ground at two points, which creates a parallel path. The fact that there are no neutral loads served does not change the existence of this parallel path.

However lack of neutral loads means that no current would be injected into this parallel path, so I suspect that having this particular parallel path is not a significant safety issue.

-Jon

THERE IS NO NEUTRAL as we know it in a Ungrounded Delta or a Conor phase Grounded Delta! Look it Up. In a Conor Grounded Delta one of the Phase conductors (Is) the grounded conductor.Soares Has plently of Details and Photos of these in there Book Soares Book on Grounding 7th Edition.

George
Is there a Neutral Present (480/277) not being used?? Or is there No Neutral at either source?? If there a Neutral Present Even if its not being used(No load) then all Neutral rules apply EVEN if theres no Load. If there No neutral Period (Straight 480v) then Different as I;ve said in other posts.

I guess George needs to clarify what this service is. Is the delta grounded? Where is the grounding connection made? If it is corner ground that kicks a couple of other issues, not the least of which is what color is this phase?
If there is no grounding at all we wouldn't have the question.

There is no neutral load. I don't know if the system is a wye or a delta, I don't know if the delta is corner grounded or ungrounded. My guess is that it is corner grounded. But I do know for sure that there is no neutral load and due to the fact that grounding is optional at 480v. We have this issue we are talking about.

If this is corner grounded you are switching the neutral, assuming 3 phase transfer equipment. Bond it in the service disconnect on the load side of the TE and you are good to go.
You also need to be sure the grounded phase is white.

Okay Greg- I agree, So what do I do in the transfer switch and what do I do at the generator?

I do think it would behoove us as professional code persons to use the term "grounded conductor" for this issue instead of "neutral" since the term "neutral" is not a true code term.

You are right about the term "neutral" if this is corner grounded delta. That is the best example of a place where grounded and neutral are not the same thing.
Go ahead and bond the "white" at the generator too (if this is really "corner")
If it is being switched in 3 pole transfer equipment. You could be using 2 pole equipment tho. Then we are back to worrying about parallel paths.

I would insure that is the same phase we are grounding but the "white" indentification should enforce that.

George;

To find out what type of incoming (PoCo supplied) Power system you have, check Voltage to Ground at the Service Equipment - or a Panelboard relatively close to the Service Equipment.

First, test Voltage With a "Wiggy":
<OL TYPE=A>

[*] Test between Line "A" and Metallic Enclosure of Panel / Gear,


[*] Do the same for Line "B",


[*] Do the same for Line "C"
</OL>
If any of these L-G tests give a solid reading, the system is Grounded at some point.
If the Wiggy acts like it's going to kick in, but quickly stops and shows nothing - on all 3 L-G tests, the system is Ungrounded.

Next, test Voltage with a DVM:
<OL TYPE=A>

[*] Test between Line "A" and the Metallic Equipment, as was done in the first tests,


[*] Do the same for Line "B",


[*] Do the same for Line "C"
</OL>

If the Wiggy tests showed a Grounded system, see if the DVM's readouts are somewhat equal on all 3 Lines.
If they are within 20-25 Volts of each other, the system is a 4 Wire Wye, and the Common Grounded Conductor is carried to the Service Equipment for bonding purposes.

If there is one Line with a noticably higher Voltage than the other two, the system is a Center Tap Grounded Delta. Most likely, the Ground Bonding was done only at the Transformer, but the Conductor from the Center Tap may also be brought to the Service Equipment, then bonded at both the Transformer and the Service Equipment.

I have seen them done both ways for 480VAC 3Ø 3 Wire Grounded Deltas - Grounded only at the Transformer (via center-tapped, center transformer), and with the Conductor brought to the Service Equipment.

If you find that one Line has zero Volts to Ground as read by the Wiggy, and the DVM shows like 10 - 25 Volts from the same Line to Ground, + the remaining two Lines have a high solid reading to Ground; you have a Corner Grounded Delta.

If the Wiggy shows no readings, and the DVM shows really high ones - like 300 - 500 Volts to Ground, you have an Ungrounded Delta.
You can verify this by having someone place the Wiggy's leads L-G as you have the DVM's leads L-G, and observe how the Voltage readings on the DVM drop drastically.

If this Project is of a good square footage size, and was built circa 1950 to 1970s, you most likely have an Ungrounded Delta.
If there are customers sharing the Transformer which have Commercial Lighting loads, the system may be a 4 wire Wye.

I doubt very much that you will find any 480V Corner Grounded Deltas, but the 480V Grounded Center Tap Delta "Oddball" may test out as would an Ungrounded Delta (most of the time it will not, unless the PoCo's tap and pole mounted grounding electrode conductor have a crappy connection going on!)

Good luck.

Scott35

P.S.: In the above descriptions, with the term "Voltage To Ground" or "Line To Ground", the meaning for "Ground" is to reference Metallic Enclosures - like Switchgear and Panelboards and such; NOT to any Earth Ground or Metallic Water Piping systems.

S.E.T.

Scott- Your information is very valid but in my case I was not given that information in the question I was fielding over the phone. I don't have any idea where the job is as it is not in my jurisdiction.

D'OHH!!!

Sorry for that, George! Somewhere in the middle of the thread, I convinced myself that you were doing this Genny Install, instead of just responding to a previously asked question.

That's what lack of sleep can do to a person

Scott35