Utility & Owner N-G Bonds

[billyzee]

There will be a 2000kVA (12.47-480/277) utility owned transformer located just outside of a building. The user owned service gear will also be located outside of the building and very close to the utility transformer.

The utility will bond the neutral to ground within the transformer enclosure. The NEC requires that the neutral be bonded to ground in the service gear.

There are several questions associated with this arrangement:
1. This arrangement does not create "objectionable" ground current and violate NEC 250.30(A)(1) Exception #2 if the equipment is located on separate concrete pads. However it seems to violate 250.50 which states that the grounding electrode system needs to be bonded together.
2. Since the utility owns the transformer is it Ok to put blinders on and let them do what ever they want. Couldn't this arrangement result in unbalance neutral current flows over ground grid which could falsely trip ground fault devices?
3. What if the equipment were on the same concrete pad.

All comments appreciated.

 

[tryinghard]

?There are several questions associated with this arrangement:
1. This arrangement does not create "objectionable" ground current and violate NEC 250.30(A)(1) Exception #2 if the equipment is located on separate concrete pads. However it seems to violate 250.50 which states that the grounding electrode system needs to be bonded together.

I believe the utility transformer is under different codes and guidelines and with this NEC is not the determining code.
I think parallel current is common because of the duel bonding jumpers, because of this in my opinion the XO should bond only once at the serving source transformer and not at the service disconnect, of course the secondary would need an equipment grounding conductor 250.118. I'll eat my hat if this ever changes!

2. Since the utility owns the transformer is it Ok to put blinders on and let them do what ever they want. Couldn't this arrangement result in unbalance neutral current flows over ground grid which could falsely trip ground fault devices?

I certainly will allow parallel paths on all common routes, I don?t know if this would trip ground fault devices I don?t think it would?

3. What if the equipment were on the same concrete pad.

I don?t think this changes the installation, bonding jumper at serving transformer and bonding jumper at service disconnect.

 

[iwire]

It is fine, almost every service in the US is bonded at both the utility transformer and the service disconnect. You will not run an EGC between these two bonds so there is no objectionable current.

As far as the bonding together of the utility electrodes and the premises electrodes that will be done via the grounded conductor.

 

[iwire]

II certainly will allow parallel paths on all common routes,

The earth will be the only parallel path unless they choose to use RMC the whole way. And if they do ............ well that is also the case on tens of thousands of services and is just the way it goes.

In my experience we run PVC to utility pad mount transformers so there is no issue at all.:smile:

 

[billyzee]

OK, then. Thank you very much. It Made My Day.

 

[weressl]

...... in my opinion the XO should bond only once at the serving source transformer and not at the service disconnect, .....I'll eat my hat if this ever changes!.

Ansolutely, and can I have my hat with sugar on top?!

I certainly will allow parallel paths on all common routes, I don?t know if this would trip ground fault devices I don?t think it would?.

The sensor will see the sum of parallel currents, it will see all the GF current, so it will(should ) trip.

 

[jghrist]

Bonding at the transformer and service may interfere with ground fault sensing at the transformer, but it won't affect anything on the load side of the service bond. In all likelihood, there will be no ground fault sensing at the transformer.

 

[tryinghard]

Check out 90.2(B)(5), also 250.6.

 

[George Stolz]

I'll eat my hat if this ever changes!

I know someone just crazy enough to try.

 

[iwire]

I am having a hard time following this.:-?

Are some of you saying there is a problem with the overwhelming majority of services in the USA because there is a bond at the utility and the service disconnect?

If that is so it is time for some code proposals to go in.:smile:

Very few utility transformers have any GFP on the secondary side ahead of the service disconnect.

 

[George Stolz]

I am having a hard time following this.:-?

Are some of you saying there is a problem with the overwhelming majority of services in the USA because there is a bond at the utility and the service disconnect?

I don't have a problem with it, but I didn't have a problem with 250.32(B) either. We see how that turned out.

Very few utility transformers have any GFP on the secondary side ahead of the service disconnect.

It's early, but I'm not picking up on the significance of that statement.

 

[weressl]

Check out 90.2(B)(5), also 250.6.

Unfortunately 250.6 will not support the argument for single point system grounding, unless you have an objectionable current flowing during normal operations. Perhaps on a larger system the capacitive coupling of your insulation will drive enough current to accomplish this, but you will still be faced with the problem that your single grounding point is under somebody else's - POCO - control and they are not required to maintain it or to notify you that it being disconnected, repaired, etc. Perhaps the code should call for grounding integrity monitoring at one point in the future.

 

[billyzee]

What if..?

What if..?

If the system is wye connected and the user as some imbalances then there will be neutral current. Therefore, if the utility transformer and service gear are both grounded there will be some ground current. Is this objectionable? Not to me. The service gear GFP sums everything up down stream of the parallel path so it should work fine.

But what if the utility transformer has a ground ring around it. And assume the switchgear is nearby, outdoors and also has a ground ring around it. Should these ground rings be solidly bonded together thus preventing step potentials. But this would increase the "objectionable" current because of the reduced resistance in the ground connnections.

Just wondering

 

[tryinghard]

Yours would be a paralleling path to source, 1) grounded neutral conductor and 2) earth, between serving transformer and service disconnect only. It would not be dangerous touch voltage. I understand your secondary is in RNM PVC.

 

[charlie]

I don't know if this helps or not but in general the grounding from the service equipment downstream is done with an equipment grounding conductor. Upstream from the service equipment to the electric utility, equipment is grounded (bonded) to the neutral (grounded conductor). The electric utilities follow the NESC and bond to the MGN (multi-grounded neutral). Where you are grounding meter enclosures, junction boxes, and other equipment ahead of the service equipment, you are essentially following the NESC. The service equipment is where the grounding conductor is created and the GEC is connected to the neutral and grounding conductors via the main bonding jumper. Keep in mind that these are broad strokes and will not hold up to scrutiny. :smile:

 

[Cold Fusion]

billy -
Keep in mind that my experience is all industrial, some light commercial in buildings associated with the industrial. Little to no residential (just mine)

There will be a 2000kVA (12.47-480/277) utility owned transformer located just outside of a building. The user owned service gear will also be located outside of the building and very close to the utility transformer.

The utility will bond the neutral to ground within the transformer enclosure. The NEC requires that the neutral be bonded to ground in the service gear. (cut)

Interesting that the wise men of Gotham (NFPA code panels) think that the physics change depending on which side of the service point you are on. Also interesting that the rules are the same for 2000kva industrial as it is for 20kva residential.

The design concept that changing the owner of the utility transformer changes safe construction practices is past poor science - its a joke.

1. This arrangement does not create "objectionable" ground current and violate NEC 250.30(A)(1) Exception #2 if the equipment is located on separate concrete pads. However it seems to violate 250.50 which states that the grounding electrode system needs to be bonded together. (cut)

I would bond the utility pad to the disconnect pad. As you said the two are close together. Now deal with the neutral current splitting between the neutral conductor and bonding conductor. Since the two are close together, maybe it doesn't matter. And if it does matter, well, the utilities I deal with do not demand poor design practices.

2. Since the utility owns the transformer is it Ok to put blinders on and let them do what ever they want. Couldn't this arrangement result in unbalance neutral current flows over ground grid which could falsely trip ground fault devices?

Yes, one could put on blinders. But that is a pretty poor design practice if one wants reliability.

As for the "over whelming majority of services": If one leaves out all of the residential, light commercial 208V, light industrial (<2000kva); I have no idea what is a typical installation. However, some I have dealt with have xfm differential protection. And yes, having a bonding jumper on both ends on the secondary feeder could easily cause a trip.

3. What if the equipment were on the same concrete pad.

An excellent reason for you and the utility to not submit to a twisted translation of a document that self-proclaims, "not intended as a design specification".

Lets take a look at the bizzare idea that the laws of physics change at the service point:

Some companies purchase their own substation transformers. That makes the secondary an SDS - can only be grounded and bonded in one place, xfm end, disconnect end, or anywhere in between. And any ground mats need to be interconnected.

Now, let's take the same installation and and have the utility own the transformer. It's suddenly safer to bond both ends of the secondary feeder? And now the interconnected ground mats provide a parallel neutral path?

As iwire alluded, in the residential world it is likely, "Who cares?" (I'm assuming that when iwire was speaking of the "overwhelming majority of services in the USA ..", he meant residential.)

But in the industrial stuff I work with it can be a big deal. Luckily the utility companies are generelly not morons and will allow/participate in good design practices.

For those still having a hard time with this concept, Riddle me this Batman:

Suppose the utility owns the transformer, but by contract the metering is on the primary side. There is a primary cb under control of the customer and the utility. Where is the service point? Is the xfm considered an SDS?

cf

 

[tryinghard]

?Suppose the utility owns the transformer, but by contract the metering is on the primary side. There is a primary cb under control of the customer and the utility. Where is the service point? Is the xfm considered an SDS?

I?d say the service point is at the service equipment at the end of the secondary.

If the XFMR is utility owned it may not be a SDS because sometimes they bond the grounded conductors primary and secondary. Also notice NEC definition of SDS, ?power is derived from a source?other than a service?, it?s ?A premises wiring system?.

 

[tryinghard]

If the secondary is in metallic conductive items like RMC which terminates at SWG that fastens to a metal building, the neutral paralleling current will also travel throughout the whole building frame system and all equipment grounding as it routes through earth and to source.

 

[don_resqcapt19]

250.24(A)(2) Outdoor Transformer. Where the transformer supplying the service is located outside the building, at least one additional grounding connection shall be made from the grounded service conductor to a grounding electrode, either at the transformer or elsewhere outside the building.

While in this case the service equipment is not in a "building", it is clear that when the transformer is outside and the service equipment is at or in the building, bonding is required at both locations.

 

[iwire]

It's early, but I'm not picking up on the significance of that statement.

From the opening post.

Couldn't this arrangement result in unbalance neutral current flows over ground grid which could falsely trip ground fault devices?