System Grounding

[VTer]

We have primary utility service and a 25kV-480Y/277V outdoor located pad-mounted transformer. The natural X0 bushing is bonded to ground at the transformer. In addition, 480/277V 4W switchgear lineup in the building also has the system bonding jumper between the neutral bus and ground bus. The condition that concerns me is that we have the green equipment grounding conductors connected between the switchgear ground and transformer ground. The equipment grounding conductor will carry portion of the normal neutral current. Is this allowable by NEC in this scenario or is it a code violation? Attached is a simple grounding diagram of the existing setup for your reference

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[wroggent]

Is this required by 250.24(A)(2)? Take a look at exhibit 250.8.

 

[VTer]

The outdoor transformer neutral has to be grounded per 250.24 and I also believe that the indoor switchgear has to be bonded. My question is that in this case, the equipment grounding conductor creates a parallel path with the neutral for normal neutral current which my understanding is a violation of the code.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[Mbrooke]

Who owns the pad mount? Considering its 5 wire (TN-S earthing was the goal)I would guess the facility owns it and thus follows the NEC. In that case, the neutral to ground bonding jumper needs to be removed at the switchgear or transformers but preferably the switch gear. Keeping the bond will cause the grounding system to carry currents which is both an NEC violation as well as opening the door to problems with sensitive electronics.

 

[VTer]

We own the pad mount. I was thinking about the removal of one system bonding jumper but I think the code requires both the outdoor transformer and indoor switchgear to be bonded. In addition, we have double-ended switchgear with modified differential ground fault scheme that has CTs on the neutral to ground bonding bus bar- so that bond in the gear would have to remain. My concern is on the equipment grounding conductor if it can stay as is or if it has to be disconnected? My opinion is to remove the equipment grounding conductor between the gear and transformer and remove the parallel natural path but I am not sure what the code requires in this scenario.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[dpc]

If you own the transformer, then this is a separately derived system. Take at look at 250.32.

 

[Mbrooke]

I 2nd that. It is considered a separately derived system under NEC rules. One of the bonds must be removed. I would also check with the specs on the differential ground protection and see what it recommends. I would imagine protection can be configured to allow the removal of one of the bonding jumpers.

 

[SteveWag]

I have done the following: If the conduit is steel from the Service to the pad mount, I run an equipment grounding conductor connected to Service ground at the Service end and connected to the conduit stub up end in the low voltage compartment of the pad mount, not the Neutral or transformer ground. I use a bushing with a ground screw. If the conduit is steel with PVC in the run somewhere I either do the same or I connect the steel conduit entering the pad mount compartment to the pad mount ground and do away with the grounding conductor. Either way the equipment grounding is connected at only one point and no Neutral current can flow except maybe in a Fault condition.
Steve

 

[Kiribanda]

VTer,
Multiple grounding of Neutral terminal is not allowed. If there is a zero seq. CT used for ground fault protection on the 4-core cable between the outdoor transformer and indoor switchgear, then that CT will not see the full ground fault return current flowing through it back to the transformer neutral. As a result depending on the magnitude of the ground fault, the sensitive ground fault (50/51S) relay might trip and isolate the transformer. Therefore, one of the two Neutral groundings (outdoor or indoor) to be romoved. Outdoor is better.

 

[dpc]

http://www.iecoregon.org/wp/wp-cont...y-Derived-Systems-Complying-withe-the-NEC.pdf

This might help (very SLOW loading). Basically, you need to comply with 250.30(A). You can have the bonding jumper in one location or the other, but not in both locations.

 

[VTer]

There is no easy way for me to remove the bonding jumper inside switchgear because of the CTs that are located on the bonding bus bar jumper. If I remove this bonding jumper than I will defeat the modified ground differential scheme and no longer have proper GFP.

The outdoor transformer natural has to be bonded per NEC at the transformer so I cannot remove that jumper.

Both solutions above solve one code violation and cause another. I think I only have two options (maybe):

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1. Remove bonding jumper in switchgear and relocate and install the (2) modified ground diff CTs at the bonding jumper at outdoor transformer location and run ~100' of secondary CT wiring back to the switchgear - technical and economical challenges.

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2. Remove and isolate green equipment grounding conductor between the switchgear and the transformer and leave the two bonding jumpers - the code does not allow this for separately derived systems. However, what confuses me is that if the pad-mount is utility owned they will typically bond their XO bushing at the transformer but the code then also requires the customer to bond the neutral in their gear. In those cases only the phases and the neutral (grounded) conductor are brought in the building and the green equipment grounding conductor is not installed between the two pieces of equipment – why can we not do the same for the customer owned pad-mounts or can we?

The better solution is option 2 but I cannot see any code exceptions to be able to do this. Does anyone think that option 2 would meet NEC and tell me where this exception or section is located and does anyone see another possible option to correct this?


"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[dpc]

The outdoor transformer natural has to be bonded per NEC at the transformer so I cannot remove that jumper.

What NEC article are you basing this statement on?

 

[VTer]

I do not have the code book in front of me now, but i believe it was in 250.24. I will confrim later.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[VTer]

dpc, the requirement is it in 250.24(A)(2) and in 250.30(C)

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[wroggent]

If you install per 250.30(A)(1) Exception No. 2 then can you remove the equipment grounding conductor by 250.30(A)(2) Exception. ?

 

[VTer]

wroggent, that is the same thing that I am not clear about. It seems to me that my option 2 listed above to disconnect the equipment grounding conductor might be acceptable.
Exhibit 250.5 showing the 208Y/120V service does not indicate the equipment grounding conductor.
To remove the equipment grounding conductor would be the easiest solution to remove parallel paths as required by 250.30(A)(1) Exception No. 2. However, the question then arises on the metal conduit being a parallel path as well. Do you have to go as far as to isolate the metal conduit as well?


"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[dpc]

250.24(A)(2) applies to a SERVICE. By NEC definition, a service comes from a utility. This is not a service, it is a separately-derived system. 250.30.C is in reference to grounding electrodes, not the system bonding jumper.

Read 250.30 (A)(1), I think this is the most applicable section.

 

[VTer]

Thank you dpc. If 250.30(C) does not require the XO bushing to be bonded to ground at the outdoor transformer, I will remove the bond there. My system bonding jumper will remain in the switchgear and this way I do not have to disconnect the equipment grounding conductor because there will not be any parallel paths.

For future and educational purposes, if you were to follow 250.30(A)(1) EXCEPTION 2, then equipment grounding conductor would not be allowed between the two pieces of equipment and nonmetallic conduit or metal conduit isolated at the equipment would have to be utilized. Is this correct?


"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[dpc]

That's the way I read it. It is equivalent to a utility service, I guess. They have their ground rod and give you a grounded neutral. This may be in the NEC because some pad-mounted transformers have the X0 bushing internally grounded to the tank. That's something you'll want to verify.

 

[jghrist]

Is the transformer delta-wye or wye-wye? If it is wye-wye, then it is not a separately derived source. You do not want to isolate the secondary neutral from the grounded primary neutral.