System Grounding
System Grounding
(OP)
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






RE: System Grounding
RE: System Grounding
"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
RE: System Grounding
RE: System Grounding
"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
RE: System Grounding
RE: System Grounding
RE: System Grounding
Steve
RE: System Grounding
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.
RE: System Grounding
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.
RE: System Grounding
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):
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.
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
RE: System Grounding
What NEC article are you basing this statement on?
RE: System Grounding
"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
RE: System Grounding
"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
RE: System Grounding
RE: System Grounding
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
RE: System Grounding
Read 250.30 (A)(1), I think this is the most applicable section.
RE: System Grounding
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
RE: System Grounding
RE: System Grounding
RE: System Grounding
RE: System Grounding
his drawing shows delta-wye.
dpc,
I believe jghrist is correct about the wye-wye - as long as the two neutrals are solidly connected.
From 2014 NEC:
"Exhibit 250.12 A 208Y/120-volt, 3-phase, 4-wire system that has a direct electrical connection of the grounded circuit conductor (neutral) to the generator and is therefore not considered a separately derived system."
"Exhibit 250.13 A 208Y/120-volt, 3-phase, 4-wire system that does not have a direct electrical connection of the grounded circuit conductor (neutral) to the generator and is therefore considered a separately derived system."
Article 100 Definitions "Separately Derived System. An electrical source, other than a service, having no direct connection(s) to circuit conductors of any other electrical source other than those established by grounding and bonding connections. Examples of separately derived systems include generators, batteries, converter wingdings, transformers, and solar photovoltaic systems, provided they have no direct electrical connection to another source. The earth, metal enclosures, metal raceways, and equipment grounding conductors may provide incidental connection between systems. This definition clarifies that those systems can still be considered to be separately derived systems as long as the separately derived systems have no direct electrical connection to service-derived systems. The grounded circuit conductors are not intended to be directly connected."
RE: System Grounding