Parallel Ground Path

gastoor

Member
Application on a separately derived system. We have both ungrounded and grounded systems at our plant. A contractor is hooking up a new separately derived system with a 25KV primary and 480V secondary 1500KVA WYE-WYE transformer. The primary is internally grounded to HO. Contractor ran 4 wire plus EGC from transformer and hooked both the neutral and EGC to the same ground buss on a 3 phase 3 wire switchgear (switchboard). The switchgear has a 2500 amp main and does not have a separate neutral buss and therefore no CT on neutral to detect the fault and trip the main. The contractor says since both are hooked essentially to the grounding electrode system then it makes the system grounded and the main should trip on a phase to ground fault or phase to phase fault. I believe he created a parallel path to ground which can cause equipment damage and electrical hazard to personnel. We do not have electrical engineers on site, and my emails to some have not returned. Are there any code violations here, so at least I can use that to stop the project before the system is energized. Help.....
 
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brian john

Senior Member
Location
Leesburg, VA
Application on a separately derived system. We have both ungrounded and grounded systems at our plant. A contractor is hooking up a new separately derived system with a 25KV primary and 480V secondary 1500KVA WYE-WYE transformer. The primary is internally grounded to HO.
I assume you own the transformers?

If the primary is wye with a 4 wire connection, then on the primary you have set up a parallel path for ground/neutral current assuming the Neutral of the medium voltage system is grounded. NOTE I seldom to never work with Wye - Wye transformers and others might correct me on this.

Contractor ran 4 wire plus EGC from transformer and hooked both the neutral and EGC to the same ground buss on a 3 phase 3 wire switchgear (switchboard). The switchgear has a 2500 amp main and does not have a separate neutral buss and therefore no CT on neutral to detect the fault and trip the main. The contractor says since both are hooked essentially to the grounding electrode system then it makes the system grounded and the main should trip on a phase to ground fault or phase to phase fault. I believe he created a parallel path to ground which can cause equipment damage and electrical hazard to personnel. We do not have electrical engineers on site, and my emails to some have not returned. Are there any code violations here, so at least I can use that to stop the project before the system is energized. Help.....


If the EGC and XO/Neutral are bonded In the transformer and in the switchgear then you have a parallel path.

He hooked the neutral to the ground bus? As you state this is a 3-phase, 3-wire board?

The main will trip on a Ground Fault but with a parallel paths and this can affect the tripping response of the GFPE system.

ARE THERE NEUTRAL LOADS? Why was no GFPE Neutral ordered with the gear?
 
If done correctly, you will not have a parallel path. Recall that you have two options for bonding a separately derived system. Mike Holt has a good graphic, I bet you could find it if you google image search "mike holt separately derived system bonding" or similar. Since you do not have a separate ground and neutral bus in the gear, it seems you are committed to not bonding at the transformer, setting up the gear like a service, and running a SSBL from the gear back to the transformer to bond it. There would be no parallel paths.

I dont follow the contractor's argument about the system being grounded - seems he thinks this is a substitute for the GFPE requirement? It is not; it is still required.

I cant comment on the primary side as I dont know MV well. Although connecting the primary neutral to the wye point in the LV world is typically a no no, it seems to be common/standard in the MV world, but I am not sure wye ;)
 

big john

Senior Member
Location
Portland, ME
...Contractor ran 4 wire plus EGC from transformer and hooked both the neutral and EGC to the same ground buss on a 3 phase 3 wire switchgear (switchboard). The switchgear has a 2500 amp main and does not have a separate neutral bus....
Why would anyone pull a 4 wire feeder to 3 wire switchgear? The simple answer is that if the neutrals and equipment grounds are bonded at both ends it's a violation of 250.30(A)(1) as well as creating objectionable current over the EGC.
...The contractor says since both are hooked essentially to the grounding electrode system then it makes the system grounded and the main should trip on a phase to ground fault or phase to phase fault. I believe he created a parallel path to ground which can cause equipment damage and electrical hazard to personnel.
Multiple system bonds don't change the nature of whether this system is grounded, nor decrease the likelihood of clearing a ground-fault: Even with only one bonding point, it's still a grounded system.

Dual bonds just make it more likely you will have nuisance tripping of your GFP in addition to being a code violation.
 
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Smart $

Esteemed Member
Location
Ohio
I think its a SSBJ not a EGC
I was thinking if he had a service that didn't require a neutral he didn't need to bring one to his gear.

But I could be misreading this, yes, if there's an SSBJ and X0 is unbonded in the transformer this would be fine.
I think there's a little confusion here.

The 4 wire feeder are 3 hots and 1 neutral... plus the SSBJ for 5 wires total (perhaps multiple sets, being the main is 2500A).

For SDS, the grounded conductor is not required to be brought to the disconnecting means, but that's mean all the grounding and bonding has to be done at the transformer. Where grounding and bonding is done at the gear, the grounded conductor must be brought there.
 

Smart $

Esteemed Member
Location
Ohio
. The switchgear has a 2500 amp main and does not have a separate neutral buss and therefore no CT on neutral to detect the fault and trip the main. The contractor says since both are hooked essentially to the grounding electrode system then it makes the system grounded and the main should trip on a phase to ground fault or phase to phase fault. ...
He is correct as long as the grounded [neutral] conductor and GRD are only bonded at the gear.

For current to return to the transformer from ground fault, it travels through the grounding system back to the gear, through the system bonding jumper (which is the ground bus of the gear), then through the grounded neutral conductor back to the transformer. The GFPE should sense the imbalance of ungrounded current leaving/returning and trip, if it is a resistive ground fault and not enough current to trip any overcurrent device.

NOTE: The above assumes there are no line to neutral loads.
 

jaggedben

Senior Member
Application on a separately derived system. We have both ungrounded and grounded systems at our plant. A contractor is hooking up a new separately derived system with a 25KV primary and 480V secondary 1500KVA WYE-WYE transformer. The primary is internally grounded to HO. Contractor ran 4 wire plus EGC from transformer and hooked both the neutral and EGC to the same ground buss on a 3 phase 3 wire switchgear (switchboard)....
I'm sure I'm the least qualified person commenting on this thread, but it seems to me that red doesn't match blue. Is any of it incorrect information?
 

Smart $

Esteemed Member
Location
Ohio
I'm sure I'm the least qualified person commenting on this thread, but it seems to me that red doesn't match blue. Is any of it incorrect information?
Your red WYE is the primary. The (red) 4-wire feeder and (blue) 3-wire gear are on the secondary side.

EDIT: At least that's the way I'm reading it.
 

jaggedben

Senior Member
Your red WYE is the primary. The (red) 4-wire feeder and (blue) 3-wire gear are on the secondary side.

EDIT: At least that's the way I'm reading it.
Huh. The way I read it, all the items I colored are on the primary side. But reading it again, I think it's vague.
 

big john

Senior Member
Location
Portland, ME
We need a lot of clarification, but it sounds like OP is also discussing that a neutral was landed on H0.

This is separate from having a 4 wire system coming off the low side and connecting to 3 wire gear.
 
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