Objectionable Current (Art. 250.6)

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harrycan

harrycan

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I am looking for direction on trying to elliminate about 7 amps of objectionable current on the secondary side grounding (building steel) conductor of a 75 kVA isolation (208 Delta-208/120 Wye) transformer.

Upon troubleshooting it was found that by opening the secondary side disconnect (3-pole) - current remained at 7 amps, while opening the primary side disconnect (3 pole) causes current to drop to zero. Why would this occur considering that in a normal grounding/bonding scenario the equipment (case) grounds, neutral, output ground through the conduit have the exact same path energized and de-energized. What is different in these two conditions (besides the transformer being energized) that would cause current to flow on the grounding wire?
 
harrycan said:
I What is different in these two conditions (besides the transformer being energized) that would cause current to flow on the grounding wire?
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There are a couple of causes but the most lilely reason with this much current is the the grounded circuit conductor is bonded somewhere downstream from the service disconnect.
 
Secondary was de-energized.

Secondary was de-energized.

dereckbc said:
There are a couple of causes but the most lilely reason with this much current is the the grounded circuit conductor is bonded somewhere downstream from the service disconnect.
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I agree that there is/are parallel paths for the "Grounding" conductor in this situation, and that is the most likely cause of the 7 amps on the building steel (grounding electrode conductor) BUT we de-energized all the loads (secondary) of this transformer and STILL had 7 amps. It was not until we shutdown the transformer (opened primary disconnect) that the 7 Amps went away. That is the conundrum.
 
Is the entire building fed from this transformer? It sounds like only when you disconnected all sources of supply that you eliminated the current. You will need to shut down each source until the current is eliminated in order to determine which is causing the problem.
 
grounding electrode conductor

grounding electrode conductor

petersonra said:
how do you know this is objectionable?
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What else would current (of that magnitude) be called that exists on the grounding electrode conductor, such as in this case the building steel ground?

I thought it was "objectionable" because it is current found on grounding and bonding paths.
 
My apologies - clarification provided.

My apologies - clarification provided.

haskindm said:
Is the entire building fed from this transformer? It sounds like only when you disconnected all sources of supply that you eliminated the current. You will need to shut down each source until the current is eliminated in order to determine which is causing the problem.
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Sorry I did not clarify: This is a NEW transformer that is downstream of the building service (208) that is an isolation means to a Hospital Operating Room (recently rennovated) and it's equipment. This is why this current is striking a nerve.

The transformer supply (208V 3-pole fused disconnect) to this transformer is about three feet away and located in a new main distribution panel (again, not the main service). The output (sec) disconnect is right beside the Transformer and is also a three pole (with Neutral fed through).
 
How can there be a "fed through neutral" if the supply is a Delta?


Roger
 
roger said:
How can there be a "fed through neutral" if the supply is a Delta?


Roger
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Ooops, nevermind, I see you are just talking about the neutral feeding through the disconnect.

Roger
 
Sorry... Clarification - Again....

Sorry... Clarification - Again....

roger said:
How can there be a "fed through neutral" if the supply is a Delta?


Roger
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The Neutral is fed through the output disconnect to the load distribution panel. It is a Delta-Wye Transformer as previously stated. I guess I confused the issue, adding that comment. Sorry
 
I would leave the secondary disconnect off, then cycle the primary on/off with each cycle having a different ground wire connected. That should tell you which ground wire (or all?) is bringing in the 7-amps.

You might just have all the grounds floating and check for a voltage between grounds?

I'm curious, does that lug that the 3 grounds are connected to have a listing for more than 1 conductor?
 
In fact, you _do_ have a primary neutral connection: that EGC coming in with the primary feed. I would suggest using an amp-clamp to look for current on the _individual_ EGC conductors, with the transformer in different operating states.

Also, in addition to the lug being used for too many conductors, the EGCs are looping up into the region where you are not supposed to have any conductors because of temperature rise.

-Jon
 
One of the green conductors is going to steel (GEC). I would be willing to think that is the conductor providing the 7 amp. There may be a neutral to ground bond on the primary side, and the steel is carrying it. Just a guess. Try disconnecting the conductor going to steel from the said transformer (with the secondary switch open) and check for the current at X/O - then measure the conductor from steel.
Let us know what you find.
 
would it have to be a sub panel that is bonded or could some piece of equipment close to the transformer be bonded? because if it is bonded and the new transformer comes on-line it make the path to the new transformer the easiest. when the transformer is off it is not noticable..I hope that sounds like I meant it to sound..
 
Possible you have tied on to a neutral from a separate system,
Possible the current is from separate system and using this as one of many ground path.

When load is off ground current can SHIFT, I have seen this several times, when objectionable ground is on the grounding electrode conductor.

Check all separately derived systems.
Check generator grounding.
Check neutral ground bond at main service.


You are looking for NEC compliant installations and ground current.
 
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