55 volts to ground on neutral

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domelectric

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Sarasota, Florida
Last week I was putting a branch circuit neutral wire on the neutral bar and was getting shocked when I touched the neutral bar. The panel is a 200A three phase supplied through a fused disconnect fed from a 480/240 112.5KVA transformer. I opened up the disconnect and found the neutral and ground separated. I opened up the transformer which is fed from a 480V three phase breaker in the MDP. Three ungrounded conductors and an egc coming into the transformer and leaving the transformer are three ungrounded conductors, the same egc (bonded to the transformer case) and a neutral wire on XO. There is no continuity between the bonded egc and the secondary neutral. Since this is a separately derived system I though the neutral and the egc should be bonded together at the disconnect. Also, there was no gec to the building steel from the transformer. Any input?
 
You are corrrect.

There needs to be a system bonding jumper installed on the secondary side of the transformer and also a connection needs to be made to the GES of the building.

Take a look at 250.30 for the requirements for grounding and bonding of a separately derived system.

Chris
 
domelectric said:
Okay, thanks. Now should the gec be connected to the egc in the transformer, to the transformer neutral, or both?
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The grounding electrode conductor must be connected to the secondary grounded conductor of the transformer. This connection can be made on the terminal where the system bonding jumper is attached to the transformer frame and the equipment bonding jumper or to the XO of the transformer.

Check out 250.30(A)(3).

Chris
 
Thanks again for the input and code reference. I had already installed a gec from XO in the transformer to the gec then to the building steel. I haven't bonded the egc and grounded conductor in the disconnect since I did that in the transformer. I'm not sure if that's typical, or harmful, but now I have no neutral to ground voltage. I'm surprised the oversight by the original installer went unnoticed because the panel in question supplies branch circuits to CNC machines. 55 volts on the neutral can't be good. Thanks again!
 
domelectric said:
Thanks again for the input and code reference. I had already installed a gec from XO in the transformer to the gec then to the building steel. I haven't bonded the egc and grounded conductor in the disconnect since I did that in the transformer. I'm not sure if that's typical, or harmful, but now I have no neutral to ground voltage. I'm surprised the oversight by the original installer went unnoticed because the panel in question supplies branch circuits to CNC machines. 55 volts on the neutral can't be good. Thanks again!
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You are permitted to install the system bonding jumper either in the transformer or the first disconnecting means but make sure you don't install one in each location.

Chris
 
MNake sure the secondary's GEC is sized for the secondary, which will lilely be larger than the primary's EGC.

augie47 said:
any breakers trip after the "correction" ?
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domelectric said:
No breakers tripping. Should I be concerned?
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Only if there was a line-to-ground fault that the missing MBJ allowed to remain undetected. He was being funny. ;)
 
domelectric said:
No breakers tripping. Should I be concerned?
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Your eyebrows should be raised a little.

You measured 55 volts when the thing was in the illicit state, and it shocked you, so there is at least a little current leaking from somewhere. The question is how much?

I'd want to get a clamp around the neutral / ground bond to see, and if its amps, I'd want to know where its coming from.

The reason I'd be concerned is that when there was no bond, there was very little leakage current. Now that the leak is running into a short, that leak may be insignificant, or may be a sign of trouble to come.
 
I wouldn't be surprised to get a shock from a floating system. Capacitance and/or inductance can take it almost anywhere.
 
LarryFine said:
I wouldn't be surprised to get a shock from a floating system. Capacitance and/or inductance can take it almost anywhere.
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Plus, being in a ten by six room with one MDP, one 112.5 KVA xfrmer, a 200A disconnect, and a main panel (and FL relative humidity this afternoon was 98%) , sweating, I was a great conductor. Not fun being a path to ground...
 
dbuckley said:
Your eyebrows should be raised a little.

You measured 55 volts when the thing was in the illicit state, and it shocked you, so there is at least a little current leaking from somewhere. The question is how much?

I'd want to get a clamp around the neutral / ground bond to see, and if its amps, I'd want to know where its coming from.

The reason I'd be concerned is that when there was no bond, there was very little leakage current. Now that the leak is running into a short, that leak may be insignificant, or may be a sign of trouble to come.
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I will clamp the bond. Good point.
 
Just a point to add:
Your OP didn't say if the transformer is a "Y" primary.
If it is, and if you bond the primary X/O you can have problems with transformer over heating and the primary EGC getting hot, never bond a Y primary X/O only the secondary X/O.
 
hurk27 said:
Just a point to add:
Your OP didn't say if the transformer is a "Y" primary.
If it is, and if you bond the primary X/O you can have problems with transformer over heating and the primary EGC getting hot, never bond a Y primary X/O only the secondary X/O.
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Nah, it's delta/delta. Secondary has a high leg. Primary three wire 480
 
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