5V N to I.G.

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barclayd

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Colorado
Tech was "shocked" (heavy tingle) plugging in a computer to a power strip that was plugged in to an I.G. Receptacle in some systems furniture (cubicles). Preliminary investigation showed 5 volts between the neutral and isolated ground in that receptacle, and all the other I.G. receptacles in that cluster of cubicles. Similar cubicles nearby showed 0. Panel is about 30 feet away.
Electrical troubleshooter is due today or Monday.
Any quick, easy answers? Do not really know the qualifications of the electrical guy that's coming.
I think the I.G. is a dedicated circuit with several (4?) receptacles, but the 'regular' circuits in the cubicles may share a neutral. I believe they are Steecase cubicles.
(by the way - I was previously unable to convince others that the isolated ground system was unnecessary)
I'll keep you posted
thanks
db
 
You always have some voltage drop between neutral and ground. There is current flowing in the neutral so there is going to be a voltage drop across the neutral.

That 5V is not your problem.
 
I think the circuit is #10's, but even with 12's, the current would have to be over 40 amps to get anywhere near 5v drop. I know you always have some, but this is higher than normal for the panel only being 30 feet away.
Somebody suggested that 'something is plugged in' that is causing it. We'll see - maybe it's just a bad (low-bid chinese) power strip.
thanks
db
 
Five volts is not high enough to drive enough current through a person's body to give a "heave tingle." The problem lies elsewhere. I would ask the tech to reenact the event (without actually plugging anything in). I would want to know exactly what each of his (or her) hands were touching at the moment of the shock, whether an arm was also touching the computer case or some other part of the cubicle, and exactly where the shock was felt (i.e., in the hands, the arms, only one hand or arm, etc.). That would help pin down the source of the shock.
 
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barclayd said:
I think the circuit is #10's, but even with 12's, the current would have to be over 40 amps to get anywhere near 5v drop. I know you always have some, but this is higher than normal for the panel only being 30 feet away.
Somebody suggested that 'something is plugged in' that is causing it. We'll see - maybe it's just a bad (low-bid chinese) power strip.
thanks
db
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The IG is not typically run back to the local panel board. It would be pointless to do so. It usually goes back to the service point. You may be looking at the voltage drop to a neutral connection that is far, far away.
 
What voltage did you get between the N and regular equipment ground? Regular ground and the isolated ground? One possibility the isolated ground is truly isolated and insulated from the actual service ground. (WRONG) This is a very common mistake made when isolated grounds are installed. It is possible the equipment the tech was plugging in had a fault to ground, causing the shock, since the isolated ground was not actually grounded. Quite a few possibilities.
 
hillbilly1 said:
What voltage did you get between the N and regular equipment ground? Regular ground and the isolated ground? One possibility the isolated ground is truly isolated and insulated from the actual service ground. (WRONG) This is a very common mistake made when isolated grounds are installed. It is possible the equipment the tech was plugging in had a fault to ground, causing the shock, since the isolated ground was not actually grounded. Quite a few possibilities.
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I don't think they checked anything other than N-IG.
He was sitting on the carpet under a work service. He pushed down on the plug with the palm of his hand. That's when he felt the tingle. There were other plugs already in the power strip. hoping it's a be power strip.
db
 
Is the isolated ground connected as required by the NEC? Many are truly isolated and a safety and code violation.
 
barclayd said:
That's one thing they need to check. There is a dedicated transformer & Panelboard for the IG system. (I think you mentioned "pointless").
db
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I was the one that said "pointless", but it would appear that given this is an SDS that running the IG where it went is correct.

I remain unconcerned about the 5V difference between the IG and N. How was this measured BTW? With a digital meter? You can get all kind of flakey readings with high impedance volt meters. Get yourself a 10k or so resistor and run it between IG and N and see what the voltage drop is.

I think it is far more likely that you have some kind of wiring or equipment problem.

The thing is that if the guy could feel it, it was probably enough current to trip a GFCI. Maybe install a GFCI on this circuit until such time as you can figure out what is going on.
 
petersonra said:
...I remain unconcerned about the 5V difference between the IG and N. How was this measured BTW? With a digital meter? You can get all kind of flakey readings with high impedance volt meters. Get yourself a 10k or so resistor and run it between IG and N and see what the voltage drop is. ...
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Assuming everything is correctly connected, I see it as an indication of excessive voltage drop, either from poor connections or excessive load. I don't really see a problem using a high impedance meter for this measurement. Most of the "flakey" readings are when one or both of the wires you connect your meter to are not connected to anything.
 
don_resqcapt19 said:
Assuming everything is correctly connected, I see it as an indication of excessive voltage drop, either from poor connections or excessive load. I don't really see a problem using a high impedance meter for this measurement. Most of the "flakey" readings are when one or both of the wires you connect your meter to are not connected to anything.
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Most of the time. But make sure what you have. I think he said they ran #10. I don't see how you can get 5 Volts drop on a 30' length of #10 wire for a 20A circuit.

He might also want to see what the resistance between the neutral and IG is.
 
barclayd said:
The isolated ground system was developed at a transformer 30 feet away from this cubicle.
db
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My concern is if the transformer secondary is properly connected to grounding electrode system. Many people have a misconception of "isolated grounding" and sometimes think it means to connect the grounding conductor to a ground rod, or otherwise isolate it from the rest of the equipment grounding system. This will leave a high impedance between the 'islolated' conductor and other grounded conductors. It still must connect to the grounded conductor at the source or service but can bypass connection to other grounding conductors in panels and other enclosures on its way to the point where it does connect to a grounded(ing) conductor.

Do you have voltage between this 'islolated' grounding conductor and other grounded surfaces? If so the 'isolated' grounding conductor is probably not bonded at any point to the grounded conductor at the source.
 
Both the transformer gec, and the isolated ground conductor run up the wall together to building steel. I do not know if they are both connected to xo. There are separate ground buses in the panelboard. Hope to find out something today.
thanks
db
 
barclayd said:
I don't think they checked anything other than N-IG.
He pushed down on the plug with the palm of his hand.
Click to expand...
Could it have been a pinched nerve in the palm of his hand, instead of a shock?

I remember once thinking I was being shocked by the seat belt latch in my car.
 
barclayd said:
Both the transformer gec, and the isolated ground conductor run up the wall together to building steel. I do not know if they are both connected to xo. There are separate ground buses in the panelboard. Hope to find out something today.
thanks
db
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Shouldn't the IG go to the PB ground bus and not the steel?

Are you saying you have a ground bus and a seperate N bus in the panelboard?
 
There is a ground bus, and an isolated ground bus in the panelboard. Do not know if they are bonded together in the panelboard. Do not know if they are both attached to XO. Both conductors run to building steel in the electrical room.
There is also a neutral bus in the panelboard. Do not know where it is bonded to ground.
So many questions
db
 
barclayd said:
There is a ground bus, and an isolated ground bus in the panelboard. Do not know if they are bonded together in the panelboard. Do not know if they are both attached to XO. Both conductors run to building steel in the electrical room.
There is also a neutral bus in the panelboard. Do not know where it is bonded to ground.
So many questions
db
Click to expand...

I might start by doing some investigating as to what exactly you have and sketch it up so you can figure out just what might be going on. It is hard to even know where to start.
 
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