Ground to Neutral Voltage

[jg3317]

I have a situation that I have come across at with a commercial customer that we do electrical work for. It deals with the Ground to Neutral voltage. This is the situation I have.

3rd floor of a production facility in a office area. office personel complain of a slow computer system, they called the IT department and the IT specialist comes down and the first thing he does is plug in a power diagnostic tester that basically has about 15 light on it and it test for high and low voltage, properly grounded, properly wired and then the one that always causes us problems is the ground to neutral voltage. His test instrument has 3 lights for this catagory <.5v gives OK light, <1v gives yellow warning and >1v red danger light. We always get called in when it is a yellow or red light situtation basically ground to neutral voltage >.5v

When I first went into the room I had 2.3 volts from ground to neutral. As the person was clearing out of the office for me to work I retested voltage after she left and I had .3 volts ground to neutral. After some searching and testing I discovered that the coffee pot would cause my G-N voltage to go from .3 to 1.3volts. She also had a floor radiant heater when turned on caused a G-N voltage of 2.5 volts.

We have been running individual circuits from each recepticle back to the power panel to eliminate this problem. Now we are running into the problem of them plugging the coffee pot into the same yoke as the computer which caused the same problem.

Am I missing something or is this typcial for neutral impedence on the neutral. Is this common and it is only noticed when dealing with electronic equipment?

Jim Gavin

 

[scott thompson]

Re: Ground to Neutral Voltage

This is normal.

What you are seeing is the results of the Phenomina known as:

Voltage Drop!

Ask the IT Guru(s) why the N-G Voltage is there, and you will hear many fabulous stories!


Ask the same Guru(s) what the hazard is to have an N-G Voltage of 2-5 VAC, and get even more fantastic stories!


My 2?
Scott

 

[dereckbc]

Re: Ground to Neutral Voltage

As Scott stated it is normal I*R voltage drop along the length of the neutral conductor. The voltage varies with the length of the circuit from the N-G bond and the amount of current. So in essence the longer and/or the more load the larger the voltage drop.

IEEE Emerald book, the bible on power quality, recommends no more than 5% voltage drop on any circuit that includes feeders and branch circuits together. Typical designs allow for up to 2% on feeders and up to 3% on branch circuits. 5% of 120 VAC is 6-volts between L-N, or 3-volts N-G.

Please ask your IT nerds why the N-G voltage needs to be so low and post back their answer. I would like to hear it myself. I only know of one reason why, and I bet they do not have a clue.

Hint: It has to do with RFI filters and MOV's installed from N-G in the equipment and injecting noise back into the EGC circuit. It used to be a problem when RS-232 and coax protocols were used to network computers that used ground as a signal path. However the problem is eliminated with ethernet and optical protocols.

 

[petersonra]

Re: Ground to Neutral Voltage

I think the IT guy does not have a clue. It's not unusual to have a few volts potential between neutral and ground for the reasons another poster mentioned.

The idea that this would cause a computer to run slow is just ludicrous.

[ May 19, 2005, 04:34 PM: Message edited by: petersonra ]

 

[jg3317]

Re: Ground to Neutral Voltage

I wrote the question late in the evening and it was the user of the computer in the office that was complainting of a slow running computer, but the first test the IT person performs is the power diagnositic that lights the led lights when N-G voltage gets high.
The IT person stated that the computer is running slow because of multiple applications, spyware and other computer related problems. He said that he has had problems in the past when the PC will shutdown,(powerdown)unexpectedly and when he goes to check it out he has a high N-G voltage on the recepticle.
I have order 500w isolation transformer to install in the office that we have a voltage of 2.5v N-G on the recepticle that the computer is plugged into, I believe this will satisfy the IT guru, I hope.
We have in the past satisfied the IT peron by running individual home runs to every recepticle that a computer is plugged into, but they move the office around and now you have to do the same thing again because the plugged into a different recepticle and the N-G voltage is high again.
What would be the acceptable voltage N-G on a normal desktop computer wired into a network?

One place that we always have a problem and that is if they install a new zerox machine, and if there happens to be any computer on that circuit it has a fit, high N-G voltage, it seems to be when the zerox machine is warming up that causes the computer to "powerdown". We have just wired home runs to every zerox machine they install.

Thanks for the responses it has been a great help
Jim

 

[karl riley]

Re: Ground to Neutral Voltage

Possible scenario: computer crashes. Operator is stressed, plugs in coffee maker. While coffee is brewing, checks N/G voltage.

Karl

 

[charlie]

Re: Ground to Neutral Voltage

It is interesting to me that most of the computer problems are located between the keyboard and the back of the chair.