customer is having problems with their photocopier, copier service guy comes out with some sort of fancy tester that records voltage from neutral to ground he says they have 6v n to g 2000 times in 4 hrs and this is causing the photocopier to break down repeatedly, i check the conduit from the panel its a 1 inch with 4 ccts and 2 neutrals in it 3 phase the service in the bldg is relativly new 1200 amps bldg is full of servers etc and no problems a 2 wire bx is connected to one of the 4 ccts and on its own neutral this bx feeds the copier. the copy guy says this 6v problem must be resolved in oreder for the copier to work.I think the copier is a piece. can anyone offer advice on how to resolve this problem?

trevman

6 volts can be a pretty big difference and could cause trouble on any computer controlled device. Check that the secondary of a local distribution transformer is bonded and grounded. If there are no distribution transformers the problem might exist right at the main service. It is likely there is a poor bonding or neutral connection somewhere in the system. You should see less than a volt or 2 difference between the neutral and ground but always more than 0 since that would indicate a bootleg ground or other accidental connection between groung and neutral.

You are always going to see something from neutral to ground considering that the neutral is current carrying and the ground is not. Any voltage drop on the neutral is going to be reflected as a difference to ground. This gets even more pronounced with sub panel feeders and heavy line to neutral loads. I wouldn't doubt that when the heating element in a laser printer (or copier) cycles it causes a 6 volt spike. I also doubt that it is having an effect on the copier. Matter of fact I wonder if it is the copier itself that is causing the spike.

What I would do is run a dedicated back to the service panel. Don't take it from a sub if there are any.

-Hal

In my opinion, I would try a different copier, (or perhaps a different service guy) before starting rewiring the building. Any installation with lots of computers, etc, is going to have high neutral currents, and hence measurable neutral voltage, particularly due to third harmonics from switching power supplies, but its hard to see why that should cause any equipment to actually fail. If it really is so susceptible an isolation transformer might be more economic than special wiring.

Iwas also thinking of installing a iso xfrmr right at the receptacle, but i am unsure if a iso xfrmr will lower the n to g voltage. sharp photocopiers say any thing more than 1v n to g voids warranty this is almost impossible to have less than 1v. does anyone know if the iso xfrmr will work. i bet its a coffeee pot or something thats causing the up to 6v spikes

Take the building ground straight through to the copy mc. (as well as to the transformer case/frame of course). From the transformer secondary you have the choice of leaving it floating or grounding the neutral end. There will definitely be no neutral/ground volts in the latter case!

I take it this is a 120v copier. This is precisely why we never brought a neutral into a computer room. Everything was L/L loads or on an isolation transformer.
I am still kind of surprised they are using the neutral for anything but one side of a switcher power supply that effectively does isolate it.
I bet fixing this problem does not fix the copier.

Thank you, gentlemen. I learn constantly from these forums. Now I understand why all the new copiers in a State of Maine office building (loaded with servers and networked PC's) have iso xfmrs.

"I learned something today," as the fella at Big Orange said when I had to explain his own stock to him.

If this is 120 volt circuit and you have 6 volts between the neutral and the ground, you have a circuit with excessive voltage drop. The voltage read between the neutral and the ground is nothing more than the voltage drop on the neutral between the main (or system) bonding jumper and the point of measurement. There will be additional voltage drop on the ungrounded conductor. It is very possible that the voltage drop on this circuit may exceed 10%. Note it also could be less if the excessive voltage drop on the neutral is caused by a poor connection and not just by the load.
Don

retired,what do those iso xfrms look like?
do they look like a big door bell type one, could you put it in a 6x6 box?

If he's dropping 6V on the neutral, he's also dropping 6V on the hot, which is a 10% voltage dip, and I'd think THAT would be causing the problems, and not 6V on the neutral.

My first reaction was inrush current on something else nearby, but 2000 times in 4 hours is a bit much for in-rush.

Lemme guess- circuit is overloaded just shy of tripping the breaker, and consists of 150' of #12?

Which of the 4 circuts is the copier on? There are two variations of the 4 circut systems one is a 3+1 where only one circuit is on the second neutral and a 2+2 where two circuits are shared by the neutral in the later case this has led to problems when a regular sized neutral is used for the second feed.

Trevman: I'm having to reach back into memory now, but to the best of my recollection the xfmr was in what looked like an OEM box, as if it were supplied as part of the copier. I thought at first it was a surge suppressor, but it didn't look quite right for that. It was completely enclosed, and resembled the xfmr's you see on PC printers, but it was, I believe, heavier. I wish I'd thought to pay more attention. The copier was an unfamiliar brand, and many people remarked on the unusual, even bizarre, logic programmed into things like paper size selection. It all struck me as not only foreign-made (what isn't these days), but hastily adapted for the USA. I wish I could be more helpful.

6 volts does sound a rather excessive voltage drop on the neutral of a 120V circuit, certainly if it implies a 12V/10% drop overall.

I'd add my vote to the opinion that it's more likely the overall low voltage causing the problem than an actual 6V difference between neutral and ground.

Maybe that iso-xfmr will also need to boost the voltage a little?

Steve,

Only if the cause of the voltage drop is excessive current on the circuit. You can have a high voltage drop on the neutral without one on the hot in cases where there are poor connections on the neutral.
Don