# Ground - Neutral voltage less than 0.003v ???



## pranana (Mar 1, 2018)

Got a manufacturer of a dental x-ray machine that says that the G-N voltage on the dedicated circuit needs to be close to 0.003v. The G-N readings at the plug are between 0.02v and 0.15v depending on what they have on in the office at different times of day. 

The circuit is dedicated, with an isolated ground and is fed by a sub panel with a floated ground, but sufficient feeds back to the service entrance where the G-N are bound. 

From what I can find, it seems impossible to get down that low due to the impedance of the 30-40ft run. 

They already have an isolation transformer hooked up to the machine but they are saying they need 0.003v G-N at the plug.

I think their machine failed and they are trying to blame it on the power rather than a faulty unit. But before I tell them that, anybody else think that 0.15v is something that can be improved upon?


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## MadSparky (Mar 2, 2018)

If the machine is hooked up to an isolation Transformer then the machine no longer has a neutral or a grounded conductor. That's the whole point of an isolation Transformer

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## brian john (Mar 11, 2007)

electrokinetix said:


> If the machine is hooked up to an i*solation Transformer* then the machine no longer has a neutral or a grounded conductor. That's the whole point of an isolation Transformer
> 
> Sent from my A574BL using Tapatalk


Depends, as all transformers with the exception of Buck-Boost offer isolation. If it is a true isolated neutral 60 VAC to ground 120 VAC between L1 and L2 that is different.

With a dedicated IG the voltage with no load would be close to or "0" volts. With load, there will be a difference of potential.

How are you measuring this with load or without load?

Where is the transformer located in regards to the machine?


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## frenchelectrican (Mar 15, 2007)

Ya did not tell us what size conductor ya ran it out to the Xray machine.

and what type of voltmeter you used to read it?


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## brian john (Mar 11, 2007)

And unless you place the transformer at the x-ray machine you are going to have N-GND voltage.

But I think the levels they are quoting are based on engineer BS.


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## pranana (Mar 1, 2018)

French -I didn't run the circuit, was put in a few years ago with their old machine. 10awg though I think. Measured about the same on an old fluke and another digital multimeter.

Brian - how close to 0 do you think you can get? I am getting 0.1 and 0.02. It is an isolated ground to the panel, but the ground is floated back to the service and I read 0.05 G-N on the subpanel. So there still has to be some potential on the G-N just due to the resistance of the wire and the current of other loads on the panel right? So how close to zero can you realistically maintain? I just haven't looked for such small potentials on G-N before.

The isolation transformer is a plug-in powervar conditioner but they want the G-N voltage at that low level without that plugged in.

electrokinetix - that is what I understand, but the manufacturer keeps saying that the G-N potential is still causing problems. I think they are trying to pass the buck, but not completely sure yet.


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## pranana (Mar 1, 2018)

brian john said:


> And unless you place the transformer at the x-ray machine you are going to have N-GND voltage.
> 
> But I think the levels they are quoting are based on engineer BS.



Thus is the problem... and they won't get the engineer that is spouting it on the phone.


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## macmikeman (Jan 23, 2007)

pranana said:


> French -I didn't run the circuit, was put in a few years ago with their old machine. 10awg though I think. Measured about the same on an old fluke and another digital multimeter.
> 
> Brian - how close to 0 do you think you can get? I am getting 0.1 and 0.02. It is an isolated ground to the panel, but the ground is floated back to the service and I read 0.05 G-N on the subpanel. So there still has to be some potential on the G-N just due to the resistance of the wire and the current of other loads on the panel right? So how close to zero can you realistically maintain? I just haven't looked for such small potentials on G-N before.
> 
> ...


Remove the grounding / bonding and try the thing out again to see if the manufacturer's are full of chit or not......


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## pranana (Mar 1, 2018)

macmikeman said:


> Remove the grounding / bonding and try the thing out again to see if the manufacturer's are full of chit or not......


Here is the problem. The brand new x-ray machine is toast and they won't install a replacement until this electrical "problem" is resolved.


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## RePhase277 (Feb 5, 2008)

Digital meters will always read some ghost voltage unless there is some kind of load resistance in the circuit. Get a 100 kOhm or 1 megohm resistor and put it between ground and neutral and measure again.

3 millivolts has never toasted anything, ever.


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## MadSparky (Mar 2, 2018)

I suppose you could pull an oversized neutral to the outlet box. That would at least reduce the resistance of the conductor run. I'm still confused about the isolation Transformer. Are they talking about neutral to ground voltage on the primary or secondary side?

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## brian john (Mar 11, 2007)

I would set a transformer right at the equipment or a true double conversion UPS.

http://www.newark.com/tripp-lite/is...m8LEX0w1Xrdgb5SKcYL-t4QkwQZxS7ZhoCdFYQAvD_BwE


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## paulengr (Oct 8, 2017)

With an isolation transformer you connect the neutral to ground at the transformer and it’s a couple feet from the machine. Neutral-ground voltage is a poor way to write a spec but the issue is usually ground resistance/impedance which causes all kinds of common mode issues. X-rays are above microwave frequencies do grounding is crucial. You really need a ground conductor that is at least as good (no undersizing) as the phase conductors, short run, etc. Basically the manufacturer cheaped out by not providing their own isolation transformer. Or maybe based on the goofy spec they’re lab/design/instrument people so they have no idea how power distribution works. The goofy spec and the source tell you that.

An isolation transformer as with any transformer magnetically couples power so DC is blocked. As an inductor it blocks high frequencies too. A special transformer built for isolation can also use phase shifting to cancel harmonics. A top of the line brand is islatrol. Many manufacturers also buy islatrols and incorporate them in their equipment. For instance SOLA and ITI sell surge arresters with an islatrol as a packaged clean power unit.


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## joebanana (Dec 21, 2010)

If their machine "requires" .003v. N-G to operate properly, their machine/design is F-ed up.


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## MadSparky (Mar 2, 2018)

paulengr said:


> With an isolation transformer you connect the neutral to ground at the transformer and it’s a couple feet from the machine. Neutral-ground voltage is a poor way to write a spec but the issue is usually ground resistance/impedance which causes all kinds of common mode issues. X-rays are above microwave frequencies do grounding is crucial. You really need a ground conductor that is at least as good (no undersizing) as the phase conductors, short run, etc. Basically the manufacturer cheaped out by not providing their own isolation transformer. Or maybe based on the goofy spec they’re lab/design/instrument people so they have no idea how power distribution works. The goofy spec and the source tell you that.
> 
> An isolation transformer as with any transformer magnetically couples power so DC is blocked. As an inductor it blocks high frequencies too. A special transformer built for isolation can also use phase shifting to cancel harmonics. A top of the line brand is islatrol. Many manufacturers also buy islatrols and incorporate them in their equipment. For instance SOLA and ITI sell surge arresters with an islatrol as a packaged clean power unit.
> 
> ...


What he said

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## brian john (Mar 11, 2007)

RePhase277 said:


> Digital meters will always read some ghost voltage unless there is some kind of load resistance in the circuit. Get a 100 kOhm or 1 megohm resistor and put it between ground and neutral and measure again.
> 
> 3 millivolts has never toasted anything, ever.


Not only that but I would bet that falls outside the meter accuracy


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