What is your N-G voltage?

[mcduman]

Was catching up with grounding threads (takes a while) and remembered that some tech people told me at the time I had a high neutral-to-ground voltage differential in the office which could potentially harm the servers they were installing.

it dawned on me to measure it at home last night (takes 2 secs with a multimeter) and read 0.3V. It appears the electrical code allows for a 5% (or 6V) drop in the US. I was curious to know whether the N-G differential was a factor on the efficacy of some of the grounding equipment. Anyways, did any one measure theirs and what was it?

 

[Speedskater]

It's not about the grounding equipment, it's about the Neutral conductor and the current flow.
That is, assuming that the Safety Ground/Protective Earth is correctly connected to the Neutral at the service entrance/main breaker box. (this is not always true)
Quick example:
100 feet of Neutral 12AWG wire has an end-to-end resistance of 0.16 Ohm.
Voltage drop equals current times resistance.
20 Amps times 0.16 Ohms equals a 3.2 Volt drop.
There is no drop on the ground conductor.

Note that the stake in the garden has absolutely nothing to do with any of this.

 

[mcduman]

i fail to see why you do not have the same 3.2 V drop on the ground conductor in this picture, you have the same 100 feet ground wire. Should not N-G, theoretically at least, measure 0?


And why do you have 20A in your neutral?

 

[amirm]

Before I comment, let me note that sticking meter probes in mains supply can be highly dangerous. We are talking about voltages that can shock and potentially kill you. Also, many knock-off multi-meters lie about their safety ratings and should a surge arrive while you are doing this measurement, it can lead to the meter blowing up or worse. So do this at your own risk.

On measurements, in my listening room right now without the equipment be on, the differential is 0.2 volts between ground and neutral. Powering up just one of my monoblocks, brought that up to .6 volts.

That should answer your last question. The drop is specific to how much current you draw on the neutral line as Kevin explained. The ground wire in properly wired systems, should not have any load on it so will not experience that drop. Yes they are both connected at the meter but they are not at the equipment end where you are measuring them.

The causes of excessive drop could be undersized wires, poor terminal connections, meter probes not making secure connection, etc.

The proper way to measure this is with a device that draws that maximum current and then measure the drop. This is what I use which also mitigates the safety issues of using a multimeter:

Unfortunately at $300 is not cheap.

 

[mcduman]

amirm, did you also measure the potential difference between your equipment (ie between your pre and power amps or dac and pre)? ignoring the R of the interconnects they should also come out at c. 0.2 and 0.6V, is that right?

 

[amirm]

No, I measured the power at the socket. Explain again how you want me to measure that and I will do. I connect the one lead at the chassis and the other one where?

 

[Speedskater]

Voltage drop is caused by current flow through a conductor. Under normal conditions the Safety Ground/Protective Earth should not have any current flow thus no voltage drop. If it's an old building with an all metal water pipe system the SG/PE to water pipe voltage should be about zero.

Also note that in older city neighborhoods that still have all metal water pipe systems (but these are rapidly disappearing) it's normal to have some Neutral currents flowing through the house to house water pipes.

So some water pipes can be involved in day-to-day power as opposed to the stake in the garden which has nothing to do with normal operation.

 

[Speedskater]

It's not uncommon to have Neutral / Safety Ground swaps in a US type system. (it's wrong but not uncommon) So if there is a voltage drop on a Safety Ground then there is an error.

This from an Audio Precision note:
[h=1]Notes from the Test Bench[/h] By Bruce Hofer, Chairman & Co-Founder, Audio Precision


Recently, AP celebrated the 15th anniversary of our custom designed building. Photo albums were displayed showing the construction and the move into our facility back in 1998. (My, how some people have changed in appearance!) Among my many memories of that time, I was reminded of a particularly nasty problem we experienced as we restarted operations in our new production environment.
Almost immediately after moving we began to experience failures of certain bench tests that are performed by our technicians during the course of product assembly and adjustment. After some research, we discovered our new building had some extremely large magnetic fields in the production area, almost as if it was haunted. These fields coupled high levels of hum into our products that were causing the test failures. AP products are designed to reject reasonable levels of stray magnetic fields that would typically be encountered in a lab or production environment. However the magnitude of the fields we faced were at least 20 dB worse.
We ultimately discovered that several of our AC outlets had been wired incorrectly, having their neutral and ground connections swapped. This is a big no-no from the safety viewpoint, but it also caused all of the neutral currents in a particular circuit (outside of our production area) to return through the safety ground connection and ultimately through plumbing and drainage pipes. Some of these plumbing pipes were located in the space directly above our production area while the main drainage pipe was buried directly below; thus our production area was effectively inside of a huge coil. Our electrical contractor was embarrassed but confirmed our diagnosis, and the problem was quickly fixed.
Sometimes one has to think “outside of the box” to correctly perceive or understand a given problem. In this case, our new building plans provided the necessary insight to recognize the inadvertent source of our unwanted magnetic fields (plumbing that formed a coil around our production area). Perhaps you might want to check the neutral and safety ground connections of the AC outlets in your own work space—there could be some ghosts present!
Enjoy this month’s edition of our newsletter…
Bruce

 

[mcduman]

No, I measured the power at the socket. Explain again how you want me to measure that and I will do. I connect the one lead at the chassis and the other one where?

I measured my n-g voltage which appeared to be the same as the voltage between the chassis of my pre and dac. I wonder if you can replicate this

 

[amirm]

I measured with my music playing and everything on. The differential *at the outlet* is about 1.2 volts now.

Measuring chassis to chassis between the amp and Pre/DAC, there is about .003 volts or essentially nothing as it should be.

What type of mains cable do you have on your audio gear? Any of them lacking a ground connection (i.e. double insulated)? I would definitely investigate what is going on there.

 

[Speedskater]

In a double insulated component (appliance), that's one with the box in a box symbol, the chassis is not connected to ground. Connecting a Safety Ground to the chassis would void the double insulated rating.

 

[mcduman]

I measured with my music playing and everything on. The differential *at the outlet* is about 1.2 volts now.

Measuring chassis to chassis between the amp and Pre/DAC, there is about .003 volts or essentially nothing as it should be.

What type of mains cable do you have on your audio gear? Any of them lacking a ground connection (i.e. double insulated)? I would definitely investigate what is going on there.

Chassis to chassis I am 0.3mV, so that should be OK. I am still bothered by the existence of any difference between neutral and ground though.

 

[Speedskater]

I am still bothered by the existence of any difference between neutral and ground though.

If current is flowing through the Neutral then there will always be a difference.
It's just Ohms Law.
If you are pulling a fair amount of current and there is no voltage difference at the outlets between Neutral and the Safety Ground then you have a wiring error that needs to be corrected.

 

[amirm]

Chassis to chassis I am 0.3mV, so that should be OK. I am still bothered by the existence of any difference between neutral and ground though.

As Kevin says, that differential is normal and load specific. That said, having heavier gauge wiring reduces that as would proper connections to the outlet and such.

 

[mcduman]

If current is flowing through the Neutral then there will always be a difference.
It's just Ohms Law.
If you are pulling a fair amount of current and there is no voltage difference at the outlets between Neutral and the Safety Ground then you have a wiring error that needs to be corrected.

Thanks.

This article says there is a regulatory cap of 3V, plus some specific installations require 0.5 V or 1V. it looks as if it is something that should be minimized. balanced power? multi or mono phase? isolation transformer? how? and much N-G do forum members measure?

http://ecmweb.com/power-quality/clearing-neutral-ground-voltage-confusion

 

[Speedskater]

It's an Ohms Law thing. Each persons reading will depend on the end-to-end resistance of the Neutral conductor and the amount of current being used at the moment of the measurement. With a Class AB amp playing music, the reading will jump all over the place.
E=IxR (simplified)

A large permanent transformer (3000 to 5000 Watts) installed near your listening room and wired as a separately derived system, will lower the reading a lot. It will also lower the component to component Safety Ground resistance which is great. It has the important advantages of a balanced power system without the high cost and complications.

 

[microstrip]

An excessive voltage between mains neutral and ground can also be due to a broken/faulty ground and/or an apparatus leaking current through the ground connection. A friend of mine has an AC voltmeter monitoring it permanently - as soon as it starts increasing he knows it is time to arouse his flower bed!

 

[mcduman]

It will also lower the component to component Safety Ground resistance which is great. .

kevin,

is this what entreq does?

 

[Speedskater]

kevin,

is this what entreq does?

Not even close!

 

[mcduman]

why or why not?