2 Volts Between Ground and Neutral

If you have current flow on the neutral you will have a voltage difference. Check and see if you do have current on the neutral. If so try to re-balance the load so the neutral current is at a minimum.

Thanks, i will check, but i expect a neutral current due to the fact that i now the 230VLine to neutral loads are not balanced.

What is the resistance between the ground (where you're measuring the 2V difference) and a "known good" ground? (should be < 0.04Ω for 50' of 8 AWG - a PAT tester or Megger should measure this). How about neutral to ground?

DC resistance is a good 1^st check. If you've really got 2V to ground (with 40mΩ) that's 50 amps going somewhere.

Excellent advice, except for the Megger. In the usual use of the term, Meggers are used to measure very larger resistances, greater than one meg-ohm, hence "megger". Megger, the company does manufacture instruments for measuring very low resistances, general a ground resistance tester can be employed to do this.

As you point out, it appears that there is 50A of unbalance,, that seems a lot.

You have joined together grounding trafo and building, right ?. Im not so expert in here but according to some paper i read, grounding power suplly eg trafo, turbine, genset should separate with grounding building or equipment.

I would start by confirming the star point of the transformer is solidly grounded. If it is floating or high impedance grounded then the 2 volts is nothing. That level is quite common in power systems when the neutral is not grounded.

If it is solidly grounded and the neutral and ground are common at that point, then you should measure zero potential difference at the ground point.

Now proceed to the point of utilization. Is the neutral and ground still at the same potential?

Has the ground been interrupted in a junction box? Has it shifted to building ground or is it just a piece of conduit?

Are all three phases and ground wire run in the same conduit or cable?

Can you run a separate feed from the transformer distribution to the machine?

I have seen long runs of high amperage conductors (approx 1000 amps) run in parallel without transposition create sheath currents in the 100's of amps--- it melted the aluminum jacket right off!)

A second point, I believe your phases are relatively well balanced. I would find it hard to believe that the imbalance would pose a problem to your equipment unless there were large (10 to 100 times your offset) noise spikes coming through.

It sounds like there are equipment failures and if it can be blamed on the supply there is no warranty.

Hi Mesi, not sure if what I am saying is right, but you did state that this transformer was an isolation transformer, correct me if I am wrong but isn't an isolation transformer a transformer that works with no earth return? That is why they are called an isolation transformer isn't it.

Usually an isolation transformer was used in the good old days before RC D's to protect persons from electric shock. It was used to eliminate the voltage potential between phase and earth.

I would suspect that the voltage between neutral and earth is just an impedance, because you are using an isolation transformer, and then you have installed and earth to the machine, but where does this earth connect to, is it the main earth or is it the isolated earth on the isolation transformer.

Is the main 600v Earth and the secondary 400/230v earth bonded, if so it is no longer an isolation transformer. If they are not bonded then you should check where you have you earth wiring connecting to, it should not be connected to the main earth.

It should be connected to the neutral of the 400/230v secondary side of the transformer and not bonded with any other equipment.

As I said at the start of this post, don't quote me on this, but I think what I am saying is correct.

The other alternative is if you do not need an isolation transformer is to bond the Neutral And Earth to the Main Earth, but that is another issue that needs to be thought through.

Best Regards

Joe

Mr. Joe,

May I say you are partially correct about isolation transformer?. You are right about that here is no longer "hot or neutral after the isolation transformer, but ground is and should still be available to take care of any noise in the line and other safety related issues. [p]

Although isolated, there still exist currents flowing that is referenced to the ground, this what is known as 'leakage current' of any electrical devices. Depending on how good the copper insulations and type of windings used during the manufacturing of the isolation transformer, will determine how much this leakage current will be. On a normal 115 /230 volt service system, the magnitude of such leakage current normally will be in the range of micro-Amperes to several milliamperes, maybe miniscule, is still considered as shock hazards.[p]

VS

Mr. Joe,

I forgot to include the real culprit behind about all this leakage current are the capacitances existing between windings or capacitance that exists between any two current carrying electrical conductors.

Two issues have to be certify here;

1)Check the Load current on each of the Line because a flowing current at Neutral means unbalance load.

2)Also check the earthing of the neutral at the transformer & other relevant places;My Computer was always damaging until one day i realize they was floating current from my stabilizer which was later traced to my generating-set earthing system,then it was rectified

<...The equipment manufacturer is suggesting that the voltage between neutral and ground is 'severe' and may be causing electronic issues....>

What "issues", please?

In case I missed it, is the measured voltage AC or DC? If there is DC with AC ripple it could be a cathodic protection system impressing a voltage on the earthing system. If it is DC with no ripple it could be an electrolytic reaction between the earthing system and ground.

If it is AC the issue could be from a neighbouring transformer that has a poor (possibly burnt off) neutral connection.

Is your local system a Multiple Earthed Neutral system (MEN) I have found all the above faults affecting MEN systems and repaired each problem. Finding and fixing the issue may not be easy. In the case of the neighbouring system the better your earth bond the worse the problem becomes which is confusing until you work out the reason. If the neighbouring system has a poor neutral link back to its tranny the neutral current may partially flow via your eathing system first having made it into the general mass of earth via the faulty system's MEN at a switchboard. I have seen 2 VAC and more in this situation.

For an impressed Cathodic system expect more like 0.8VDC with a ripple that reflects the source of current be it 3 phase or single phase (rectified).

When the earthing system is itself causing a DC half cell it can vary a fair bit depending on the materials involved, just to make it trickier there could be some AC impressed as well from other faults or imbalance. What will seperate this from Cathodic issues is the frequency of the AC. Actual AC impressed by a faulty neutral will be at normal mains frequency, cathodic will be at twice mains frequency for single phase or 6 times mains frequency for 3 phase source.

Try and source an oscilloscope. Once you see the waveforms try turning things on and off. If the voltage or components of it are still there with everything in your control switched off then suspect outside sources.

One last thing if can use a current sensor to feed one channel of the scope your job will be easier.

Hope that helps.