from the Automation List department...
Neutral to earth voltage
Information resources, documentation. topic
Posted by P.K. John on 7 November, 2006 - 12:31 am

Being a regular visitor of your site, we look forward to your assistance in the following.
Upon installation of a computer to control an automated nondestructive test system, we have checked the electrical circuits in the building and have observed that the neutral to earth reading is around 8 volts. We have made a new ground pit and grounded the circuit and later found the neutral to earth reading is around 3 volts. Considering the instructions of the control card manufacturers, we have approached the Electricity board for assistance. They have provided an earth to the neutral from the main supply point. This has not changed the reading. Having difficulty in achieving lower reading, we have contacted local suppliers of major online UPS brands. We were told that the online UPS wouldn't solve this problem. Some suppliers mentioned of an "Isolation transformer", but could not come up with any literature (brochure) on this.

We would request your valuable assistance in resolving this issue. We do hope your technical personnel can provide us with answers for the following.

1. Can we eliminate this problem by providing an earth to Neutral? If so, are there any consequences and how this would reflect in the monthly electricity billing?

2. Can the Electricity board do something on their side to lower this voltage?

3. Is there any economical products available for the same purpose?

4. Is it possible to achieve a Neutral to earth reading as low as 250milli volts as required by the card manufacturer?

Thanking you,

P.K. John

Posted by Rick Macy on 9 November, 2006 - 4:58 pm
Hello P.K,

It sounds like you're in the research industry.
Answers to your questions...
1-A YES... But we need further info!
1-B NO
2- "Electrical Board" I trust it's your electricity supplier, They'll do nothing!
3- Yes, but we need answers to your question 1A
4- Yes, but Contact me offline.

Rick Macy
BC Electronics

Posted by Vic Peņaranda on 9 November, 2006 - 7:06 pm

To answer some of your concerns:

1. Can we eliminate this problem by providing an earth to Neutral? If so, are there any consequences and how this would reflect in the monthly electricity billing?--> As you had said, as I understand it right,your Electricity board has provided an earth to the neutral bus on the main supply point. This is correct.However, this will not solve your problem of having voltage difference between earth & neutral on the point where your equipment is connected.I suggest to tie also the end of your neutral bus to earth, meaning, earth & neutral is tied (connected) at both end of the wiring installation. These should have no effect on your monthly electricity bill, because, the voltage you are actually measuring is the noise picked up by the wiring on a long run.

2. You can also install an isolation transformer in between your conventional power supply and the equipment. You can buy a standard isolation transformer from your electrical stores but you should know the transformer rating required for your equipment. To get an approximate rating, you can get the power required by your equipment (in VA) X 1.25, then that's the transformer rating that you should buy.

Kindly let me know if you have further question.


Posted by Michael Griffin on 9 November, 2006 - 7:19 pm
With most data acquisition boards, the requirement is to limit the voltage difference between the signal common and ground to prevent ground loops. The board will be connected to neutral via the computer back plane (possibly through a rather obscure path). If the signals are also grounded in the field you will get an undesirable current flow, possibly enough to damage the board.

Normal practice is to isolate the field signals via 3-way (not 2-way) signal isolators (or 3-way isolated signal conditioners), and to then "float" the field signals. The signal conductors between the signal conditioners (or isolators) are not tied to ground, but are left to float, except for whatever connection is provided by the board itself.

Where people usually have a problem is when they connect the field signal directly to the board without any isolation (possibly through a non-isolated signal conditioner) and one end of the signal is "grounded" in the field. This can be done, and it is cheaper (at least from a bill of materials point of view), but it is often the source of endless troubles.

Have a look at the signal wiring in your machine, and talk to the board manufacturer about the actual intent of their specification. You may find that what you really need is signal isolation.

Posted by Curt Wuollet on 14 March, 2007 - 12:11 am
For this you could get an isolation transformer large enough to supply your test equipment and simply ground one side of the secondary and call it the neutral. Isolation transformers are sold for standard voltages and by the VA required. It overcomes a lot of regulatory problems if you can mount it inside the equipment rather than supplying outlets in which case many jurisdictions have problems with multiple neutral to ground connections. It's fine most places if you isolate inside a cabinet and earth ground the cabinet. Before you get too excited, you might want to use a scope to see just what the difference voltage spectrum is. I've seen a merry chase to find power leakage and ground potential differences that turned out to be fluorescent lighting hash and noise with no real power available, that is, the power frequency component was very low and quite acceptable and the rest was killed with a $10 line filter.


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