Diff. voltage between NULL and EARTH


Thread Starter

Masoud salimi

we have some problem with EARTH in our company.
there is a difference voltage about 18VDC between null and earth.we don't know what is the correct method for reduce it?
it seems that easiest way is make a connection between null and earth, but is this method correct or no?

Jose Ignacio

It is, but you will offer a path for a lot of current to flow so you must be careful to asses your existing grounding instalation. Also the neutral cable which is where the 3d harmonic current is about to flow. A simple rule of the thumb for office buildings filled with computers say neutral shoud be sized 1.7times the load of the phase cables.

An Electrical Engineer

We have the same problem at our factory. We used a high reactance coil to short both. U may use a primary winding of a Power Transformer along with a suitable fuse to conect it with the earth. So that potential difference may became negligible. Use a suitable resistance in series too.

Hope this may serve the purpose.

An Electrical Engineer

Steve Myres, PE

I don't know what "null" is but if it is the same as neutral, then no, you shoudn't connect them other than at the point of derivation of the neutral. It can actually be dangerous. First take a voltage measurement between the ends of the "null" and earth wires. IOW, using a tracer wire, check the voltage between the two ends of the null wire and then check between the two ends of the earth wire. (Measure from the point where you know there is a difference to the point where the two are tied. This will tell you in which wire the 18v is being developed (or in what combination). If it is the groundING conductor (earth), 18V is too much and indicates poor ground continuity, inadequate wire size, or excessive ground leakage. In the groundED conductor (neutral), 18v is still high, depending upon system voltage, though less worrisome than in a ground, as neutral is intended to carry current and will see SOME finite voltage drop. Most of the same issues can be checked (wire sizing vs. design and measured current, continuity including connection integrity, etc.)