Device Detecting Potential Difference Between Neutral and Earth

Depends on your desired current and voltage limit.

Easy is a simple potentiometer circuit with a small AC relay on the cursor. If only potential may be detected, everything has to have a high input impedance. A small transistor or Op- amp circuit can help you out. I didn't see any devices with variable threshold setting,( most of safety units, work on current limits, because the current is more important ) straight from the shelf yet, but I live very remote. Perhaps one of the others can fill you out whit that.

There is no need to measure it.

Earth is a safety conductor. Its job is to disconnect the circuit protective device(s) in the event of a fault. Ordinarily, there is no current carried by Earth. Adding some sort of detector changes all that....

Is this question is due to an instrumentation issue where you're measuring something like terminal connection temperatures on appliances using thermocouple and galvanometer means?

If so the easiest solution is to tie earth to neutral at the lab sub board.

This is only normally done at the council connection, but if you are in a large plant with a lot of intermittent high current activity, potential between earth and neutral can fluctuate enough to blow instruments.

You need to check with your electrician. You may have to run an independent feed to the lab. Distances and drops and circulating currents are to be considered.

Other wise, if you just want an alarm - you need to build something (as above - opp amp and transistor) or tap out of a volt meter that has a settable threshold alarm.

Kyzine

<...potential between earth and neutral can fluctuate enough to blow instruments...>

How does that arise?

In large buildings, neutral wiring and "earth" voltage differences can be significant. In essence, you could be working with very long test leads. And you may only be looking at these voltage differences over a short distance. Regardless, you might consider using a graphical recorder to measure the neutral/earth AC voltage differences for long periods of time. A computer version of a graphical recorder could be implemented. Often it's not simply the detection of exceeding a set limit, but also the time the exceeded event occurred. I once worked in a building with voltage problems.