Good day experts,
got a question related to measuring the isolated conductor. it is mentioned in the standard under the note on clause 8.3.2, that care must be taken in using NC voltmeter or a field meter. meaning we can use to measure isolated conductor using a field meter as well. my question is why field meter is also applicable where in fact we are measuring the voltage potential on the surface of the isolated conductor. my understanding is that a field meter measures a field induced by the material. can you give me clarifications on the application of field meter and voltmeter in this case.
Although the non-contact voltmeter and the field meter can be used for the measurement of the voltage on an isolated conductor, the preferred tool is the contact voltmeter. Non-contact voltmeters and field meters can be used but you would need to understand the drawbacks in getting an accurate reading. The field meter usually has a 3 inch field of view so if the isolated conductor is any smaller than this, which it often is, the filed meter will be averaging in the electric field of the space around the isolated conductor. In order for the field meter to measure the actual voltage on the isolated conductor, it also has a specified measuring distance, usually one inch, which can affect the accuracy especially if the isolated conductor is not flat.
Although non-contact voltmeters have a more narrow field of view and are not as affected by the distance from the isolated conductor, they still are not as accurate as a contact voltmeter. Both non-contact voltmeters and field meters can also be affected by any stray electric fields from sources like insulators near the isolated conductor.
One of the big reasons the standard allows the use of non-contact voltmeters and field meters is because most small facilities don’t have the budget for a contact voltmeter which can cost over $5,000 new. The non-contact voltmeter and field meter can at least give a good estimate of whether there are any charges on the isolated conductor. If a facility finds an isolated conductor with any charge close to the limits of +/- 35 volts with a non-contact voltmeter or field meter it should consider obtaining a contact voltmeter to better understand the isolated conductor(s) in question.