How do I select test voltage for an insulation test?

Choosing voltage for an insulation test used to be simple in that it didn’t involve many choices. Over the years, test instrumentation has increased in capabilities, making it both harder and simpler: harder in the fact that there are more choices, but simpler in that the operator can do a lot more with all the testers that are on the market.

Building wiring, and the equipment that runs off of it, can almost universally be tested at 500V. Some simple insulation testers offer no more than that. Old instruments would add a 1 kV selection in order to do a stress test, and lower voltages like 250 V and 100 V for testing more sensitive equipment and communications.

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Initially, only one voltage selection read direct (usually 500V), and the others had to have the reading adjusted by a multiplier that was often printed on the selection position; X2, ÷2, etc. Models with variacs also came on the market, allowing continuous voltage adjustment from zero up. But the tradeoff was that these read directly at only one or two most fundamental test voltages, while a bewildering exercise was necessary to correct readings at any other position.

Improvements over the years

Improvements made over the years to instrumentation have led to direct reading at multiple voltages.  Today, a direct reading can be done at almost any voltage over a wide measurement range. A high-quality tester might now enable voltages to be set in 1V increments all the way up to 1 kV. This capability enables testing to the exact measurements and requirements that have been set by a wide range of standards and operating procedures, instead of the closest possible approximation.  These standards continue to multiply throughout the industry and the insulation responses to changes in voltage can now be studied with precision rather than by wide leaps.

Similarly, the actual voltage applied by the oldest instruments had to be presumed and then selected. But test voltage actually tends to float against the load. Modern instruments display the actual applied voltage, which is generally a bit above selected. This is ok; you’re getting what you wanted plus a bit more. What you don’t want is a tester that loads down and fails to deliver selected voltage, thereby giving a reading with considerable error over what was desired.

Loading up isn’t a problem

While loading up is not a problem, it does tend to confuse operators who selected, say, 500 V and got 522 V. Quality testers now have tightened this window to only a few volts, making test reporting and standards conformance more coherent and compliant.

Choosing a tester and test voltage

The variety of testing to be performed, level of familiarity by the operators, and degree of assuredness about future assignments should all be considered when selecting a model of insulation tester.

For some basic testing, if relatively new or apprentice personnel are to be the operators, less is better. Most instrument manufacturers still offer testers with only one to three selections. Testers are still available that test at only 500V, nothing else. These models may be more efficient if only pass/fail testing is in order; less setup and no concern that the tests may be performed at the wrong voltage.

Some industries, telecom in particular, tend to be nervous about 1000 volts applied across their equipment, and so many manufacturers offer models without this top voltage setting. For even more sensitive applications, there are specialized models that only go to 100V.

At the opposite, there are full-featured models that offer wide voltage options. These are the models of choice for broad, generalized applications. For troubleshooting, a 1 kV selection is useful for spotting weak, though not failed, insulation, while the 500V selection is used for routine maintenance and record keeping. But many industries are becoming more specialized and rigorous in their testing requirements. For applications like solar, marine, nuclear, heat trace cabling, and many more, the ability to select any voltage across a broad spectrum is becoming indispensable.