Selection of usage scenarios for high voltage testers and megohmmeters

Should I use a high voltage tester or a megohmmeter?

When should you use a high voltage tester and when should you use a megohmmeter? This seemingly simple question does not have such a simple answer, because the difference lies not in the function of each tool, but in the expected results.

Let’s first take a look at the functions of each device:

A high voltage tester (short circuit way of saying high potential or high voltage) is the term used for electrical safety testing instruments used to verify the electrical insulation in finished appliances, cables or other wired assemblies, printed circuit boards, motors, and transformers.

A megohmmeter is a special type of ohmmeter used to measure the resistance of insulators.

Essentially, both the megohmmeter and the high water meter do the same thing, but the results are very different. Both instruments apply a relatively high voltage across the insulation and draw a leakage current through the insulation, commensurate with its insulation properties and conditions. However, the megohmmeter provides the operator with a measurement, while the high water meter takes action.

When to Use a Megohmmeter

A megohmmeter measures very small currents, typically in nano-amperes, making it through imperfections and degradation of insulating materials. Following Ohm's law, it converts this into a resistance reading. These are usually in megohms. Anything less than 1MΩ is generally considered unusable.

Once the operator obtains a reading with a megohmmeter, he/she must decide whether the device under test should continue to be used, cleaned, repaired, or scrapped. Because it uses only a few milliamps of test current, the megohmmeter has limited power and will not damage insulation. This is important because effective use of a megohmmeter requires repeated, routine testing in order to establish lifecycle trends and perform preventative maintenance.

When to Use a High Voltage Tester

In contrast, a high voltage tester uses much higher voltages and currents, and while it can provide measurements, its primary function is to identify and kill weak currents; for example, taking marginal equipment out of service by breaking weak insulation before a costly failure occurs at work. Testing can be done at either DC or AC voltages.

The item under test should be able to withstand the voltage applied; however, compromises can be made in "tip" testing. The tester has a voltage and current meter, and both are expected to rise in sync as long as the insulation is "tough". When the current begins to rise above the voltage, the test is stopped abruptly before increased damage occurs. The item under test then needs to be repaired.

Choosing the right test equipment

Choosing between a high voltage tester or a megohmmeter for testing really comes down to what you are testing, the voltage and current you need to test, and some personal preference.

If you are looking for measurements to track life cycle trends of your equipment, a megohmmeter is the tool for you.

However, if you wish to really stress your device under test to determine if it still meets standards and operates safely, high voltage testing will help you take more decisive action.