What is Insulation Resistance Testing - RS Components

An insulation tester or Megohmmeter can be used to determine the insulation resistance of a cable, installation, motor, or appliance.  The principle is to apply a known DC voltage and then measure the flowing current.  Ohm’s Law would then apply in deriving an effective DC resistance.

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Ohm's law states - the voltage across a conductor is directly proportional to the current flowing through it if physical conditions and temperatures remain the same. In the equation, Resistance is R, and has units of Ohms, with the symbol Ω.

The value of insulation resistance measured is important.  The minimum acceptable value for most applications is to have more than 1 megohm (million Ohms) for every volts of the intended operating voltage.  Modern insulation testers have digital readouts and work by applying high voltages to conductors that are insulated.

The PAT testing insulation resistance limits are 1 megohm (1 million Ohms) for every V of intended use.  For example, if we were to expect an insulated conductor to operate at V then we would expect the insulation resistance value for that cable to be over 5 megohms.

There are three categories of domestic electrical appliances:

• Class 1 (has live neutral and earth)
• Class 2 (has live and neutral and no earth, usually a double insulated appliance)
• Class 3 (Low voltage appliance less than 50V)

Let's assume that we would like to PAT test a Class 1 appliance. We would:

• Plug the appliance into the PAT tester socket
• Connect the flying earth probe to the appliance's metal body
• Press the Class 1 test button

The PAT tester would then display earth continuity, insulation resistance and load leakage on the display indicating either pass or fail.

You would then record the results on an equipment inspection and test record and a pass-fail sticker would be applied to the appliance that has been tested.

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There are many PAT tester accessories available such as printers, labels, and cable attachments.

Testing insulation resistance is essential to keeping wires and motors working properly. Megohmmeters provide a quick and easy way to determine the condition of the insulation on wire, generators, and motor windings. A megohmmeter is an electric meter that measures very high resistance values by sending a high voltage signal into the object being tested. Typically, however, this is the only function a megohmmeter performs.

While megohmmeters are often informally referred to as insulation testers, strictly speaking this is not accurate. Why? What’s the difference between a megohmmeter and an insulation tester? An insulation tester performs the basic measuring function a megohmmeter does — measuring very high resistance values by sending a high voltage signal into the object being tested — and it often does much more; it usually performs more functions, including more complex testing and recording of measurements.

A full-featured insulation tester can perform high-voltage insulation resistance tests and much more

What makes insulation testers different

For example, unlike megohmmeters, insulation testers can also measure voltage and current. The Fluke FC insulation multimeter, for example, can perform insulation tests up to volts, and it is a full-function digital multimeter. The Fluke C FC can generate up to volts for insulation tests.

Insulation testers can also perform more complex tests, such as compensating for ambient environmental conditions like humidity and temperature during a test, which provides information about how motors perform in changing conditions. Because environmental conditions and/or chemical contamination accelerates the rate at which insulation degrades, it’s critical to compare insulation resistance test results that are corrected for different test conditions.

Insulation testers like the Fluke FC and the Fluke C FC offer another advantage over megohmmeters. Memory storage through Fluke Connect® saves measurements to your or the cloud so you don’t have write down results. This saves time, reduces errors, and saves data for tracking over time.

Choosing between an insulation tester and a megohmmeter depends on your business needs. A meg test may be all you need. But if you want increased power, convenience, prevention, and safety, an insulation tester may be your best choice.

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Fluke insulation resistance meters and testers