Identifying Faulty Cables With an IR Tester

Insulation begins to age straight away, and its ability to insulate effectively diminishes the older it becomes. This process is only further accelerated if it is installed in a harsh environment such as those extreme temperatures or contamination by chemicals.

It is therefore important to identify any deterioration in order to be able to take corrective measures, this is done with an IR tester.

What is the IR test?

The IR (Insulation Resistance) test measures the amount of resistance that exists between any two points that are separated by electrical insulation. The test, therefore, determines the effectiveness of the insulation in regards to resisting the electrical current flow.

IR tests can be very helpful to check the insulation quality both immediately after manufacture and also over the course of the time period in which the insulation is actually being used.


The IR tester that is generally used is known as the megohmmeter, which is not dissimilar to a digital multimeter, although it comes with a much higher output than the latter. Voltages range from 100 all the way up to 10,000 volts, though the most common are 500V and 1000 volts. Higher voltages are made use of to stress insulation to greater degrees in order to achieve more accuracy in the results.

Another difference between a megohmmeter and a digital multimeter is the extent of its range. A multimeter measures in ohms, but a megohmmeter’s range is actually in megohms.

Megohmmeters also have a fairly high level of internal resistance, which helps to make it a lot less dangerous to use even with the much higher voltages.

Megohmmeters tend to have three terminals, the LINE terminal, the EARTH terminal and the GUARD terminal.

The LINE terminal connects with the conductor of which you are measuring the insulation resistance.

The EARTH terminal connects to the ground conductor, which is on the opposite side of the insulation.

The GUARD terminal offers a return circuit that is actually able to bypass the meter.

The tests

A megohmmeter can be used as an IR tester in three different ways.

The simplest test is to test insulation resistance. The test voltage is applied for a minute following the necessary connections, during which time the resistance should either stay steady or drop. There will be a steady decrease with bigger insulation systems, though smaller ones should stay steady. After the minute, the resistance value can be read and recorded.

The Dielectric absorption rate test recognises that there will be a gradually increasing IR in good insulation following the application of the test voltage. The IR is read after either thirty or sixty seconds, or after sixty seconds and ten minutes, with the later reading divided by the first.

The step voltage test can evaluate damaged or aged insulation has not necessarily been contaminated or feature moisture. The IR test should be done at a lower voltage, with the test specimen being discharged and then the test redone at a much higher voltage. If there is a difference of more than 25% between the readings, the insulation may be damaged or have suffered age deterioration.

If such tests are performed on a regular basis, insulation failure can be detected before it happens, preventing user accidents and expensive repairs to products.

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