Theory behind doing an IR test

[parm]

Hi Guys,

Can anyone explain how an IR test works for me please? I understand how to do it and the readings I should get but I'd like to understand why I'm getting those readings please. Probably a pointless question but would like to be enlightened...

 

[Jonesy83]

What is "insulation resistance testing" (IR)?

In insulation resistance testing you are trying to answer the question "Is the resistance of the insulation high enough?" You apply a voltage and very carefully measure the current. You then calculate the insulation resistance using Ohm's Law (R = V/I).


Why high voltage test?

You use a hipot test to make sure you have good isolation between the parts of a circuit. Having good isolation helps to guarantee the safety and quality of electrical circuits. Hipot tests are helpful in finding nicked or crushed insulation, stray wire strands or braided shielding, conductive or corrosive contaminants around the conductors, terminal spacing problems, and tolerance errors in IDC cables. All of these conditions might cause a device to fail.
lerance errors in IDC cables. All of these conditions might cause a device to fail.

Hipot Testing FAQ - Introduction to Hipot Testing

 

[Sparky Ninja]

Insulation Resistance Testing - Insulation Resistance Testers and Test Equipment

This test is the same as the withstanding voltage test in that it is mandatory to prevent electric shock and fire accidents from using the equipment and that it checks the functionality or performance of the insulator. The withstanding voltage test detects insulation defects by checking whether dielectric breakdown occurs. The insulation resistance test detects insulation defects by measuring the resistance.

After absorbing the moisture of the equipment (sometimes this is not done), a specific DC voltage that is 5 to 10 times higher than the normal voltage is applied, and the resistance is measured from the amount of current that flows. If the insulation resistance is sufficient, the equipment meets the requirements for preventing electric shock and fire accidents.

Why DC voltage is used to perform insulation resistance testing?

The insulation resistance test measures the resistive component of the insulator. The capacitive component is ignored. The equipment is only safe if at least a given insulation resistance (a value specified by a standard) is maintained. The insulation resistance test is performed to check this resistance. If the insulation resistance test is performed using an AC voltage, we end up measuring the impedance of the capacitive component and prevents us from obtaining the required insulation resistance. This is the reason why the insulation resistance test is performed using a DC voltage.

 

[telectrix]

good answer from widdler, but, to put it simply, if you, for example, used a multimeter ( on ohms ) across phase/neutral/cpc, it would only show up a dead short due to the low voltage applied. when applying 500V DC any leakage due to trapped Phase/Earth, neutral/Earth or moisture problem will show up . This will tell you if when upgrading an installation whether or not RCD will trip and also if you have a potential safety/shock problem.

 

[parm]

Thanks to you both for this comprehensive explanation, its one thing doing the test and its another knowing why your getting the reading for the test. Thanks again, you brought back painful memories of A-Level Physics, I knew it would come in useful one day