An insulation test is carried out to test the integrity of the insulation between conductors.
To test it you use an insulation tester (ofter called a Megger, one of the manufacturers, same way we call a vacuum cleaner a hoover). An insulation tester is a piece of electrical equipment with two leads coming off it, this tester is used to measure the resistance between the conductors.
To do the test you ensure the cable is disconnected from any devices (insulation testers nomally work on 500v, 1000v testers for higher voltage testing, this can damage sensitive equipment). Ensure both ends of the cables are seperated from each other (connect one end in terminal strip if necessary). Now connect the megger to two conductors, push test and you will get a resistance readout, this is the resistance of your insulation.
If the insulation resistance is too low then you are going to get a potentail for power to travel between cores, leading to equipment damage, blown fuses and potentail harm to personnel (imagine putting power onto the earth).
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