Lesson 16 of 21
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What is Insulation Resistance?

NAC-Trainer March 1, 2020

This test should be carried out on all appliances before and after a repair. Insulation resistance is defined as the resistance to current leakage through and over the surface of the insulation material surrounding a conductor. It is measured in Ohm`s and its value represents a very important factor to electricians working within the electrical industry.

The purpose of an Insulation Test is to ensure that there is no leakage to Earth.

The insulation resistance test is an electrical test that uses a certain type and level of voltage (500V d.c. for low voltage installations i.e.: 230V) to measure insulation resistance in Ohms. 

The measured resistance indicates the condition of the insulation between two conductive parts. An infinite resistance would be the perfect result, but no insulator is perfect, so the higher the reading the better. 

The table below shows the required test voltage and the minimum required resistance in accordance with BS 7671.

Nominal Circuit VoltageTest VoltageMinimum Resistance 
Between 0 V and 50 V a.c.250 V d.c.0.5 Mohm
Between 50 v and 500 V a.c.500 V d.c.1Mohm
Between 500 V and 1000 V a.c.1000  V d.c.1 Mohm

An easy way to understand this test is to think about it as a form of pressure test. When the test voltage of 500V d.c. is applied to a regular 230V circuit, we will see if there is any leakage from one conductor to another through the insulating material.

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