Insulation Resistance Test of a Three-Phase Induction Motor

To prolong the life of electrical systems and motors, regular insulation resistance is required. Over the years, after many years of cycles of operations, electric motors are exposed to environmental factors such as dirt, grease, temperature, stress and vibration. These conditions can lead to insulation failure, resulting in loss of production or even hazards. An effective motor insulation resistance system has high resistance, usually (at an absolute minimum) greater than a few mega ohms (MΩ). A poor insulation system has lower insulation resistance. The optimal insulation resistance for an electric motor is often determined by the manufacturer’s specifications, the criticality of the application where the motor is used, and the environment where it is located

When to Test an AC Motor Insulation Resistance

The Insulation resistance resistance of a motor requires periodic checking i.e. testing at regular intervals depending on the frequency of usage, the criticality of the application of the motor, and the environment where it is used.

Insulation Resistance Test  

Insulation test measures the total resistance between two points separated by an insulation. 

This test determines how effective the dielectric(insulation) is in resisting the flow of current through it. Performing such test at regular interval can help detect impending insulation failures before they occur.


Test and Measurement Methodology


The measurement of the insulation resistance is carried out by means of a Fluke Insulation Resistance Tester 1550C and 1555 has a high resistance range ohmmeter. To measure insulation resistance, a DC voltage of 500V or 1000V is supplied between the windings and earth (motor frame) of the motor. It is advised that during the test or measurement and immediate afterwards, do not touch any of the motor terminals, can be fatal because some can carry dangerous voltages.


The minimum insulation resistance of motor measured to ground at 500V can be measured at a winding temperature of -15°C to 20°C. The maximum insulation resistance at 500V with operating windings temperature of 80°C - 120°C depending on the motor type and efficiency.

A general rule-of-thumb is, the insulation system of an electric motor is said to be in good condition if: measured insulation resistance is greater than or equal to 10MΩ.



PowerPro Company Limited testing services and products offerings 

​Powerpro is a leading test, measurement, diagnostic and energy management company in Nigeria. We provide continuity and insulation resistance test for electric drives/motors for client in the manufacturing and oil and gas industries. In addition, we are second to none in providing facility/power audit, electric drives/motor installation and turkey maintenance projects.

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