Measurement of insulation resistance

This measurement important to determine the winding condition for electric motor,electric cables,electrical installation,transformer,circuit breaker,heater and electrical equipment.

From the data of insulation resistance,we can make a perfect and clear decision either the electrical equipment in good condition or not.

The insulation resistance testing also can avoid serious damage,fire or electric shock to public and personal.It also can protect and pro long the life span for electrical equipment.We can detect any abnormal condition earlier and rectify quickly and avoid major damage.

Method of measurement for insulation resistance

This time i want shared about how to perform insulation resistance testing for common electrical equipment such as motor,transformer and cables.I explain detail step by step how to perform the testing for each equipment.

1) Electric motor

For electric motor,we used a insulation tester to measure the resistance of motor winding with earthing ( E ).

(a) For rated voltage below 1000V, measured with a 500VDC using insulation tester meter.

(b) For rated voltage above 1000V, measured with a 1000VDC using insulation tester meter.

(c) In accordance with IEEE 43, clause 9.3, the following formula should be applied:

(Rated voltage (v) /1000)+ 1

From the table below,compare the measurement of resistance value with minimum level of winding resistance, we can determine the winding condition is good or not.

2) Transformer

To measuring the insulation resistance for transformer,we also used insulation tester.For testing single phase transformers,We need to test winding to winding and winding to ground ( E ).

For three phase transformers,We need to test winding ( L1,L2,L3 ) with substitute Earthing for Delta transformer or winding ( L1,L2,L3 ) with earthing ( E ) and neutral ( N ) for wye transformers.For determining minimum insulation resistance use the formula below :

3) Electrical cable and wiring

For insulation testing,we need to disconnected from panel or equipment and keep them isolated from power supply.The wiring and cables need to test for each other ( phase to phase ) with a ground ( E ) cable.The Insulated Power Cable Engineers Association (IPCEA) provides the formula to determine minimum insulation resistance values.

R = K x Log 10 ( D/d )

R – MΩs per 1000 feet (305 meters) of cable.Based on dc test potential of 500 volts, applied for one minute at 15.6 °C (60 °F) temperature)
K – Insulation material constant.(For example: Varnished Cambric-2460, Thermoplastic Polyethlene-50000,Composite Polyethylene-30000)
D – Outside diameter of conductor insulation for single conductor wire and cable ( D = d + 2c + 2b diameter of single conductor cable )
d – Diameter of conductor
c – Thickness of conductor insulation
b – Thickness of jacket insulation

For more detail how to used insulation tester,please read my last post : How to use insulation tester?


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  2. Can you give correction factors for formulas at various temeperatures.?

  3. Please kindly furnish me the necessary steps in transformer maintenance.

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