The Voltech Handbook of Transformer Testing

2.25 IR - Insulation Resistance

Transformer windings are often insulated from each other and from the core by insulating tape, enamel on the wire or by plastic moulding on the bobbin.

The quality of the insulation can be assessed by applying a DC voltage across the insulation and measuring the value of resistance.

The voltage applied is usually greater than the voltage that the insulation will see in normal use.

Typical test voltage is 200V to 500V DC, and the insulation resistance expected will be of the order of one hundred to several thousand MOhms.

Where Used

An Insulation Resistance check is recommended as good practice for most transformers to check the integrity of the insulation between separate windings, or between a winding and a screen.

It is used where winding isolation is a part of the functionality of the transformer, but not part of a safety requirement, for which a Hi-Pot (EHT) test is preferred.

It would therefore be used, for example, between two primary windings, or between two secondary windings.

Measurement Conditions

To measure insulation resistance, the tester applies a dc voltage between two groups of windings, with the windings in each group being shorted together. Each group may contain as many windings as you wish.

The voltage and current are measured; dividing the voltage by the current gives the insulation resistance.

You can also command the tester to perform a compensation measurement on the empty fixture at the start of a test run. This is recommended as it will allow the tester to subtract the errors due to the fixture from subsequent measurements, to give more accurate results.

The test voltage may be chosen in the range from 100V to 7kV. (500V AT1600).

Normally however, you would perform the test with a voltage just greater than twice the highest possible peak working voltage on the winding. Therefore, for example, to check the isolation between the two primary windings of a line frequency transformer, which could be connected in series and operated at 240V, 800V would be a suitable test signal.