6.4 Insulation Resistance (IR)
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 resistance. The voltage applied is usually greater than the voltage that the insulation will see in normal use.
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 windings, or between a winding and a screen.