how to test insulation resistance of a transformer?

To test the insulation resistance of a transformer, use a megohmmeter. First, disconnect the transformer from the power supply and ensure it is de-energized. Connect the megohmmeter leads to the transformer’s windings and ground. Set the megohmmeter to an appropriate voltage (typically 500V or 1000V) and take the reading after a minute. A resistance value above 1 M? is generally acceptable, but refer to manufacturer specifications for specific requirements.