Abstract: An electrical insulation testing device has a voltage source 12 for applying a test voltage through terminals 14. Measuring devices 16 take measurements to enable the resistance of the insulation under test to be determined. Results are shown by a display 20. The controller 24 prevents the display of results until the voltage source 12 has been turned off and discharge circuit 22 has discharged the capacitance of the insulation under test. This prevents the insulation being left charged after the test. Measurement is commenced by operation of the switch 28. Only one measurement can be taken each time the switch 28 is closed.