Solenoid voltmeters draw appreciable current in operation. This makes them useful for testing residual-current devices (GFCIs) because the current drawn trips most RCDs when the solenoid voltmeter is connected between the live and earth conductors. Also, when testing power supply circuits, a high-impedance connection (that is, a nearly open-circuit fault such as a burned switch contact or wire joint) in the power path might still allow enough voltage/current through to register on a high-impedance digital voltmeter, but it probably can't actuate the solenoid voltmeter. For use with high impedance circuit applications, however, they are not so good, as they draw appreciable current and therefore alter the voltage being measured.