Many moons ago.... Electricians would lay/throw a crobar across a circuit they were working on to protect them while working on the equipment. If the power switch was in a different location (sometimes miles away) and an unknowing person re-energised the circuit, the "crobar" shorting the circuit would protect them by blowing the fuse. Now days in electonic circuits (power supplies) the "crowbar" is generally replaced by an SCR that is designed to have a very high peak current. Most often seen as a protection circuit to kill the power supply in case of overvoltage malfunction. The advantage of the SCR is that once triggered it will continue to conduct untill the current through it reaches a VERY low threshold, even with the trigger removed.
No "lack of power" indicator is totally foolproof. Lamps burn out, voltmeters can as well. A voltmeter is the best check, but, just to be on the safe side, after checking with the voltmeter seeing the voltage is low, "crowbar" that cap before you touch it.
Steve