In order to definitively determine whether or not fuse "1", shown in the illustration is blown using an on-line testing technique, across what points would you connect the voltmeter leads? Illustration EL-0062

Correct Answer: Option D — the bottom of fuse "1" and the top of either fuse "2 or 3"

Testing a fuse 'on-line' requires measuring the potential difference across the fuse. If the fuse is blown, the voltmeter will show the full line-to-line voltage because the circuit is open. Measuring from the bottom of the blown fuse to the top of a healthy phase confirms the voltage drop.