If a fuse is installed and energized, which method is appropriate to test it?

• A. Test across the fuse using a voltmeter set to the system voltage, using proper PPE.
• B. Test across the fuse with an ohm meter while energized.
• C. Remove the fuse and test with a continuity tester.
• D. Visually inspect the fuse.

Answer: (A)

The key idea is to verify the fuse’s electrical status without creating unsafe conditions on a live circuit. Testing across the fuse with a voltmeter set to the system voltage lets you observe the voltage drop across the fuse while it remains in place and energized. If the fuse is good, it behaves like a low-resistance link, so the voltage difference across it is very small. If the fuse is blown and open, the downstream side is de-energized and you’ll see the full system voltage across the fuse. This method confirms whether current is actually flowing through the fuse without applying further current that could damage components or cause arcing, and it keeps the protective device in place.

Using an ohm meter or continuity tester on a live circuit injects current and can damage the meter or fuse, and poses safety hazards. Removing the fuse to test with a continuity tester breaks the circuit and creates an arc or shock risk. Visual inspection alone cannot reveal whether voltage is actually present or whether the fuse is conducting.