There are many different ways to test a fuse without a multimeter. These include swapping, visual inspection, use of light bulbs, and more. Using a multimeter is typically the safest and easiest way to test a fuse but it’s not always an option.
If another part of the equipment you are working on has an identical fuse then a quick swap can be a great way to determine the integrity of a fuse.
Remove the suspect fuse from the circuit. Make sure to keep track of which fuse is which.
Identify an identical fuse and figure out what circuit it is protecting. Take the known good fuse out of the working circuit and replace it with the suspect fuse. If the circuit still works with the suspicious fuse that proves the fuse is good and the problem lies elsewhere in the system.
If the previously good circuit doesn’t work with the fuse, that means the fuse is blown and needs to be replaced.
Don’t put the good fuse into the suspect circuit unless you are prepared to have that fuse blow as well. Sometimes fuses get weak and blow, but most often there is another reason the fuse has blown.
Visual checks are great when testing a fuse where the link is visible, such as a glass fuse.
Typically glass fuses contain a single metal fuse link. Safely remove the fuse and give it a thorough inspection under good light. Sometimes the link will have a hard to spot break, and other times the glass will be blackened from the fuse blowing.
The fuse link will vary in size based on the amperage of the fuse. It’s much easier to spot a break in a 10A fuse than it is to find it in a 250mA fuse.
A magnifying glass can make this task easier, or even zooming in on a picture taken with a phone camera.
Using a Light Bulb
As long as the voltage of the circuit with the blown fuse is known, an incandescent light bulb with a socket can be a quick stand-in for a multimeter. Always follow your companies safety policies and wear the appropriate PPE while working on live circuits.
A finger safe terminal with the correct voltage bulb is the safest configuration for testing.
Make sure the makeshift tester is wired in a way that there is no exposed metal that could shock you or cause a short circuit. Put one wire to neutral/common, and one wire on the line side of the fuse. If voltage is present the bulb will light up. Now move the wire from the line side to the load side of the fuse. If the fuse is good the light will come on.
If the bulb doesn’t light up the fuse is blown and needs to be replaced.
This test is also a great way to see if a voltage reading is real or “ghost” voltage when you don’t have access to a solenoid type meter.
Although it’s not often practical, an IR gun can sometimes pickup bad fuses. If the normal operating temperature of the circuit is known then seeing a cold fuse with the IR gun could indicate a problem.
A non-contact voltage tester could also be placed on both sides of a fuse to check for voltage. Oftentimes non-contact voltage testers give off false readings so this test could potentially send you down the wrong path when troubleshooting.
Ultimately, using a multimeter will always be the best method for testing fuses, but there are other techniques that can be used in a pinch.