Knowing how to test a blown fuse is an important skill for any electrician or maintenance technician. In this article, I’ll explain how to test a fuse using a digital multimeter.
Fuses come in all shapes and sizes but luckily for us testing them is straight forward.
Testing fuse continuity
Checking a fuse for continuity when it has been removed is the most accurate test you can preform.
- Before removing the fuse remember to isolate the power and lockout as required. For easy and safe fuse removal use insulated fuse pullers.
- Place the fuse on a non-conductive surface and set your multimeter to the continuity setting.
- Place your leads on either side of the fuse. If the fuse is good you will hear the continuity tone and see a very low ohmic value, typically 0-2 ohms.
- If the fuse is bad you will not hear a tone and your meter will display OL (Over Limit).
Can you test a fuse without removing it?
Yes, it is possible to test a fuse without removing it from the circuit. This can be done with voltage or continuity checks.
Testing fuse without removing – using voltage
Testing fuses for voltage has the advantage of allowing you to test without de-energizing the circuit. Remember to wear the correct PPE and follow your company policy when testing live circuits.
If you’re unfamiliar with testing voltage check out our guide.
- Set your meter to the correct voltage setting, AC or DC depending on the circuit. Some meters like the fluke T5-1000 have an auto voltage setting that can properly detect AC or DC voltages.
- Place one lead on the line side of the fuse you’d like to test and your other lead on ground or neutral/common point.
- Verify that the correct line voltage is present, for example, 120v.
- Now place your leads on the line and load sides of the fuse.
- If your meter reads 0v that means that the fuse is good.
- If your meter reads line voltage, 120v in our example, that means the links in the fuse have blown open and the fuse needs to be replaced.
Testing fuse without removing – using continuity
To test a fuse for continuity while still in place requires a few extra steps than testing with voltage. Multimeters cannot read continuity/resistance when there is voltage present in the circuit.
If there are capacitors in the circuit it is possible for them to discharge while reading continuity so it is important to verify that no voltage is present. There is also the potential to get readings from different parts of the circuit that you aren’t intending to meter.
- De-energize the circuit.
- Set your meter to voltage and place one lead on the line side of the fuse and the other on a ground/neutral/common point.
- Verify that no voltage is present.
- Set your meter to continuity and place your leads on the line and load side of the fuse.
- If the continuity tone sounds and your meter displays a low resistance value the fuse is good.
- If your multimeter displays OL and no tone sounds the fuse needs to be replaced.
Can a fuse test good but be bad?
Yes! It is possible for fuses to test good but actually be bad under certain uncommon circumstances.
One night I got a trouble call about a three-phase hydraulic pump that wouldn’t run. I went to the motor control center, opened up the cubicle, and began troubleshooting.
Initial testing was performed with the power on. Everything seemed ok, line voltage was present, the fuses tested good, the control circuit was functioning properly, but the motor still wouldn’t run when the starter pulled in.
At this point, I locked out the cubicle and tested the fuses again, this time using continuity. I found one of the fuses was bad!
While removing the fuse I discovered that the welds attaching the top screw tab to the fuse had broken. This fuse was quite old and the welds had likely broken due to the high vibration in the area.
There was enough of a connection between the tab and the fuse to allow my meter to read “ghost” voltages, but not enough of a connection to let current flow through the circuit. Because multimeters send a small current through the fuse when testing continuity, I was able to discover the issue with the fuse.
This situation is uncommon and because the top of the fuse was above my eye level I wasn’t immediately able to see the broken welds.
I’ve heard from other electricians that they’ve seen similar situations when fuse links blow open but are still close enough together for ghost voltages to show on their meters.
Because of this possibility, I use continuity to test fuses whenever possible. Another option would be to test fuses using a solenoid type voltmeter.