How to Test a Fuse with a VOM or DMM

A VOM (short for “Volt-Ohm-Milliameter”) or a DMM (Short for Digital Multi-Meter) is a handy device to have around. A DMM is a digital version of a VOM. Some people use the terms interchangeably, but although they serve the same functions, they are somewhat different in the details of their operation. One thing you can do with either type of device is to test a fuse. Most common fuses are either good, or blown. The way you test them is to measure whether electricity can flow through the fuse or not.

Difficulty: Easy

Instructions

Things You'll Need:

• A fuse to test
• A VOM or a DMM

1. Turn the power off to whatever device your fuse is in.

2. Remove the fuse from the device. There are different kinds of fuses. Some you pull straight out of the socket. Others you will need to unscrew from the socket. Still others have a cap that you press in, and then turn.

3. Identify the two electrical contacts on the fuse. On a glass cylinder fuse, these are the two caps of the glass tube. For an automotive fuse, it is the two metal pieces that are the base of the fuse. For the type of fuse that you screw into a home fuse box, one of the connectors is the center of the base, and the other is the metal part with the screw threads on it.

4. Set your VOM or DMM to one of its resistance readings. This may be signified by the term “resistance”, or, “Ohms”, or something like Rx1K, Rx10K, etc. It doesn’t really matter which you choose as long as it is measuring resistance. Rx10K is a good setting for this application.

5. Test that the meter is working and on the correct setting by touching the two leads together. On a VOM, this should make the needle on the meter deflect to full scale (indicating no resistance). On a DMM it will display “0” or some very small value like “0.007”.

6. Touch one of the leads to one of the contacts on the fuse, and the other lead to the other contact. It doesn’t matter which lead goes to which, but avoid touching the metal parts of the lead.

7. Look at the results on the meter. A fuse that is good offers very little resistance to the electricity coming from the meter. A fuse that is blown offers infinite resistance. Thus, on a VOM, if the fuse is good, the needle will deflect to full scale just like when you touched the leads to each other. If the fuse is bad, the needle won’t deflect at all. On a DMM, a reading very close to “0” indicates that the fuse is good. A very large number indicates a blown fuse.

8. Replace the fuse with a new one if your test indicates that the fuse is blown. Replace the fuse you removed back into the circuit if your test indicated that it is good.

Tips & Warnings

• Some DMMs have a setting called “continuity” or “diode” where the device emits a tone when there is very little resistance. If your meter has a setting like that, test it by touching the leads together. The tone should sound. Then, if you test a fuse and the fuse is good you will hear a tone. If the fuse is blown you will not hear the tone.
• Always turn the power off to the device you are removing the fuse from BEFORE you remove it.
• Be sure to turn your VOM or DMM off between uses to avoid wearing out the battery.