How To Test A Fuse With A Multimeter (4 Step Guide)

We will get into the detail on how we will test the fuses shortly. But, before we get into that please read the table of contents below.

What You Need To Know About How To Test A Fuse With A Multimeter

To test a fuse effectively is not very difficult as you will see in the forth coming section. In fact its very common for electricians and mechanics to perform the tests I am about to describe below. 

Before we delve it it -  worth noting and understanding the different kinds of fuses, how they work and what typically cause them to fail.

This understanding will help you in keeping your fuses healthy, and better equip you to test them effectively with a multimeter to diagnose any faults.

Different types of fuses

There are so many different types, so I will touch on the most popular you will come across when testing with a multimeter.

Plug Fuse

These types of fuses are typical in standard electrical circuit fuse boards. There are primarily two types that are used.

Namely, the Edison base (Type T fuse) and the rejection base type (Type S fuse).

What is interesting about these types of fuses is that they are correlated with a specific type of socket.

For Edison base type T fuses, you can interchange them between different sockets.

The rejection base type fuse (The Type S) can only work with a socket of the similar current rating as the fuse.

So for example, if you use a 20A Type S fuse, you will only be able to connect it with a 20A socket.

This additional layer of protection on the rejection base type fuses ensures that there is no mis-match between the plug fuse and the socket.

Mismatch creates two problems.

First – the plug fuse is under-rated as compared to the rated current through the socket. This causes nuisance tripping during normal operating conditions. When the fuse should actual not trip at all.

So to put this in an example imagine connecting a 15A fuse to a 20 A socket. This fuse will blow even when there is nominal steady state current throughout the circuit. This is obviously not good.

Second – If the fuse is oversized for the actual circuit, you could find yourself in a situation where, if there is a fault that occurs, the fuse will not blow in time and this could lead overload on the conductors, possibly causing a fire.

Therefore, That is why Type S fuses are considered as safer options as opposed to the Type T, because they will not fit into a socket that is a different size to the current rating of the fuse.

Read More:>>> Find Perfect Inexpensive Multimeter to test fuses with.

Automotive Fuse

These type of fuses are primarily there to protect the electrical circuitry of your vehicle. Since automobiles primary use Direct Current (DC), all the car’s fuses especially the blade type fuses operate of DC current.

Automotive fuses are typically rated between 30 to 80 Volts and generally have a current rating from 0.5 to about 500 Amps.

The reason that these ranges are so wide, is simply because there are so many different types of automobile fuses, such as the blade type, torpedo fuse and the strip fuse.

Fundamentally all of these fuses practically work the same way. So when testing them with a multimeter for functionality there wont be any major differences.

Glass Type Fuse

These are the most common type of fuses that you would find in consumer electronics. Especially in your musical amplifiers or pre amps.

At first glance, these fuses look the same however you would be interested to know that there are two types:

• Fast acting fuses; and
• Slow acting fuses.

It takes a bit of practice, to know which one is which, but if you take a super close look at the internal wire, you will see that for fuses that are fast blow, you will have a thin straight wire.

Whilst for fuses that are slow blow, you will have a wire that is more coiled.

Not to get into too much more detail, but the ratings of the fast and slow type fuse, is dependent on the inrush current that the fuse will be exposed to in the beginning of a circuit's operation.

So although a fuse is fast acting, there is still a slightly delay (in milliseconds) to ensure that the fuse does not blow for the normal inrush current.

Read More:>>> Learn how to bias tube amp with a multimeter

Equipment you will need to learn How To Test A Fuse With A Multimeter

To be able to test your fuse correctly, you will need:

• A simple multimeter that is capable of measuring resistance.
• Fuses (Obviously), that you are trying to test.
• A clothe/paper towel (not so obvious).

Step by Step on How To Test A Fuse With A Multimeter

Step 1 – Setup area to test fuses

The very first step involves setting up the area for testing correctly. By this point you would have safely removed the fuses from which ever circuit it was connected.

Place the cloth/paper towel on a flat surface such as a table.

The reason we do this is because we are trying to ensure that as we measure with the multimeter there is no interference between the measurement points, the surface and our readings.

This step might sound a bit odd or weird, but you will not believe how measuring midget or inline fuses particular on a metal table can produce results of continuity when in actual fact the fuse has blow.

So in essence the table cloth acts as insulation between the flat surface and the measurement terminals of the fuse.

Step 2: Set Multimeter to Ohm measurement. 

Now that you have setup to measure your fuse, setup the multimeter to measure ohms. Multimeter dials can be autoranging or manual ranging. If your multimeter is of the manual ranging type, set it to about 20KOhm measurement.

In the case that the multimeter is of the autoranging type, set it up for Ohm readings.

Step 3: Ensure continuity function works

Once you have setup you multimeter correctly, you should see a number on the screen, most modern digital multimeter show a 1 or the symbol OL (open circuit).

This practically means that the measurement is completely open circuit. To test if the ohm reading works touch the multimeter leads together.

You should get a reading of 0.00 ohms or something very close to that.

Upon completion of this test and step, you will then know that your leads and multimeter are functional and you can now go ahead onto Step 4.

Step 4: Test Ohm readings of the fuses.

This is the most simple step of them all. A fuse no matter the type or make has two opposite terminals

Connect the multimeter leads to both of these terminals.

If you get a reading of 0.00 Ohms or something very close to zero, then you can rest assured that your fuse is operating and still functional.

If you get a high reading or a reading of 1 – this is a good indication that the fuse has blown and needs to be replaced.

Sometimes you might get unstable readings from your multimeter. This tends to happen when the multimeter either hasn’t been calibrated in a while, or it is very old and no longer functions as it should.

In this case restart the multimeter and measure again. This should usually clarify the results.

And that is it!

This is a very simple test to undertake, and usually it is black or white (i.e. the fuse is either completely blown or it is functioning. You will never get anything else in between).

Extra tips on How To Test A Fuse With A Multimeter

Do not put your fuse on a metal/alimunium table

I mentioned this extra tip in one of the steps above, but it is worth reiterating.

You wont believe how frustrating it can be to misdiagnose a faulty fuse as a functioning one due the fact that the contact had continuity but not through the wire.

This happens so often that it is not funny, even seasoned DIYers and electronic practitioners can often forget this small detail.

To avoid this frustration do not place your fuse on a metal table. Use the cloth/paper towel as insulation.

Disconnect the fuse from the circuit you are measuring

This is very important in terms of safety. 

Never test a fuse over a live circuit. Always remove the fuse and perform measurements of the device in isolation.

Testing over a live circuit could cause current spikes and surges through your multimeter and depending on it’s safety rating you could damage the multimeter all together.

Read More:>>> Find the best pocket multimeters perfect for testing fuses

Final Thought on How To Test A Fuse With A Multimeter

I hope that this guide has given you clarity on how to go about testing a fuse with a multimeter.

It is not difficult to do, but there are a couple of simple pitfalls that you should avoid whilst performing this test to ensure that you do not go through the typical frustrations that most people face when performing this test for the first time.