How To Test A Fuse With A Multimeter
Electrical circuits are peculiar in the sense that its the smallest components within them that play the biggest roles.
The fuse is one of these small components, serving as a self-sacrifice that prevents unexpected power surges from rendering the entire circuit useless.
Is a device within your home or car not receiving power? Do you suspect that the problem is from the fuse box? How do you know if a fuse hasn’t blown, which could be causing your problem?
In just a few simple steps, you will learn how to test a fuse with a multimeter from this guide.
Let’s get right in.
How Does A Fuse Work?
Fuses are simple components designed to protect electrical circuits from power surges or overloads.
They mainly consist of a tiny metal string or wire that melts or “blows” when excess current passes through it. The current a fuse may hold is known as its ampere rating, which goes from as low as 10A up to 6000A.
The most common type of fuse found in various electrical devices is the cartridge fuse, which is cylindrically shaped, usually transparent, and comes with two metal tabs at both ends.
The metal string is located inside it, connects these two tabs, and burns out from excess current to prevent electrical flow between them.
Tools Required To Test A Fuse
To test a fuse, you need:
- Fuse puller for car fuses
How To Test A Fuse With A Multimeter
Set the multimeter to the 200 Ohm range to measure resistance, place the negative and positive multimeter probes on each end of the fuse, and wait for a reading of zero (0) or a value close to zero, which means it is good. If you get an “OL” reading, then the fuse is bad and needs to be replaced.
We will go into each of these steps, as well as every other important step in detail.
- Take Out The Fuse
The first step is to take the fuse out of the electrical circuit it is in. Of course, how you take the fuse out depends on the electrical circuit, device, or type of fuse it is.
Regardless, you want to disconnect the power supply before doing this to avoid fatal shock hazards. You should also be careful while taking the fuse out so you don’t damage it.
- Set The Multimeter To Ohms
Testing fuses for faults requires you to test their resistance. To measure resistance using the multimeter, you turn its dial to the Ohms setting.
The Ohms setting is represented by the Omega Symbol (Ω) on the multimeter and, as you will see, also has multiple ranges (2MΩ, 200kΩ, 20kΩ, 2kΩ, and 200Ω).
The 200 Ohm limit is the appropriate range you set the multimeter to, as this is the closest higher range that gives the most accurate result.
Alternatively, you may also set the multimeter to continuity mode, which is usually represented by a soundwave symbol.
Now, although the continuity mode is also appropriate for checking whether the metal string is broken or not, it doesn’t provide you with a detailed diagnosis.
The Ohm setting is the best as it tells you whether the fuse is bad even when the metal string isn’t broken. Give the Ohm setting priority.
To check whether the multimeter is appropriately set, place the positive and negative leads on each other.
If set right, you get a value of zero (0) or close to it with the Ohms setting or hear a beep from the multimeter in the continuity mode. If you get these, proceed to the next step.
- Place Multimeter Probes On Each End Of The Fuse
Here, you simply place the multimeter leads on each contact end of the fuse, regardless of any polarity.
Measuring resistance doesn’t require you to strictly place the positive or negative lead on a particular end, so you don’t have to worry about this. Once the leads have made proper contact, check the meter screen for a reading.
- Evaluate Results
The results are pretty much straightforward. With the continuity mode, if the multimeter beeps, then there is continuity between the two multimeter leads (the metal string is good). If you didn’t get a beep, the fuse has blown and should be replaced.
However, even when the multimeter produces a beep, the metal string may still have some defects, and this is where the resistance test is useful.
If the multimeter is in the Ohm setting, good fuses are expected to present you with a resistance value of zero (0) or close to zero.
This means there is an uninterrupted path between the two multimeter leads (the metal string is still good), and additionally tells you that current can easily flow through it when required.
A value higher than 1 means there is too much resistance within the fuse, which could be why enough current isn’t flowing through it.
The multimeter may also present you with “OL”, signaling that there is no continuity within the fuse at all (the metal string is blown) and the fuse should be changed.
Testing Car Fuses With A Multimeter
Car fuses are peculiarly shaped, as they have “blades” rather than tabs on the two sides of them. They also last longer than regular fuses and are located in a fuse box.
To test a car fuse, make sure the vehicle is switched off, check your vehicle fuse diagram to locate the particular fuse for the device that isn’t working, then take out the fuse using a fuse puller.
Now, you visually inspect it for any dark spots signaling a burnt or blown fuse, or try to check whether the string is broken, in case the fuse is transparent. These signal a bad fuse that should be replaced.
In case you don’t find anything wrong after a visual inspection, proceed with the regular multimeter fuse check process. Set the meter to the 200 Ohm range, place the multimeter probes on the two “blade” contact ends of the fuse, and check the screen for a value after making proper contact.
If you get zero, a value close to zero, or a beep, the fuse is in good condition. An “OL” reading or any other value means the fuse is bad and should be replaced.
Lastly, when replacing fuses, make sure you use a new fuse with the same ampere rating as the faulty fuse. You do this to avoid installing a fuse that allows more current to go through than needed, which may damage the device or circuit it is meant to protect.
You can find this whole process in our video guide:
Although you may test a fuse without a multimeter, the digital multimeter is the easiest way to identify if a fuse is bad. It is also useful for making other electrical diagnoses.
Testing fuses with the use of a multimeter is one of the most straightforward electrical diagnosis procedures as long as you follow our tips. You simply place the multimeter probes on each end and wait for a beep or value close to zero.
Remember to take out the fuse from the electrical device before testing and also replace a bad fuse with one with the same ampere rating.
How Can I Tell If A Fuse Is Bad?
A fuse is bad if you detect that its string is broken or there’s a dark spot on it, signaling a burn. You also use a multimeter to measure its continuity or resistance, which should be close to zero.
What Is The Most Common Cause Of A Blown Meter Fuse?
The most common cause of blown fuses is circuit overloading. A circuit overload is when there are too many appliances drawing current within the circuit through the fuse, causing its string to burn.
Alex Klein is an electrical engineer with more than 15 years of expertise. He is the host of the Electro University YouTube channel, which has thousands of subscribers.