How To Tell If a Fuse is Blown

When an electrical device in your home or vehicle stops working, you almost immediately think that a major wiring or component in it is damaged.You fear that you may need to spend big to repair it or even replace the whole unit. 

On the brighter side, a blown fuse may just be the cause of your problems. Having a blown fuse means you simply install a replacement and your device starts working again.

Our blog post seeks to comprehensively show you how to identify if a fuse is blown, so you never worry about simple problems.

Let’s get right in.

blown fuse

How To Tell If a Fuse is Blown

Checking the state of a fuse depends on the type it is.  For transparent fuses, you visually inspect whether the metal wire in it is broken or melted. With others, you may search around for dark burn marks. The most accurate method of testing a fuse is to use a multimeter to check its continuity.

how to tell if a fuse is blown

To know how to check properly, you need to have a basic understanding of how electrical fuses in your home work. They have a wire inside that melts or blows out when excess power passes through it, which discontinues the electrical pathway.

This is the general principle used in ensuring the safety of other components. 

Now, there are different types, with each coming in its specific shape and having its own safety mechanism. The most basic types of electrical fuses used in your home are cartridge fuses. 

Cartridge fuses have an internal thin strip, wire, or “link” that makes contact with their two ends. When there is excess power, the wire melts or pops, stopping current from going through since there is a break in the circuit.

  1. Cartridge Fuse Visual Inspection

If the electrical fuse in your home is transparent, you may simply visually inspect it to see whether the link has melted or there is a break in it.

Sometimes, it may look foggy inside due to the smoke when melting or have a dark brown stain from burning out or blowing. 

In the case that it isn’t transparent, this dark stain may seep out from the ends or even break the cartridge container.

cartridge fuse visual inspection

For the cartridge types common in your home, these are the only visual pointers that help to identify if they are blown or not.

  1. Testing Cartridge Fuse With a Multimeter

The most accurate method of telling if fuses are damaged or not is to test it with a multimeter. Here, you will be testing for continuity between its two ends. 

Remember that the link connects the two ends and melts when there is an overcurrent. At this point, there is no continuity between its two ends and the multimeter can help to quickly and easily identify this.

testing cartridge fuse with a multimeter

Check out our complete blog guide on testing fuses with a multimeter from the comfort of your home. 

Electrical fuses used in cars are a little more common and deserve a whole section of their own. 

How To Tell If a Car Fuse is Blown

To diagnose a car fuse, you simply take it out of the box and look through the fuse’s plastic cover. If the link inside the plastic looks broken or there are dark marks or metal residue in it, then the fuse is blown. You may also use a multimeter to check for continuity between the blade terminals.

how to tell if a car fuse is blown

The type used in cars is also called an automotive, blade, or knife fuse. These fuses have a peculiar look that features two short blade terminals at both ends plugged into a box.

The automotive types are designed in a way that they are easy to remove from the car. 

When you suspect a device in your car to be faulty due to a damaged electrical fuse, it is good to check your car manual to identify the exact type that works with it.

This is because picking it out may be difficult as you have multiple identical automotive fuses plugged into the same box. 

  1. Car Fuse Visual Inspection

Once you identify the exact unit you need to check, you pull it out of its slot. Although car fuses are covered in colored plastic, they are still fairly transparent.

The link is typically a flat solid metal and when it blows out, the short gap created is also visible.

Closely check the transparent plastic for any break in the link, foggy appearance, or any dark marks. A burning link may cause these. You may also see some residue in the plastic, which is part of the link that broke off.

car fuse visual inspection
  1. Testing Car Fuse With a Multimeter

Just like the cartridge types, however, the multimeter is also the most accurate tool to use to diagnose the blade types for faults. Run a continuity test between the two blade terminals to see if the link is broken or not.

If the multimeter does not beep, it is bad and you need to replace it.

testing car fuse with a multimeter

Sometimes, testing other types of electrical fuses with a multimeter may not be so easy. Thankfully, these different types typically have specific visual pointers that help you know whether they are blown or not.  

For example, a drop-out type has a holder that disconnects from a contact and drops away from the body when the link is blown. A striker fuse, on the other hand, shoots out a pin when triggered.

Check out the specific type you have installed and see if you may test it with a multimeter or if there are visual indications that point to a problem.

What Causes a Fuse To Blow

A fuse blows when there is more current or voltage flowing through it than it is rated for. Electrical or mechanical faults may cause this overcurrent in the circuit, and examples include grounding faults, short circuits, arc faults, wiring faults, or design faults.

what causes a fuse to blow

Whether in your cars or devices in your home, a damaged electrical fuse is a common signal to a deeper problem centering around overcurrent. It is an indication that the current or voltage supplied is experiencing a spike due to some electrical or mechanical faults. 

For example, it may blow due to an overload. This overload in the circuit may have been due to causes like the hot and neutral wiring coming into contact or too many devices drawing power through the electrical fuse. In these cases, you repair the circuit by finding bad wiring or removing devices from it. 

Electrical fuses could also blow if misdirected live wiring touching a conductive surface causes a short circuit or ground fault. You find this and apply the necessary fix. 

The bottom line of these is that when you see problems with an electrical fuse, you try to figure out the cause of the overcurrent that made it blow in the first place. This is the only way to properly solve any problem related to it, rather than just finding a replacement. 

Replacing a Car Fuse

When you need to replace a faulty car fuse (or any other type in your home), always make sure that the replacement has the same features and specifications as the old auto fuse.

This means the new unit should be an auto fuse with the same size, current, and voltage rating as the old auto fuse. 

What happens if it does not have the same rating?

Well, in the more subtle case, if the replacement is of a lower rating, it blows when power passes through it. Your devices are still in safety here. 

If the replacement is of a higher rating, however, it allows more power than usual to pass through it. When there is a spike, the device it protects gets damaged from overcurrent. You see that your device is not protected here.

This is why it is important to go for the same type of electrical fuse.  

Also, make sure the plastic cover of the replacement is the same color as the cover of the old fuse. This makes it easier to follow your guides in case you need to replace another unit in the future. 

Thankfully, you don’t have to worry too much about replacing one in your vehicle all the time. Vehicle fuses are designed to last up to 30 or even 40 years. 

When one blows, however, make sure you do more than just change it. A damaged electrical unit represents a larger problem in the vehicle and you find a way to repair that also. 

Video Guide

Fuse Safety Tips

Remember that even with blown fuses, circuits still have current in them. The fuses only disrupt the electrical pathway. Because of this, ensure that all the appliances and even the power source to the entire circuit are switched off before making a replacement.

This lets you avoid electrical shocks. Also, make sure the replacement doesn’t fit loosely in the circuit to avoid overheating.

Here you can find more fuse tips.


Can You Visually See a Blown Fuse?

Yes, you can visually spot whether a fuse is blown especially if it is transparent. You check if the link is broken or whether there is metal residue in it. You may also search for dark burn marks.

What Happens When a Fuse Blows Out?

When a fuse blows, the device it is connected to stops working. This is because the fuse interrupts the path through which current flows to the device when blown. The circuit is broken at this point.

Alex Klein Author


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.