How to test a fuse with a multimeter?

What do you need to know about electrical fuses?

According to Aaron Barnett of BangingToolbox: Electrical fuses are safety devices that cut off the electrical connection in a circuit during an overcurrent event, usually caused by an overload or short circuit.

Fuses ensure that the electrical wiring only carries a safe amount of current to prevent overheating which can cause fires.

Fuses come in different shapes and sizes and are made from different materials depending on the application, with each rated for a specific operating voltage, maximum current, and the amount of time it takes to blow (speed).

What are the common fuses that needed testing?

Before we learn how to test a fuse with a multimeter, let’s find out the different types of fuses.

1. Glass fuse

A glass fuse is the most common type of fuse and has a glass body with two metal end-caps acting as the contacts.

A thin metal wire inside the fuse connects the two caps and melts if energy reaches the maximum rated current, protecting the circuit and electronics in the case of excessive current from a surge.

2. Old houses fuse

Not to be confused with a circuit breaker, (Below) these are more common in older houses with a dedicated fuse box.

The glass body is replaced with ceramic, allowing the fuse to survive higher temperatures and therefore have a higher current rating flow on a circuit.

3. Car fuses or RV fuses

Unlike the first two, which are both a type of cartridge fuse, a car or automotive fuse is smaller and thinner, with plastic housing and two metal blades acting as contacts.

Car fuses are normal of course found inside vehicles and have a lower voltage range (up to 32 volts).

4. Thermal fuse

A thermal fuse is a special kind of fuse, that reacts to heat instead of current.

Thermal fuses come in handy with heating appliances to ensure that the appliances don’t overheat and cause fires.

5. Home circuit breaker

Most homes will have a circuit breaker that acts as a fuse except that the device can manually be reset without a replacement on a breaker.

If the circuit breaker is cutting off even with a low current draw, maybe there is something wrong with your circuit and you need an electrician to have a look, or maybe the breaker is getting old and too sensitive.

Test with a different breaker in the place of the faulty one rated for the same amps and volts.

6. A circuit breaker with an RCD

Modern homes can have RCDS built into the circuit breaker.

An RCD will be displayed on the breaker works to additionally trip and stop the flow of electricity if the device detects possible shorts, rather than just surges in current.

Rcds in a circuit breaker makes the wiring in your home safer by reducing the chance of electrocution.

What are the risks of a blown fuses?

Since fuses are basically ‘sacrificial’ devices, there’s nothing inherently dangerous about a fuse after they’ve blown, as the technology breaks as the weak point in a circuit to avoid anything else getting damaged.

However, if the circuit that the fuse is protecting is operating and safety-critical things like the headlights of a car, you may need to find the blown fuse and replace the fuse immediately so that the circuit and the appliances are operational again before driving.

The thing to worry about is whatever might have caused the fuse to blow in the first place.

Inspect the wiring that the blown fuse connects to for shorts or loose connections and fix everything by a professional electrician before replacing the fuse.

All you need for testing is a digital multimeter in your tool backpack or sitting in your tool belt.

Possibly the fuse that was installed was too small for the expected current, additionally, some fuses can wear down over time and handleless current before blowing.

For example, solar fuses might need replacing every few years.

What is a multi-meter and what settings should you know to test a fuse?

A multimeter is a general-purpose tool used in electrical work.

You can measure the properties of an electrical circuit with a simple tool and the details like electrical connection, voltage, current, and resistance.

Did I mention that you can also use a multimeter to measure the capacity of a battery too?

Depending on the model and type you use, using a multimeter may seem daunting at first with all the settings and dials, but I find multimeters quite easy to use.

Up top, we have a screen that shows the value of whatever you are measuring. Below that we have some buttons is the main dial where you can choose what you will be measuring for.

At the very bottom are some input terminals where you plug in two probes that will be used to make contact with the electrical circuit or device you are checking.

For multimeter fuse testing, there are only two things that you should be measuring:

Continuity

This simply refers to the presence of a first class connection between two points in a circuit [1]. In this case, the two contacts of a fuse. This can be measured by setting the dial of the multimeter to a symbol that looks like a diode.

If you don’t know how to test a fuse for continuity with a multimeter, you’ll have to find the diode-like symbol on the multimeter.

If you’re unfamiliar, this is the symbol with a triangle pointing to the right with a vertical line.

Multimeters usually make a loud beep depending on sound settings when the device detects continuity.

Resistance

Measured in ohms (Ω), resistance refers to how much a wire or component prevents current from flowing.

A resistance of 0 ohms signifies a first class electrical connection – which is what you’d want to see in any kind of fuse.

Visual testing of fuses

Before we get into the nitty-gritty of testing fuses with a multimeter, know that testing with a visual inspection can be an easier and quicker alternative.

Not all fuses can be tested by just looking at the outside surface though, as not all cases are see-through.

Only glass cartridges and automotive fuses with transparent plastic bodies can be visually tested.

Fuses that are in good condition have a clean and clear casing and the metal wire inside is intact.

Meanwhile, the casing of a blown fuse would have visible charring and discoloration. The metal wire within would also appear to have been severed or melted off.

This kind of test can be somewhat unreliable at times though, as some fuses can have discoloration and char even in normal conditions due to heat. This could prevent you from determining the actual state of the metal wire inside.

How to test an old house fuse with a multimeter?

1. First, check that your fuse isn’t just a breaker or RCD that can just be reset.
2. Get an electrician if you suspect that there is a fault in your wiring.
3. Cut off the power or turn off the breaker connecting the supply to the fuse.
4. Open the fuse box and remove the house fuse within.
5. Set up the multimeter. Connect the probes and set the dial to test for continuity.
6. Touch each probe on one of the two fuse contacts. Make sure not to let both probe tips touch each other [2].
7. If the fuse is not blown, or if the breaker is reset to have a connection the multimeter will emit a loud beep and will display the measured resistance of the metal wire inside. Else, if there is no sound and the multimeter can’t display the resistance value, then this means that the fuse has blown and needs replacing or the breaker hasn’t been reset.
8. However, if you are testing a breaker or RCD, there could be good connectivity with a multimeter but blowing happens because too much current has already damaged the breaker with age or bad circuitry causing the RCD to be overly sensitive to a normal flow of electricity.

How to test a car fuse with a multimeter?

1. Remove the fuse from the holder. Consult the manual from your car’s manufacturer to find where the fuses are. You don’t necessarily need to disconnect the power from your car’s battery but you can if you like.
2. Set up the multimeter. Connect the probes and set the dial to test for continuity.
3. Touch each probe on one of the fuse’s blades/contacts.
4. If the fuse is not blown, the multimeter will emit a loud beep and will display the measured resistance of the metal wire inside. Else, if there is no sound and the multimeter can’t display the resistance value, then the blown fuse needs replacing.

How to test a thermal fuse with a digital multimeter?

1. Make sure that the appliance is unpowered and unplugged before removing the thermal fuse from the holder.
2. Set up the multimeter. Connect the probes and set the dial to test for continuity.
3. Touch each probe on one of the fuse’s leads or contacts.
4. Just like with the previous types, a beeping multimeter is a first class sign that the fuse is fine. Otherwise, the fuse has blown and needs replacing. A thermal fuse may look black or burnt additionally if the component is blown.

Analog multimeter vs digital multimeter

You might be scratching your head when you’ve read that a multimeter has a screen that displays numbers while the multimeter you have available doesn’t.

It’s highly likely that you have a analog multimeter that is operated mostly the same way as a digital multimeter but displays values differently.

Instead of a screen, an analog multimeter has a needle that moves along markers to display the measured value. 

The measurement we’re concerned with here would be resistance, which is usually the topmost marker of the display and is labeled with the symbol Ω.

How to test a fuse with an analog multimeter?

Testing a fuse with an analog multimeter is a bit different since this type does not have a continuity test mode. Instead of that, you will be checking whether or not the resistance between the contacts is 0.

To do that, set the dial to measure resistance (Ω) and use the lowest possible setting with a 1x multiplier.

Touch the ends of each probe on each of the fuse contacts. If the needle moves all the way to the right (which is the 0 value), the fuse is in good condition. Otherwise, the fuse is blown or damaged and would need replacing.

How to replace a fuse?

When dealing with high-voltage circuits, especially for house fuses, disconnect the power from the fuse box before proceeding with the replacement.

You must also use the same type of fuse when replacing blown-out ones.

The replacement fuse must be close to the specification and rating of the old one.

A fuse needs to be the weak point in your circuit and handle less current than the wiring.

If you don’t know what the fuse parameters are for any reason you can do the following to get the right size:

1. Check the wire diameter or gauge.

You need to find a wire ampacity chart that you can find online to see how much current can safely carry without heating up too much.

Size your fuses accordingly, and check with a qualified electrician.

2. To determine the speed of the fuse, determine the application.

Devices and appliances with motors usually experience short current spikes during startup and a slow blow fuse is typically used with them.

Other applications such as household wiring and sensitive devices almost always use a fast-blowing fuse for greater safety.

Never ever replace a fuse with something sized larger than what was used before.

You should not also give in to the temptation of using a wrench or anything made out of thick metal as a short-term substitute while you go out to buy a new fuse.

This is dangerous and can lead to the wires connected to the fuse drawing too much current, heating up, and potentially starting a fire.

Safety is priority.

Fuses are essential safety devices that should be present in every electrical system. And are mainly used to prevent fires by keeping wires from overheating from too much current during overload and short circuit events.

Therefore, testing to make sure that fuses are the right size and installed in good condition is critical.

Replacing fuses when old, goes a long way in ensuring fuses don’t interrupt your TV show and give you safety and peace of mind with the future use of your electrical system.

While electricians can’t work without a multimeter, you can surely gift something else to an auto technician or electrician.

Hopefully, if something goes wrong with any of your electrical systems, you’ll know exactly what to look out for. If you know the culprit is a bad fuse. Make sure to check for an authentic tool for accurate measurements.

Till next time let the energy flow throughout your circuits.