Fuses are an important part of any electrical system, and they serve to protect the wiring from over-currents and faults. When a fuse blows or is removed, it can be difficult to tell its condition. This article discusses different aspects of a fuse, including testing if it’s blown and how to change it.
If you may have read about circuit breakers, you know that they serve a similar purpose to fuses – but what differentiates the two? We’ll discuss that too.
What is a fuse?
In electrical systems, a fuse is a component that provides overcurrent protection. It’s essentially a sacrificial part of the system – when it blows, the circuit is interrupted, and power is no longer supplied across the fuse.
Fuses are available in different sizes and ratings, depending on the amount of current they’re designed to handle. They are also classified in line with other factors like response time, breaking capacity, and the specific application of the fuse.
The first person to patent a fuse was Thomas Edison in 1890. Edison’s fuse was a simple piece of wire that connected the live power line to the neutral, and when too much current passed
How does a fuse work?
The construction of a fuse is simple, but it has an essential function.
A fuse is made up of a low-resistance wire in a noncombustible casing. The wire protrudes from both sides. The fuse is then connected in series to load and the circuitry. This load is usually an electrical system or appliance that needs to be protected from current surges caused by faults as short circuits. Such devices would be damaged if a high current flowed through them, so the fuse protects them from this.
In regular operation of the circuit, there is no problem, and the current flows through the fuse wire. But if there is a fault, such as an overload or short circuit, then too much current will flow through the wire.
This will heat the wire, causing it to melt and interrupt power flow across the fuse, thus protecting electrical appliances from overcurrent damage.
The fuse wire is sized according to the maximum current that it can safely handle without melting. It also has a meager resistance so that it doesn’t affect the normal operation of the connected (or fused) circuit.
Once it melts, the fuse is deactivated and must be replaced by a new one if needed.
Different Types of Fuses
There are different fuses available, but the basic idea is that when too much current passes through, it will blow and interrupt the circuit.
Fuses can be further classified by their physical size, response time, breaking capacity, and application. Fuses may be classified into four broad categories, namely:
- One-time only fuse
- Resettable fuse
- Current-limiting fuse
- Non-current-limiting fuse
One-time only fuse
These are the basic and the most common type of fuses. These are designed to blow once, and then it can’t be reset back (hence the name). They are cheap and easy to implement.
Also known as polymeric positive temperature coefficient device (PPTC, resettable fuses can be reset after ‘blowing’. These devices are designed to automatically open a circuit when the current exceeds a predefined limit.
These are designed to reduce the duration and magnitude of over-currents. They achieve this by producing a high resistance when an overcurrent flows.
Non-current limiting fuse
This type of fuse produces an electrical arc whenever an overcurrent flows. The arc yields a high amount of heat, which increases the resistance of the fuse and thus interrupts the current flow.
This device responds sluggishly, given that it must allow excess current to build an arc before interrupting the circuit.
Other types of fuses
Here are some of the most common types of fuses:
These are used in direct current (DC) circuits. They are usually larger than the AC fuses because the constant current value produces an arc that is not easy to extinguish. Their larger size accommodates this.
These fuses are more miniature. This is because an AC reverses direction depending on the rated frequency. This makes the arc less likely to be sustained.
This low-voltage cylindrical fuse uses a metal strip as the contact. It’s inserted into a holder, which makes it easy to change.
Also known as a spade or plug-in fuses, these are cartridges that have two blade terminals. These fuses can be inserted directly into the fuse holder and don’t require a holder like a cartridge fuse.
This type of blade fuse has copper lugs attached to the ends. This allows faster and secure connection and disconnection with high-current loads.
These fuses can be replaced or repaired. They are usually pre-connected to the wires for easy replacement, and they’re readily available in standard sizes.
Striker type fuse
This fuse doesn’t have a physical fuse element. Instead, it uses an electronic chip to detect an overcurrent and interrupt the circuit.
Drop out fuse
This fuse is used in transmission cables to protect outdoor transformers. When the element melts from current overload, it drops under gravity to a lower position, thus breaking the contact.
HRC fuses are short-term, high-current interrupting devices that may be used in the place of circuit breakers in low and medium-voltage networks. HRC fuses operate on drop-out principle and include a fuse holder and a fuse strip made from aluminum or copper.
How to tell if a fuse is blown?
The most straightforward way to tell whether a fuse is blown or not is to look at it. If the fuse appears burnt or broken, then the chances are that it’s no longer working. The switch is usually somewhere you can pop it out in most cases.
If you must remove the cover with a screwdriver, switch off the supply circuit at the mains or disconnect the appliance. Unscrew the fuse holder cap and inspect the fuse. You may notice a black metallic residue inside or see a broken wire pointing to a blown fuse.
If you cannot decide if the fuse is blown by looking at it, then it is time to test it with a multimeter.
How to test a fuse with a multimeter?
This is the foolproof way to test a fuse and determine if it’s blown or not. The process is simple:
- Switch off the current at the mains and disconnect the appliance.
- Remove the fuse from the appliance
- Use a multimeter and set it to measure continuity.
- Make sure that your probes are working by putting the negative and positive leads together and seeing if the meter beeps to indicate continuity.
- Place one probe on each end of the fuse. It doesn’t matter which end of the fuse touches the positive or negative terminal.
- Place the other probe on the other end of the fuse.
- If the meter beeps and shows continuity, then chances are that your fuse is intact. Otherwise, it’s blown.
You can also use the resistance setting on the multimeter to test a fuse. This will tell you how much resistance is present in the fuse. A blown fuse will have infinite resistance or ‘OL’ value.
How to change a fuse?
Replacing a fuse is a relatively simple process. You’ll need:
- A new fuse of the same rating as the blown one
- A screwdriver
If the fuse you are changing is housed in a fuse box or electrical panel, turn off the power first.
- Use your screwdriver to remove the cover of the fuse holder
- Use your pliers to pull out the blown fuse. Some fuses can pull out with just your hands.
- Insert a new fuse of the same rating as the blown one.
- Make sure that the fuse is correctly seated into its holder
- Turn on power to the circuit again.
- Turn the appliance on to check if the fuse is conducting current. If not, check the new fuse with a multimeter to ensure it is working.
Some fuses are hard to change, especially those found on motherboards and circuit boards. These fuses are best left for technicians to troubleshoot and replace.
Difference between a fuse and a circuit breaker?
Fuses and circuit breakers both protect electrical circuits from overloads and short-circuits. Both devices interrupt the circuit when they detect an abnormal current flow but do so differently.
Here are the key differences between fuses and circuit breakers:
A circuit breaker is a device that uses a bimetallic strip to cut off the flow of electricity when it detects an overload. Other circuit breakers use electromagnetism to break the circuit.
On the other hand, a fuse is a device that uses a thin strip of metal to melt and separate when it detects a current flow that exceeds the rating.
A fuse operates by letting current flow through it until it reaches its rated value, and then it melts, interrupting the circuit. On the other hand, a breaker interrupts the current as soon as the higher current induces a high electromagnetic force that breaks the circuit. Bimetallic strip circuit breakers depend on the heating effect that a current has on two metals to create this force (from unequal expansion).
Number of breaks
Most conventional fuses can only be used once, and then they need to be replaced. On the contrary, a circuit breaker, can be used multiple times before it needs to be reset.
A circuit breaker can be set to on/off, making it usable as a switch, whereas a fuse can only be used, good or blown with no switching between.
The characteristic curve of a fuse will change with time as the fuse and element age. A circuit breaker’s characteristic curve, on the other hand, will not change.
Fuses are more reliable than circuit breakers as they have no moving parts. Circuit breakers can fail due to multiple reasons, such as dirt accumulation.
Fuses operate at lower voltages, typically in homes, electronics, and appliances, whereas circuit breakers operate at higher voltages, such as those found in industrial and commercial electrical systems.
Type of protection
Circuit breakers guard appliances against short circuits and overloads, while fuses guard only against overloads.
Most fuses do not have a status indicator, whereas circuit breakers typically have a green or red light to indicate whether the breaker is on or off.
Fuses are set-and-forget devices and do not require regular maintenance, whereas circuit breakers often need to be serviced.
A fuse operates within a very short time, usually 0.002 seconds, to interrupt the circuit. Depending on the design, circuit breakers can take about 0.02 -0.05 seconds to activate.
Fuses are typically less expensive than circuit breakers.
Fuses are typically more miniature in size than circuit breakers, making them more suitable for applications with space limitations.
Fuses have no adjustability, whereas a circuit breaker can be reset to resume its function after being tripped.
A fuse will eventually wear out and need to be replaced, whereas a circuit breaker can serve the system for many years.
Advantages of using a fuse over a circuit breaker?
Ease of use and replacement
Fuses are easy to use and do not require any adjustment. Also, you don’t need to have a lot of electrical knowledge to install or replace a blown-out fuse. Circuit breakers require more expertise and attention.
Shorter response time
Fuses respond very quickly to over-currents and interrupt the circuit almost immediately. Circuit breakers can take a fraction of a second longer to detect and interrupt the circuit.
Less initial cost
Fuses are typically less expensive than circuit breakers, making them a more economical choice for some applications.
Less wear and tear
Since fuses have no moving parts, they are less likely to wear out and need replacement. Circuit breakers have moving parts that can cause premature failure over time.
Smaller footprint/space requirement
Fuses may take up less space than circuit breakers, making them ideal for applications with limited space. Circuit breakers are often bigger and require more room.
No regular maintenance
Since fuses do not require regular maintenance, they are a low-maintenance option for electrical systems. Circuit breakers often need to be serviced to keep them running correctly.
Since a fuse is either working or not working, it provides consistent protection. Circuit breakers can eventually malfunction and provide inconsistent protection.
When a fuse blows, it is cheap and easy to replace. Circuit breakers often require professional installation and can be expensive to replace.
Advantages of using a circuit breaker over a fuse?
A circuit breaker is a dead front, which means it has no exposed live parts that can come in contact with anyone or anything. Fuses have exposed live parts that can cause a shock when someone touches them if the system has been energized.
Circuit breakers are more flexible than fuses because they can be switched off, whereas fuses cannot. Circuit breakers can also be reset to resume their function after it has been tripped, which fuses cannot.
A circuit breaker may trip under either electromagnetism or if the bimetallic strip heats up. A fuse will only trip if the system has too much current flowing through it.
External status indicator
A circuit breaker will have an external indicator that tells the user if it is tripped or not. A blown fuse does not indicate whether it has blown or not–you have to inspect it manually.
It’s possible that a fuse can be replaced with a wrong one, or worse, it can be jerry-rigged. It’s improbable to replace a circuit breaker with the wrong one. It’s also not possible to jerry-rig a circuit breaker.
Circuit breakers can start up without tripping, whereas a fuse will always blow if more current is drawn than the fuse rating. This usually happens during startup. You need to oversize a fuse to avoid blowing it up when starting.
Can be used as an ON/OFF switch
Circuit breakers can serve as switches that are either ON or OFF. Fuses cannot be turned on and off. A fuse is either good (conducting) or blown(broken).
Ground fault protection
Circuit breakers can sense whether the current is going through a ground fault. They can then trip before it goes into thermal runaway and causes a fire. Fuses do not have this ability and will just blow at the first sign of overcurrent.
Circuit breakers are less likely to fail with age. Fuses can eventually become brittle and blow up in use, which is dangerous for the system.
At this point, it’s clear what a fuse is, how it works, and some of the advantages of using a fuse over a circuit breaker. Whether you use a fuse or a circuit breaker is dependent on several factors, such as the specific application, safety requirements, and budget.
In general, fuses are a good option for low-power applications where circuit breakers are overkill, and circuit breakers are a good option for high-power applications where fuses may not handle the load.