It helps power electrical and electronic devices in our homes, businesses, schools, streets, airports, and the list goes on and on. 

You might be reading this article on your mobile phone, or laptop, whose battery was charged using the awesome powers of electricity. 

However, electricity is no laughing matter. 

It can be highly dangerous to electrical and electronic devices that use it, as well as ourselves. 

So when it comes to electricity, we need to have contingency plans in place to protect the devices, as well as ourselves from its raw power.

This contingency plan comes in the form of something known as a Fuse. 

But, what does a fuse do in a circuit?

The fuse plays a vital role in electrical and electronic circuits. It’s main purpose is to fail under overcurrent conditions thereby interrupting the flow of this overcurrent which could damage electrical or electronic components.

This article will dive into more detail of what a fuse does in a circuit, as well as different applications where you find a fuse, what overcurrent is and why we need protection against overcurrent.  

What is a fuse

The fuse is an electrical device that is designed to fail when subject to excessive currents that are not nominal in that particular circuit (I say particular circuit because every circuit is different having its own current limit).

The image below shows what the inside of the fuse looks like;

The fuse has many different versions of symbols that appear in circuit diagrams and schematics. Below are some of the most common; 

You can see in the first diagram,at the heart of the fuse is the Fuse wire. 

This fuse wire is the star of the show,whose job is to melt in the event of overcurrent conditions (which shall be discussed in the next section).  

The *fuse wire is a thin wire* made of conductive material which has a low melting point (this melting point however, is not in the range of the nominal currents that it would be subject to). 

What is over-current?

Every electrical and electronic device will have a Power rating. 

This rating indicates the maximum values of Voltage and Current that devices must operate under to work effectively.

I say under because going over these values will cause damage to the device, circuit, and components. 

Over-current, is an unwanted scenario when current levels rise above their nominal levels. 

If the overcurrent lasts for a long time, it will damage components and circuits within the device.

There are multiple causes for overcurrents which include;

• Short circuits
• Excessive load
• Incorrect design
• Arc fault 
• Ground fault

What does a fuse do in a circuit?

So, we know the dangers that electrical and electronic circuits are subject to which are Overcurrents and that a fuse is a device designed to fail under high currents.

It’s a match made in heaven! 

So, what does a fuse do in a circuit?

Put simply, the fuse has one job in a circuit, which is to fail when subject to excessive currents such as in the scenario of overcurrents. 

Examples are a great way to understand new concepts so let’s take a look at a simple circuit and what a fuse does to protect it. 

Below is a circuit that has a lamp powered by a battery along with a fuse for protection. 

Remember every device has an electrical rating. So, let’s assume the ratings for each of the components;

• Battery (12V)
• Lamp (12V , 1A)
• Fuse (12V, 1A)

Under normal conditions the battery provides a stable current of 1 Amp which powers the lamp. 

Since this current value is within the rating range of the fuse, there is no interruption to flow of current. 

Now, let’s imagine that an overcurrent scenario occurs causing a rise in current as pictured below.

As you can see the current value has risen to 2 Amps. But, the lamp is only rated to handle a current of 1 Amp. This rise in current will damage the lamp if it persists for some time. 

Lucky for the lamp, the fuse is also rated at 1 Amp. This means that the little fuse wire inside the fuse is going to melt thereby stopping the flow of current and protecting the lamp. 

This is essentially what a fuse does in every circuit. Protect its peers from destruction! 

Fuses are generally connected in series with electrical and electronic components that they are protecting. 

Why does the fuse wire melt in over-current scenarios 00 

Wires come in a variety of sizes which are capable of carrying a certain amount of current. 

Wire gauge is a measurement system which indicates the diameter of the wire. The information present in a wire gauge indicates how much current a particular wire can carry, its resistance and its weight.

Without going into too much detail, when there is a flow of current a certain amount of heat is generated. 

You might have felt this when using your smartphone for long periods of time, or touching your computer CPU. 

The wire gauge system helps us select the right wire which is capable of handling a certain amount of current.

For example, say we have a 20 Gauge electrical wire. 

This wire is capable of carrying a maximum current of 3A according to the American Wire Gauge (AWG) system. 

When subject to a current of 3A or less, the electrons (current) can freely move along as the wire is able to cope with these many electrons.

If we bump up the current to say 4A, there are now more electrons, but the surface area of the wire has not changed. 

This means that electrons are now going to start bumping into each causing a greater build of friction. As you might know, a greater build of friction leads to a greater build of heat which can melt the wire and its insulation causing damage. 

The fuse wire works in the same principle. 

The size of the wire is chosen to handle a certain amount of current (and therefore heat). 

If it is subject to a higher current, the heat generated is going to melt the fuse wire. 

Different types of fuses available for circuits

Electrical and electronic circuits can be classed into two main categories; Alternating Current (AC) and Direct Current (AC).

Alternating Current circuits deal with a current that reverses direction multiple times a second at regular intervals. 

Direct Current has currents that flow only in one direction. 

AC circuits deal with higher voltages compared to circuits that use DC. 

Due to this, fuses are broken down further into two categories;

• AC Fuses
• DC Fuses 

DC Fuses

The main problem when it comes to DC applications for fuses, is the ARC that is produced between the gap of the fuse when the fuse wire melts.

An arc can form between the gap within the fuse due to constant DC value. 

By increasing the distance between the electrodes, we can eliminate this arcing problem. DC fuses are designed to be bigger specifically to combat this issue.

AC Fuses

In AC circuits, arcing is not much of a problem as the amplitude of voltage changes up to 60 times every second (for frequencies of 50Hz or 60Hz). 

Therefore, the distance between the electrodes can be smaller compared to DC fuses. 

Different packaging of fuses used in specific circuits

As there are many different types of electrical and electronic circuits, as well as diverse applications, so too are they a wide variety of packaging of fuses to suit each application.

Note, these fuses have different packaging styles as well as voltage ratings appropriate for each application and sit under either the DC or AC fuse umbrella.

Cartridge fuses

If you need to protect high voltage electrical appliances such as motors, refrigerators, air-conditioners etc, a Cartridge fuse is your best option. 

These types are rated for voltages up to 600V (AC), and currents up to 600A. 

Cartridge fuses are further divided up into two options;

• General purpose (no time delay)
• Heavy duty (with time delay)

High voltage fuses

High voltage fuses were designed to work with voltages ranging from 1500V to 13kV!

You will find them in power systems protecting the likes of power transformers, distribution transformers, instrument transformers etc. 

Copper, silver or tin are the go to materials used for the fuse wire in high voltage fuses. 

Automotive fuses

You might have come across this type of fuse in your car.It is a plastic body with two metal teeth. Used commonly in automobiles for wiring and short circuit protection.

The voltage range for automobile fuses is around 12V – 42V. 

Surface mount fuses

Electronic circuits have shrunk in size over the years thanks to Surface Mount Devices (or SMD for short) technology. 

Things like smartphones can fit in the palm of your hand thanks to SMD components. 

SMD fuses are fuses used to protect SMD circuits in DC applications. Circuits like computers, cameras, smartphones and much more. 

Rewire-able fuses

Rewire-able fuses are most commonly used in low voltage systems in industries or or home electrical wiring. 

It gets its name because of the fact that it can be easily rewired.

Thermal fuses

Named aptly due to the fact that this type of fuse is temperature sensitive. 

The fuse wire is composed of an alloy that melts under high temperatures. They are also commonly known as Thermal Cutouts.

Re-settable fuses

Last but not least, is the Re-settable Fuse.

This fuse can be used multiple times without having to replace the fuse cartridge. 

They operate in the same manner as other fuses by opening in the case of an overcurrent, but unlike their counterparts, they can close the circuit again, resuming normal operation. 

Resettable fuses are great for applications where going in manually to replace a fuse might not be safe to do so.

Applications and circuits where fuses are used

We are surrounded by electrical and electronic devices. 

There are devices that make our lives easier with things like washing machines, dishwashers, air-conditioners, microwaves and much more.

Or, they help to entertain us and add some spice to our lives with devices like smartphones, computers, televisions, dvd players, etc. 

We now know the damage overcurrents can cause, and these devices are at risk of being damaged without the use of a fuse. 

Rather than naming every single device that uses a fuse (which will be many), I shall list the different systems and applications;

• Power systems
  • Motors
  • Transformers
• Power distribution 
  • Homes
  • Businesses
  • Schools
• Entertainment
  • Televisions 
  • DVD players
  • Gaming consoles
• Portable electronics
  • Smartphones
  • Digital cameras
• Computing
  • Laptops
  • Printers
  • Scanners
  • Hard Disk Drives
• Home Appliances
  • Toaster
  • Washing machine
  • Dishwasher 
  • Microwave
• Automotive
  • Cars
  • Motorbikes
  • Scooter

This isn’t an exhaustive list by any means. There are many more applications. Pretty much anything that uses electricity will benefit with a fuse in its circuit to protect it. 

Is it necessary to use a fuse in a circuit?

If the circuit has a chance of being subject to overcurrents, the simple answer is yes! 

Some electronic circuits might be able to go about their lives without the need of a fuse, but in general, it’s probably a good practice to design a circuit with a fuse. 

Does the fuse affect the overall resistance of a circuit?

No, the resistance of a fuse does not affect the overall resistance of a circuit. 

Fuses are made from metal alloys that have very low resistance and melting point. 

They are manufactured so that the cross sectional area of the fuse is able to carry the rated current of the circuit it will be used in and have a low resistance so it is a miniscule part of the overall circuit, which means a very low voltage drop. 

What is the difference between a fuse and a circuit breaker?

You might have heard both these terms before. But, what is the difference between a fuse and a circuit breaker?

The main difference is the reusability. 

A fuse can only be used once. When the fuse wire within the fuse has melted, the fuse cartridge needs to be removed and replaced with a new fuse.

Whereas, a circuit breaker can be reset multiple times if it has been tripped.

But, how does a circuit breaker achieve this? 

When current levels rise above the threshold limit within a circuit breaker, rather than a wire melting, a little electromagnet inside the circuit breaker pulls the contact points apart thus stopping the flow or current. 

Selecting the right fuse for your circuit

Choosing the right fuse for a circuit depends on the voltages and currents the circuit will be subject to. Then the appropriate fuse can be chosen. 

The general rule of thumb is to choose a current rating that is 150% – 200% higher than the maximum current value that the circuit will see.

First you would calculate the maximum current the circuit would be subject to. Then select a fuse with a rating 150% – 200% higher than that value. 

Say you have a washing machine running a load. The motor running is rated at 754W and a voltage of 115V. So the current is 754 / 115 = 6.5 A at full load. 

Going with the rule of thumb, you would select a fuse around 10A.

The post What does a fuse do in a circuit? appeared first on Electronic Guidebook.

It is a safety device used to protect electrical/electronic components and devices from Overcurrents (current surges) that exceed a certain limit. 

Think of it as the last line of defence. 

It consists of a metal wire that melts under the heat caused by high currents.

But why is the fuse made of a thin wire? A fuse is made of thin wire as it needs to be able to melt under high temperatures caused by high currents. If it does not melt the overcurrents caused by power surges can damage the circuit. 

The thicker the wire, the more current can flow and the harder it is to melt. 

The size of wires comes down to their physical characteristics of and their ability to handle different amounts of current. 

I will discuss this in more detail in this article. 

What is a fuse and fuse wire?

To understand why fuses use thin wire, it will help to learn a bit more about them and how they work.

So, let’s take a closer look at the fuse (if you already know how they work, you can skip this section).

The fuse

In many areas of the engineering world we have fail-safes.

These are safety measures that enable an engineering system to revert back to a safe condition in the event of a malfunction. 

A fuse is one of these fail-safes used in electrical and electronic applications. 

It is a safety device used to protect components and devices from overcurrent. 

Overcurrent is a scenario where excessive unwanted currents are generated in a circuit. It exceeds the nominal currents seen in the circuit.

These large unwanted currents can lead to the generation of high temperatures which have the potential of causing fires.

The fuse wire

But, how does the fuse have the ability to protect a circuit from these overcurrents. 

The star of the show here is the Fuse Wire. 

This little unsuspecting piece of wire is the ‘thin’ line between safety and catastrophe.

As we saw earlier, high currents generate high temperatures which can lead to fires. 

In the scenario of overcurrent, the fuse wire will melt under high heats and break thus opening the circuit and stopping the flow of current.

Fuses have ratings just like any other electronic component which include the maximum voltage and current they can handle before failure.

But, unlike other components, the fuse failing is part of its operation in order to protect other components.

How a fuse works in a circuit

The best way to illustrate how a fuse works is to look at an example. So, let’s take a look at the fuse in action in a simple circuit. 

Above is a simple circuit which is designed to power a lamp.

Below are the components of the circuits as well as their voltage and current ratings respectively;

• Battery (12V)
• Lamp (12V, 1A)
• Fuse (12V, 1A).

The lamp requires a voltage of 12V and a current up to 1A to operate efficiently. 

So, the maximum current this circuit is going to be subject to is 1A. Exceeding this current value is going to damage the lamp.

Therefore, a fuse with a current rating of 1A (or 1.5A to give a little allowance) is chosen specifically for this purpose. 

Any currents above this value will cause the fuse wire to melt. 

Why is the fuse wire thin?

Alright let’s dive into understanding why a fuse is made of a thin wire. 

So, we now know that the main purpose of the fuse wire is to melt under high currents (and therefore high temperatures).

The amount of current that a wire can carry comes down to its physical dimensions, as well as the material it is made of. 

Wire Gauge is a measurement that specifies the value of the diameter of a wire.

This value gives us information about the wire such as, the amount of current it can carry, as well as resistance and weight. 

The most widely used wire gauge system is the American Wire Gauge (AWG)

Every wire has a limit to the amount of current it can carry and going past this limit is going to damage the wire. 

The main reason for a fuse having a thin wire is what happens when the thin wire is subjected to large currents. 

Say we have Wire A with a wire gaure of 20. 

According to the AWG system, this wire can handle a current up to 3A (so its maximum limit is 3A).

Now, for scenario A, say we supply a current of 2A. Here the electrons (current) can freely move through the wire as the size of the wire is such that it can handle this amount of current. 

The key here is that there is no build of friction (or little friction) caused by the electrons bumping into the wire or each other, as there are less of them.

For Scenario B, let’s increase the current past its rated AWG current to say 4A.

Now, the number of electrons has increased but the size of the wire has remained the same. This is going to create a traffic jam of electrons within the wire. 

They are going to be bumping and rubbing against each other and the wire. 

This is going to cause increased levels of friction and we know that a build of friction is going to cause heat. 

The more friction, the more heat which is eventually going to melt the wire. 

Having this happen in normal conditions in electrical and electronic circuits is unwanted, however, this is perfect for a fuse as the fuse wire needs to melt when unwanted high currents arise. 

This is the exact reason why a fuse is made of thin wire. 

What is fuse wire made of?

One of the criterias of selecting a fuse wire is it’s physical dimension as we know now that wires have a limit of the current they can handle. 

But, if we choose a material that has a high melting point this is going to defeat the purpose and render the fuse useless as it won’t melt under high temperatures. 

Fuse wires are made of an alloy consisting of Sn (tin) and Pb (lead) due to their low melting point. 

The composition consists of 62% of tin and 38% of lead.

Their melting point is 183°C ( 361.4°F).

What happens if the fuse wire is thick?

The simple answer is that the circuit and components the fuse is supposed to be protecting are going to get damaged. 

If the fuse wire is thick, it is going to be able to handle more current. 

For example, if your electrical system is rated at 3A (the maximum current it can handle), but the fuse wire is thick enough to handle 5A and an overcurrent condition occurs of 4A, the components in your electrical system are going to be damaged as the fuse wire will not melt. 

The rule of thumb is to choose a fuse wire rated at 1.1 – 1.5 times the maximum current value of the system.

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