An electric fuse is a safety device placed in electrical circuits to protect electrical components and wirings from excessive current flow. The fuse gets heated from excessive current and gets blown. This breaks the circuit and protects the expensive and important equipment. It is a self-sacrificing working principle for the fuse.
Maybe your mechanic told you that you need to replace a few blown fuses in your fuse box. Now you are wondering about this small weightless device that controls so many operations in your car.
So, what is an electric fuse and how does it work?
Now let’s get into the in-depth discussion about electric fuses and their working principle. Continue reading to marvel at this tiny piece of technological wonder.
What Is An Electric Fuse?
An electric fuse is an electrical component that protects other accessories on the circuit from electrical damage. It blows to shield other components from overcurrent flow. Moreover, it is considered an OCPD (Overcurrent Protective Device) according to the National Electrical Code.
There are some distinct characteristics of an electric fuse. Now, we will discuss them in detail.
The most distinguishable feature of an electrical fuse is its current rating. The current rating is always written on the transparent cover of the fuse. This rating indicates how much current the fuse can handle continuously without blowing.
Often the current rating is called the amp rating and the fuses’ amp rating often follows a color code system. For example, a blade fuse of 20A will have a yellow-colored transparent cover.
Fuses are selected to have an amp rating close to the system’s normal running current. Usually, if the system is running on 15A then a fuse of 15A needs to be chosen for that.
The voltage rating of a fuse denotes the AC or DC voltage where the fuse is designed to operate. A fuse can be AC only, DC only, or both. But you cannot use a DC fuse in AC applications or vice versa.
When choosing a fuse, the voltage rating of the fuse needs to be equal to or more than the system’s voltage. For example, an AC 220 V fuse can be used in an AC 220V or 110V system but not in an AC 440V system.
It is also known as interrupting rating. As we know, a fuse gets blown when exposed to short circuits or overloads. However, only the metal conductor inside the fuse case is supposed to melt and other parts should stay intact when the fuse gets blown.
An interrupting rating is a current level up to which a fuse can operate safely without explosion or rupture. When the interrupting rating is exceeded then there is a possibility of rupture.
The interrupting rating is much higher than the fuse’s current rating. For example, if a fuse has a current rating of 200A, the interrupting current rating will be 200,000A. So the explosion of the fuse is highly unlikely to happen.
Breaking capacity is always more than the maximum rated voltage but less than the short circuit current. The National Electrical Code requires fuses to have an interrupting rating capable of handling the current that must be interrupted.
Fuses have different terminal materials like lead and tin alloys. Depending on the terminal material, a fuse will behave differently. Usually, the terminal material is fixed for fuses and is not interchangeable.
The temperature at which a fuse can operate changes depending on the fuse you are using. Most automotive fuses can operate from -40 degree celsius to +125 degree celsius.
Response time is how quickly a fuse responds to an overcurrent situation. It actually depends on the fuse type. There are fast-acting fuses that will melt instantaneously when exposed to an overflow of an amp.
There are slow blow fuses that provide reliability when exposed to a sudden rush of current for a short time like in starting an electric motor or a sudden overloaded motor. These are also called time delay fuses. Time delay fuses can handle overcurrent for at least 12 seconds.
Another type of fuse is a dual element; one part protects against short circuits and the other part against small overloads. This fuse will not melt if there is a sudden surge of overcurrent like 100 or 200% of the current rating. But will get blown when the overload is sustained for some time.
At the same time, a dual-element fuse protects against short circuits while limiting the short circuit current. This fuse protects from overload and short circuits quickly. Also, there are super fast fuses that melt in 0.1 seconds.
Also, there is a type of fuse called a current limiting fuse. A current limiting fuse substantially reduces the amount of overflow current. It acts to clear the fault in less than 0.833 seconds.
This type of fuse counteracts the fault currents within the first half electrical cycle. As a result of this, different accessories and automotive parts are shielded against adverse thermal and magnetic forces.
Types of Electric Fuses
There are many types of electrical fuses used in automotive industries. Blade fuses are the majority, but other car fuses are cartridge, high current, high voltage, cable fuse, SMD, RTP, and PPTCs. Find all car fuses in detail here.
How Does Electric Fuse Work?
Let’s look at this simple circuit diagram. A fuse is denoted as the “S” shape in schematic diagrams.
So a fuse is connected to a circuit in a way that saves the load from harmful current overflow. The most common type of fuse used in the automotive industry is the blade fuse. Here is what a blade fuse looks like;
Source: RS Components
Notice the spiral metal part inside the transparent case, that part gets blown when there is an overflow of current. The current does not melt the metal, the heat energy produced due to the current melts the fuse.
Usually, the metal used in fuses has a low melting point. Tin, lead, copper, zinc, silver, aluminum, and sometimes alloys make up the metal ring part of the fuse. The thickness of the ring depends on the current flowing through it.
Most car fuses have transparent cases to make visual inspection easy when a fuse is blown. The automotive fuses are always DC fuses since it’s powered by the car’s battery.
Why Do Electric Fuses Blow?
Fuses blow for three main reasons; overload, short circuit, and ground fault. When Current flows through the fused metal part, heat dissipates. This heat increases the temp.
The amount of heat produced and how long the overflow happens determines when the fuse will blow. So when the temperature reaches the melting point, the fuse blows. We have discussed each type below.
This is when too much current is flowing through a circuit. Fuses are designed to disconnect and open the circuit in case of an overload.
Generally, during a short circuit, the current is 500-600% of the fuse’s current rating. Fuses have inverse time relationships which means the higher value of current will make the fuse blow quicker.
If you connect a heated wire with a power source and then to anything grounded like a metal pipe, electrical box, your hand contacts with ground/earth, etc. a ground fault happens. A ground fault will cause the electric fuse to blow.
Summary:A fuse gets blown due to overload, short circuit, and ground fault in the circuit that the fuse is connected to.
Are Circuit Breakers And Electric Fuses The Same?
Just like a fuse, a circuit breaker also provides protection against current overload and short circuits. A circuit breaker is considered an overcurrent protection device OCPD as well. However, there is a fundamental difference between these two.
A fuse gets blown when exposed to overflow of current and must be replaced. However, a circuit breaker can be reset after an overcurrent. In case of an overcurrent situation, the switch in the circuit breaker opens and stops the current flow.
When exposed to an overcurrent situation, there is a switch in a circuit breaker that opens up and the electricity flow gets shut down. After that, the circuit breaker needs to be reset and can be used again.
These are the difference between a fuse and a circuit breaker;
|Must be replaced after exposure to overcurrent
|Must be reset after exposure to overcurrent
|Does not require maintenance
|Needs regular maintenance
How To Know a Fuse is Blown?
There are a few ways to know when a fuse is blown. You can know when a fuse gets blown via visual inspection, testing, and automotive symptoms. Here is how to tell if a fuse is blown.
You can test your fuse using a multimeter to check for continuity and resistance readings. You could use either a digital or analog multimeter to test a fuse. Moreover, a fuse can be tested without a multimeter as well.
Is MCB A Fuse?
No, MCB or miniature circuit breaker is not exactly a fuse but a protective device against overcurrent. It’s a type of circuit breaker and unlike a fuse, it does not self-sacrifice when exposed to current overflow.
Can A Fuse Be Corroded?
Yes, a fuse can be corroded when exposed to a wet or humid environment. They will corrode at the metal parts sticking out. Corrosion of the fuse is not desired since it will cause overheating and holder failure.
What Is Immaterial For An Electric Fuse?
The length of the wire inside the fuse is immaterial. However, the thickness of the wire is important because it will determine how much current can be passed through it.
Hopefully, this article provided you with some much-needed clarifications about electric fuses and how they work. So if you have a blown fuse in your fuse box now you know exactly what happened.