Home  Fuse

Jun 10 2020

Basic Knowledge of Blown Fuse

I Introduction

What is blown fuse?

A fuse is an electrical safety device that operates to provide over-current protection of an electrical circuit. Its essential component is a galvanized wire with antimony tin alloy that blows (melts) when too much current flows through it, thereby interrupting the current. This process of self-protection is called blown fuse.

This passage introduces blown fuse in the household circuit and car, mainly including its working principle, causes of blown fuse, and how to repair it.  


I Introduction

II Blown Fuse in Household Circuit

2.1 Blown fuse in Household Circuit

2.2 Slow Blow Fuse VS. Fast Blow Fuse

III Help

3.1 Six Reasons Why Fuses Blow

3.2 Causes of Circuit fault From the Perspective of Blown fuse

3.3 How to Change Household Fuse?

IV Blown Fuse In Car

4.1 What Is A Blown Fuse In Car?

4.2 Why does a Car Fuse Blow?

4.3 What are the Symptoms of a Blown Car Fuse?

4.4 How to Replace a Fuse in a Car

V One Question Related to Resistor Classification

5.1 Question

5.2 Answer

II Blown Fuse in Household Circuit

2.1 Blown Fuse in Household Circuit

With the increase of fire and electricity consumption in modern households, the frequency of household fires is getting higher and higher, which has become a focus of public attention. And a considerable part of the fire is caused by the blown fuse. In the state of high voltage and large current, the fluctuation and surge of the grid voltage will cause the current in the power supply to increase instantaneously and cause the fuse to blow.

FuseFigure 1. Fuse


Blown Fuse

Figure 2. Blown Fuse

2.2 Slow Blow Fuse VS. Fast Blow Fuse

Literally, the difference between slow blow fuse and fast blow fuse is reaction speed. One reaction is faster and one reaction is slower. From a technical perspective, slow blow fuses are more resistant to surges than fast blow fuses.

In other words, slow-blow fuses have a higher ability to withstand instantaneous pulse currents than fast-blow wires. They can resist the surge of the surge current brought by the switch and do not act, thus ensuring the normal operation of the device. Therefore, slow blow fuse is also called surge resistance fuse.

To explain from a deeper level, slow blow fuse has a relatively large melting heat value I2t, and the energy required when the fuse blows is relatively large. So for fuses of the same rated current, slow blow fuses are much more pulse-resistant than fast blow fuses.

Because the I2t of the slow blow fuse is larger than that of the fast blow fuse of the same specification, when the total current of the circuit is too large and the fuse blows, the slow blow will be slower than the fast blow fuse. But it doesn’t mean slow blow fuse is worse than fast blow fuse.

Because if there is a fault in the circuit, the over-current will not disappear automatically, and the energy of the continuous over-current will greatly exceed the I2t of the fuse. No matter what kind of fuse, it still will be blown.

Therefore, the time difference between fast blow fuses and slow blow fuses is not so important for the device requirements they protect.

Only in the case of sensitive devices in the protected circuit that require special protection, the performance of the slow blow fuse will be affected.

Therefore,two kinds of blow fuse will be used in different circuits. For example, a purely resistive circuit, or a circuit that needs to protect some more sensitive and expensive components, fast-blow fuse is necessary. On the contrary, it is best to use slow blow fuses in the power input/output parts of some circuits.

In addition to protecting IC circuits, slow blow fuses can be used in most cases where fast blow fuses are used to improve pulse resistance; on the contrary, if fast blow fuses are used in places where slow blow fuses are used, it will often cause the power to be turned off immediately. The fuse cannot work normally.

III Help

3.1 Six Reasons Why Fuses Blow

(1) Overloaded Circuit

Too many appliances are operating at the same time,  resulting in household electricity overload, so that the fuse is blown. This situation is especially likely to occur when using air conditioners, electric heaters or adding other larger power appliances.

Solution: Cut back on all that power that’s tapping a single circuit. Find outlets on other circuits to plug into or unplug what you aren’t using.

(2) Connection Problems

In some families, although the fuse selection is reasonable and the load is not too large, it still occurs blown fuse as soon as it uses larger power appliances such as air conditioners, electric heaters, and rice cookers. The reason may be that when the fuse is installed or replaced, the fuse and the screw are in poor contact, causing fire and heating, and the screws fixing the fuse are oxidized and burned out.

(3) Short Circuit

If the fuse is replaced but still blows as soon as you closed the switch, there may be a short circuit. May be short circuit or load short-circuit. Electric kettles, rice cookers and other commonly used high-power electrical appliances and inferior electrical appliances are prone to short-circuit failure.

Solution: Make sure the faulty device is unplugged and there is no damage to the outlet. First, test the circuit. Then check for any damage on or around the electrical panel. If you see any damage, call an electrician before doing anything else with it. If there is none, flip the breaker switch back to its operating position. If it trips again, though, call an electrician.

(4) Ground Fault

A particular type of short circuit, a "ground-fault," occurs if a hot wire comes in contact with a ground wire or a metal wall box or touches wood framing members. Ground faults can be especially dangerous when they occur in areas with high levels of moisture, such as kitchens or bathrooms, or outdoor locations.

(5) Pulse

When the circuit is started or the power supply is unstable, an instantaneous large current causes the fuse to be disconnected. Also, the screw is not tightened when the fuse is installed, or the fuse is damaged, all will cause the fuse blown.

(6) The Wrong Type of Fuse Was Installed

Fuses come in many shapes, sizes, and configurations. Many look similar but actually have very different functions.

A suitable fuse must be selected and used. The fusing current of the fuse is usually 1.5-2.0 times the rated current. For example, when the total power of all appliances in the household is more than 1100W, choose a 5 amp fuse, and use a number 20 fuse with a diameter of 0.98MM. When the current exceeds 7.5 amps to 10 amps, the fuse will automatically blow to achieve the purpose of protection.

3.2 Causes of Circuit fault From the Perspective of Blown Fuse

The blown fuse is one of the common faults of the circuit. If not handled properly, it will cause a new fault. Therefore, we should learn to judge the cause of circuit failure from the fuse blown condition.


Situation: The fuse blows fast, often with a "snap" sound. Most fuses are turned into small round beads. The gap between the fuses is large, and sometimes the entire fuse is almost completely melted.

Analysis: This situation indicates that the blown fuse is caused by a short circuit in the line. Therefore, the circuit can work normally only after finding the short-circuit place, eliminating the short-circuit fault, and then connecting the fuse.


Situation: The fuse is disconnected from the middle, the gap between the disconnection is small, and a very thin residual part is left at both ends.

Analysis: This is caused by the fuse being too thin, or the passing current is too large. It should be installed with a slightly thicker fuse.


Situation: The connection between fuse and screw joints are blown out, and the disconnection gap is small.

Analysis: This is because the fastening screw that fixes the fuse is not tightened, the fuse is not in good contact with the screw, the resistance is too large when the current passes, and the generated heat is also increased. Heat doesn't dissipate as fast as it produces, so the fuse blows. In this case, connect the fuse directly and tighten the fastening screws.


Situation: The fuse blowing position is uncertain and the gap is small.

Analysis: This situation is generally caused by the longitudinal tension of the fuse, or a gap in the radial position, and the local diameter of the fuse becomes thinner. Just replace it with a new one.

3.3 How to Change Household Fuse?

• Guide

(1) The fuse of Knife Switch

① Find the main knife switch of the circuit and check the power supply to make sure that the fuse in the knife switch has blown.

② Put on rubber shoes and rubber gloves. Bring the prepared pliers and spare fuse in advance.

③ After power off, disassemble the upper and lower body shells of the knife switch, and use the pliers to remove the old fuse that has melted. Be careful not to touch the electrical circuit.

④ Replace with a new fuse, and then check if there is a short circuit. If not, you can install the case and close it.

(2) Household fuse

① Open the fuse box with a screwdriver, and then check to see if the fuse is blown.

② Install a fuse of similar length with the previous fuse. After installation, check whether it is done.


How to Replace Fuse Wire of a Blown Fuse

• Attention

When replacing the fuse, please pay attention: remove the cover of the insulation box, and do not directly connect the fuse in the insulation box. When installing the fuse, the two ends of the fuse should be wound clockwise around the screw. It should not be over-tightened or loosened, so as not to damage the fuse or cause poor contact, otherwise, it will be easy to blow.

IV Blown Fuse In Car

4.1 What Is A Blown Fuse In Car?

Just like many other appliances, the purpose of fuses in cars is to protect the electrical system within. The basic components of a fuse are one fuse element and two terminals. And the fuse itself works as a bridge between the vehicle and the particular system. If the system is overheated, the fuse element will heat up and melt, and, eventually, the circuit will stop working.

4.2 Why does a Car Fuse Blow?

The circuit of a general vehicle has gone through a very detailed experiment before leaving the factory, even at the beginning of the design. It should be fine, but why does the vehicle fuse still break?

Normally, there are two main reasons for the breakdown of a car fuse, one is that the load of the electrical appliance is too large, and the other is the short circuit in the car circuit.

The electrical appliance is overloaded. This is also the reason why most fuses blow. 

After buying a car, some people randomly modify electrical equipment, changing headlights, car audio equipment, using inferior cigarette lighters, adapters, and high-power electrical appliances. The operating power of some electrical equipment may be higher than the factory-set value. Once used, it may cause the fuse of the lamp to blow and short circuit in the automobile circuit. After the car has been used for a long time, some of the wire rubber has deteriorated, which has exposed the metal wire and caused a short circuit. Some car owners will directly flush the engine compartment with water pipes, which may cause water in the fuse box and short circuit.

4.3 What are the Symptoms of a Blown Car Fuse?

After the car fuse is blown, the most intuitive performance is that the car's related functions are disabled and the car cannot drive normally.

The battery has power but the vehicle cannot start. In this case, it is likely that the fuse responsible for starting the motor has burned out. When you find that the vehicle can not start, remember not to start continuously, which will cause the battery to discharge continuously and lead to complete power failure.

While the vehicle is running, the tachometer display is normal, but the speedometer display is zero, and the ABS warning light is on, indicating that the fuse corresponding to the ABS is burned out.

4.4 How to Replace a Fuse in a Car

(1) Turn off the start switch and all electrical equipment

(2) Disconnect the negative battery cable

(3) Figure out the blown fuse.

(4) Using needle-nose pliers or some small tweezers, remove the broken fuse.

(5) Replace the blown fuse with another new fuse of the same amperage.

If the replacement fuse is damaged, you should immediately go to the relevant car service station for inspection.


How To Safely Change A Blown Fuse

5.1 Question

How does blown fuse work?

A. By having pieces of metal wire that melt when current gets too high

B. By having an electromagnet that will break a circuit if the current gets too high

C. By having pieces of metal wire that melt when current gets too low

D. By having an electromagnet that will break a circuit if the current gets too low

5.2 Answer



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