Why the fuse and switch must be on the live wire?

A common mistake is to state that the reason for switch to be on the live wire is “so that the switch can turn on/off the appliance”. This explanation is wrong.

Refer to the view for the reason why switch and fuse must be on the live wire.

In general, in the event where the fuse is blown or the switch is open, it disconnect the appliance from the high potential of the live wire. So the appliance will no become ‘live’ and it is save to touch even though there is a fault.

 

Why do we need earth wire and fuse?

Simply put, electric appliance can be very simple with just a live wire bring in the current and a neutral wire to bring the current out. The appliance can just work like this forever (without the hassle of having earth wire, fuse etc), provided there is no electric fault developed.

Both the earth wire and the fuse are safety features to protect the user and the appliance respectively. Take a look at the video below to understand the rationale.

Recall these notes given:

 

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double-insulation

3 pin-plug

 

What will cause the fuse to melt

Function of Fuse

The 3 pin plug

How a fuse works?

A fuse is a safety device that is added to an electrical circuit to prevent excessive current flow. It has the same function as a circuit breaker. However , a fuse must be replaced once it melts (blows). A circuit breaker can be reset after it trips.

Electric symbol for fuse

Fuse is connected to the live wire. It consists of a short piece of thin wire. In the event of an electrical fault, when a current that exceeds its fuse rating flows through, it heats up and melts (fuse blows). When the fuse blows, the electrical appliance is disconnected from the high potential of the live wire (usually 240 V). Hence it protects the appliance and the user.

The choice of fuse is always slightly higher than the actual current flowing through the appliance, and it has to be a whole number. Typical household fuses off the shelves are rated at 1 A, 2 A, 3 A, 5 A, 10 A and 13 A. But in theory, we just state a fuse which is slightly higher and is a whole number.

 

SP N2007 P1 Q16 – Which events will cause the fuse to blow?

An electric cable contains three wires live, neutral and earth. The cable is correctly wired to a plug which contains a 3A fuse. The insulation becomes damaged and bare metal wires show.

Five possible events can occur.

• A person touches the earth wire.
• A person touches the neutral wire.
• A person touches the live wire.
• The live wire touches the neutral wire.
• The live wire touches the earth wire.

How many of these five events cause the fuse in the plug to blow?

A   1          B   2        C   3         D   4

Solutions: Option B

Consider the five events:

• A person touches the earth wire – As the person is at 0V, same as the earth wire, there will be no current flowing through the person. So current through the circuit will not be affected, which is lower than the 3A fuse rating. Fuse will not blow.
• A person touches the neutral wire.-  As the person is at 0V, same as the neutral wire, there will be no current flowing through the person. So current through the circuit will not be affected, which is lower than the 3A fuse rating. Fuse will not blow.
• A person touches the live wire. – The live wire is at high potential of 240 V. The person will get an electric shock. But a common misconception is that if a person gets an electric shock, the current flowing through him is very large, which is wrong. In fact, the current is very small, much smaller than the fuse rating. Assuming the average body resistance of the person is 100 000 ohms, and the potential difference in Singapore is 240 V, since I = V/R = 240/100 000 = 0.0024 A, which is lower than 3A fuse rating. Hence the fuse will not blow.
• The live wire touches the neutral wire. – This will create a short circuit as a large current which exceeds the fuse rating will from the live (240 V) to the neutral wire (0V) as that path has very low resistance. The fuse will blow.
• The live wire touches the earth wire. – This will create a short circuit as a large current which exceeds the fuse rating will from the live (240 V) to the earth wire (0V) as that path has very low resistance. The fuse will blow.

What’s the purpose of fuse and Earth wire, and how they work

How the wiring works and why fuse and switch must be on Live wire