Fuse and circuit breakers

In order to prevent excessive currents flowing into the home circuit, electrical appliances and its cables, fuses and circuit breakers are wired into the live wire and used as safety devices.

  • A fuse is usually made up of a tin-coated copper wire. When current exceeds its design rating value. The wire will overheat and melt, thus opening the electrical circuit. It will prevent further damage to the appliance or user. It cannot be reused.
  • A circuit breaker is usually made up of a reusable spring-loaded type of switch. The function of the circuit breaker is similar to that of the fuse. If current exceeds its breaking setting, it will spring open and break the circuit as in a fuse. The device can be reused by resetting the spring-loaded switch.

It is correct to fix the fuse or circuit breaker at the live wire before the appliance. When the circuit is loaded with excessive current, the fuse or circuit breaker will break and open the circuit. It will prevent overloading, burning or damaging the appliance.

  • Connecting the fuse or circuit breaker to the neutral wire is incorrect, i.e., even when the circuit is opened due to excessive currents, the appliance may still be at live potential, creating possibility of an electric shock.

The current limit through the fuse (fuse rating) can be controlled by varying the thickness of the tin-coated copper wire. Thicker the wire, the larger the heating effect needed to melt the connection, thus permitting larger current to flow.

  • Different fuse ratings and circuit breaker settings are used in different appliances according to their power requirements. The rating limits used is normally slightly higher than the normal current needed by the appliance.

Worked Examples

Example 1: Safety Features

A household has a circuit breaker that trips at a current of 20 amps. If a particular circuit in the house is running on a 240-volt supply, what is the maximum power rating of appliances that can safely be operated on this circuit without tripping the circuit breaker?

Click here to show/hide answer

To find the maximum power, use the formula $P = VI$.

Given $V = 240$ volts and $I = 20$ amps,

$$P = 240 \, \text{V} \times 20 \, \text{A} = 4800 \, \text{W} = 4.8 \text{ kW}$$

This means appliances with a total power rating of up to 4.8 kW can safely operate without tripping the circuit breaker.

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