Question

An appliance with a metal covering, rated at 2 kW, 220 V, is to be connected in a circuit. Given below are four diagrams (P, Q, R and S) depicting different circuit configurations.

1. Identify the safest circuit.
2. Write two reasons, supported by mathematical calculations, where applicable, to justify your choice.

Answer 1

Given, Power rating (P) = 2 KW = 2000 W

Voltage rating (V) = 220 V

a. The safest circuit is S.

b. There are two criteria for choosing circuit S:

1. It is earthed so that in the case of a short circuit, all the excess current can flow to earth without harming the person.
2. The current in the supply should be less than or equal to the current rating of the fuse.

Current in the circuit = `P/V`

= `2000/220`

= 9.09 A

Circuit P: As the circuit is not earthed, it is not safe.

Circuit Q:

• Circuit is earthed properly.
• Current in the circuit is 9.09 A but it has a fuse that has a current rating of 8 A so it will get damaged and hence someone can be hurt.

Circuit R:

• Because the circuit is not earthed properly, it is unsafe.
• Current in the circuit is 9.09 A and it has a fuse that has a current rating of 10 A.

Circuit S:

• Circuit is earthed properly.
• Current in the circuit is 9.09 A and it has a fuse that has a current rating of 10 A.

So circuit S satisfies all the necessary criteria for a safe circuit.