An appliance with a metal covering, rated at 2 kW, 220 V, is to be connected in a circuit. Given below are two diagrams (R & S) depicting different circuit configurations.
(a) Identify the safest circuit.
(b) Write two reasons, supported by mathematical calculations, where applicable, to justify your choice.
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Given,
- Power rating of the appliance (P) = 2 kW = 2000 W
- Voltage rating (V) = 220 V
(a) The safest circuit is S.
(b) The current drawn by the appliance is
$\text I = \frac{\text P}{\text V} = \frac{2000}{220} = 9.09 \text { A}$
Earthing — The appliance has a metal covering. In circuit S the metal body is connected to the earth, so if the live wire accidentally touches the body, a large fault current flows through the earth wire, helping the fuse disconnect the supply and protecting a person touching the appliance. Circuit R has no earth connection, so it is unsafe.
Correct fuse rating — The current rating of the fuse must be slightly more than the current drawn by the appliance. In circuit S the fuse is rated 10 A, which is just above 9.09 A, so it carries the normal current safely and blows only when the current becomes excessive. In circuit R the fuse is rated 8 A, which is less than 9.09 A, so it would blow even during the normal working of the appliance.
Hence, circuit S satisfies both the requirements of a safe circuit.