A Warm Extension Board and a 15 A House Circuit
A family uses a 230 V domestic line protected by a 15 A fuse. In one room they connect a 2000 W geyser, a 1000 W iron and a 300 W refrigerator through a 6 A extension board. The refrigerator draws an extra 5 A for a few seconds when its compressor starts. The fuse has not blown yet, but the extension board cable feels warm. A doorbell nearby uses an electromagnet with a soft-iron core. The family wants to know whether all three appliances can run together safely, why the refrigerator body is earthed, and why the fuse is placed in the live wire. They have a 20 A MCB available, but the house wiring is rated for 15 A. They also ask why the bell stops magnetising when the switch is released.
1. Calculate the steady total current and the current during compressor start. Decide whether running all three appliances together is safe.
Read the worked answer
Steady power = 2000 + 1000 + 300 = 3300 W. Current I = P/V = 3300/230 = 14.35 A. The refrigerator's running share is 300/230 = 1.30 A, so with an extra 5 A start current the total briefly becomes about 14.35 + 5 = 19.35 A. The 15 A fuse is therefore marginal even steadily and likely to blow during starting. Fitting the 20 A MCB would stop tripping, but the wiring is rated only 15 A, so that would remove protection from the cable and is unsafe. The 6 A extension board is badly overloaded; its warm cable is due to I²R heating. The safe judgement is to stagger use or provide separate circuits, not to uprate the fuse.
2. Explain the purpose of the earth wire on the refrigerator and why the fuse is placed in the live wire rather than the neutral wire.
Read the worked answer
The earth wire connects the metal body to earth through a low-resistance path. If a live wire touches the body, current flows to earth and the fuse or MCB operates, while the body stays near earth potential, protecting the user from shock. The fuse is placed in the live wire so that when it melts it disconnects the appliance from the high-potential supply. If the fuse were only in the neutral wire, the appliance could remain connected to live potential even after the fuse blew, which is dangerous during repair or a fault.
3. Why is the doorbell electromagnet made with a soft-iron core rather than steel? Relate this to the bell stopping when the switch is released.
Read the worked answer
Soft iron is easily magnetised by a current and loses its magnetism quickly when the current stops. That makes it suitable for a temporary electromagnet: while the switch is closed, the coil and soft-iron core attract the armature and the bell works; when the switch opens, the core demagnetises and the armature can spring back. Steel has higher retentivity and would remain magnetised, so the armature could stick or the bell could keep chattering. The choice is about magnetic materials, not just about making the core strong.