Electricity at home — domestic wiring, fuses, MCB, safety measures

Question

Draw a schematic diagram of domestic electric wiring. Explain the roles of fuse, MCB, and earthing in electrical safety.

(CBSE Class 10 — Magnetic Effects of Electric Current chapter / Electricity chapter)

Domestic Wiring Circuit

flowchart TD
    A["Power Station"] --> B["Transformer (step-up)"]
    B --> C["Transmission Lines (high voltage)"]
    C --> D["Local Transformer (step-down to 220V)"]
    D --> E["Electric Meter"]
    E --> F["Main Switch + MCB/Fuse"]
    F --> G["Live Wire (Red)"]
    F --> H["Neutral Wire (Black)"]
    G --> I["Parallel Circuits to Rooms"]
    H --> I
    I --> J["Each appliance has its own switch"]
    I --> K["Earth Wire (Green) for safety"]

Solution — Step by Step

Indian domestic supply uses 220 V AC at 50 Hz delivered through three wires:

Live wire (Red/Brown) — Carries current at high potential (220 V). This is the dangerous one.
Neutral wire (Black/Blue) — Completes the circuit, at roughly 0 V.
Earth wire (Green/Yellow-Green) — Connected to a metal plate buried in the ground. It provides a safe path for leaking current.

All appliances in a house are connected in parallel between the live and neutral wires. This ensures each gets the full 220 V and operates independently.

Fuse: A thin wire (alloy of tin and lead) that melts when current exceeds a safe limit. Once it melts, the circuit breaks, preventing damage to appliances and wiring. Fuse rating should match the appliance — a 5 A fuse for a 1000 W device (since $I = P/V = 1000/220 \approx 4.5$ A).

MCB (Miniature Circuit Breaker): A modern replacement for fuses. It trips (switches off) automatically when current exceeds the rated value. Unlike a fuse, it can be reset — no need to replace anything. MCBs are faster and more reliable.

Both are connected in the live wire (never in the neutral wire), because we want to cut off the high-potential line during a fault.

If a live wire accidentally touches the metal body of an appliance (like a washing machine), the body becomes live at 220 V. Touching it could be fatal.

Earthing connects the metal body to the ground through a low-resistance wire. If a fault occurs, current flows through the earth wire (path of least resistance) instead of through your body. This large current also trips the fuse/MCB, cutting off supply.

Why This Works

The parallel wiring ensures that if one appliance fails, others keep working (unlike series, where one failure breaks everything). Fuses and MCBs act as sacrificial protection — they break before expensive appliances or dangerous overheating occurs. Earthing gives fault current a safe path to ground, preventing electrocution.

Alternative Method — Calculating Fuse Rating

For any appliance, the fuse rating should be just above the normal operating current:

I = \frac{P}{V}

Appliance	Power	Current at 220 V	Fuse Rating
Bulb	60 W	0.27 A	1 A
Iron	1000 W	4.5 A	5 A
AC	1500 W	6.8 A	10 A
Geyser	2000 W	9.1 A	10 A

CBSE frequently asks: “Why is it safer to connect appliances in parallel rather than series?” Two reasons: (1) each appliance gets full voltage (220 V), and (2) failure of one does not affect others. Also, the total resistance decreases in parallel, allowing each device to draw only the current it needs.

Common Mistake

Students often place the fuse in the neutral wire in their circuit diagrams. The fuse must always be in the live wire. If it were in the neutral wire, the circuit would break but the appliance body would still be at 220 V (connected to the live wire) — making it dangerous to touch. The fuse must disconnect the high-potential side.