Dangers of Electricity

Sources of electrical hazards:

1. Overloaded cables: too many high-power appliances on one circuit → current exceeds safe limit → cable overheats → fire risk
2. Damaged insulation: bare live wire can touch a person or a metal casing → electric shock or fire
3. Water near electricity: water conducts → electric shock risk
4. Using incorrect fuse: too high a rating allows excessive current → overheating not prevented

Electric shock:

• Current through the body is dangerous (even 10–20 mA can cause loss of muscle control; 100 mA can be fatal)
• Body acts as resistance → high voltage → more current → more dangerous

UK 3-pin plug wiring (for reference):

• Brown wire → Live (L) — carries alternating voltage (230 V AC in UK)
• Blue wire → Neutral (N) — completes the circuit; close to 0 V
• Green/yellow striped → Earth (E) — safety wire; normally carries no current
• Fuse is always in the live wire

Fuse:

• A thin wire designed to melt (blow) if current exceeds a safe level
• Breaks the circuit → stops further current flow → prevents overheating
• Fuse rating: must be just above the normal operating current of the appliance
• Example: kettle uses 1500 W at 230 V → I = P/V = 1500/230 ≈ 6.5 A → use 13 A fuse (next common rating above 6.5 A)
• Disadvantage: must be replaced after it blows

Circuit breaker (MCB — Miniature Circuit Breaker):

• Automatically disconnects the circuit when current is too high
• Can be reset (just press a button) — much more convenient than replacing a fuse
• Faster to operate than a fuse

RCCB (Residual Current Circuit Breaker):

• Detects imbalance between current in live wire and neutral wire
• Even a tiny imbalance (≥ 30 mA) causes it to trip in < 30 ms
• Protects against earth leakage / electric shock even before current is high enough to blow a fuse

Earth wire and earthing:

• Metal casings of appliances (washing machines, toasters) are connected to earth
• If a live wire comes loose and touches the metal casing:
  • Without earth: casing becomes live → touching the casing → electric shock
  • With earth: large current flows: live → casing → earth wire → earth → fuse/MCB blows → circuit broken → safe
• The earth wire provides a low-resistance path for fault current

Double insulation (Class II):

• Appliances with plastic casings or two layers of electrical insulation (e.g., hairdryers, hand drills)
• No metal parts accessible to user
• No earth wire needed (nothing conducting to become live)
• Symbol: double square (□ inside □)

Choosing fuse rating:

1. Calculate operating current: I = P/V
2. Choose next standard fuse rating above this value
3. Standard ratings: 1 A, 3 A, 5 A, 13 A (UK)