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Mains can be dangerous

• The mains supply runs our homes — but used wrongly it can give a shock or start a fire.
• A few simple safety features keep it under control.
• Let's meet the hazards, the three wires, and the fuse 保险丝.

Hazards

• Damaged insulation — bare wires can give a shock or start a fire.
• Overheating cables — too much current makes a cable hot.
• Damp conditions — water lets current pass into a person.
• Overloading — too many appliances in one socket draw too much current.

Live 火线, neutral 零线 and earth 地线

• A mains cable has three wires:
• the live wire carries the high voltage;
• the neutral wire completes the circuit at about zero voltage;
• the earth wire is a safety wire joined to the ground.
• The switch and fuse go in the live wire, so switching off cuts the appliance from the high voltage.

Fuses, trip switches 跳闸开关 and earthing

• A fuse is a thin wire that melts if the current gets too big, breaking the circuit before the cable overheats.
• Choose a fuse rating just above the normal current (e.g. a $13\ \text{A}$ fuse for an appliance using about $10\ \text{A}$).
• A trip switch (circuit breaker) does the same job but is faster and can be reset.
• A metal case is made safe by earthing (a fault sends a big current to earth and blows the fuse) or by double insulation 双重绝缘 (a plastic case that can never become live).

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