Electrical Safety: Fuses, Breakers, and Grounding

Electrical safety devices protect people and property: a fuse is a thin wire that melts when the current exceeds its rating, a circuit breaker switches off automatically when the current is too high, and grounding (earthing) provides a low-resistance path to the ground so that a fault current flows away and trips the protection instead of passing through a person. In brief: the fuse is placed in the live wire and should have a rating just above the normal operating current of the appliance. A residual-current device (RCD) detects a small leakage of current (typically 30 mA) and cuts the supply in milliseconds. Damp conditions, damaged insulation and overloaded sockets are the main hazards.

Currents as small as 30 mA through the chest can be dangerous. This guide explains the hazards of electricity, the roles of fuses, circuit breakers, earth wires and RCDs, compares the devices in a table, works through a fuse rating calculation and corrects common mistakes and safety rules. It is intended for education and does not replace professional advice for electrical work.

What are the hazards?

• Electric shock: the effect on the body depends mainly on the current and the path. As a rough guide, about 1 mA can be felt, 10 to 20 mA can cause muscles to lock so that the person cannot let go, and 50 mA or more passing through the chest can cause fatal fibrillation of the heart. Dry skin has a high resistance (tens of thousands of ohms), but wet skin can have only about 1,000 Ω, so I = 230 ÷ 1,000 = 0.23 A, which is deadly.
• Overheating and fire: too much current heats the wires (P = I²R). Overloaded extension leads, many appliances on one socket, and wires that are too thin are dangerous.
• Damaged insulation, loose connections and exposed wires.
• Water and electricity: water lowers the resistance of the body and can make a path to the ground.

How do fuses and circuit breakers work?

A fuse is a short length of thin wire in a glass or ceramic cartridge. If the current exceeds the rated value, the wire heats and melts, breaking the circuit. A fuse must be replaced after it blows. It is fitted in the live wire, so that when it blows the appliance is disconnected from the live supply. Common ratings in a UK plug are 3 A, 5 A and 13 A; in other countries, ratings such as 10 A, 15 A or 16 A are used with different voltages and plugs. A circuit breaker is a switch that opens automatically when the current is too high, using a bimetallic strip (thermal trip, for slow overloads) or an electromagnet (magnetic trip, for sudden short circuits). It can be reset after the fault is cleared. Breakers in the consumer unit (fuse box) protect each circuit in a house, such as sockets, lights and cooker. An RCD or GFCI compares the current in the live and neutral wires. If they differ by more than about 30 mA, it means that current is leaking, perhaps through a person, and the device cuts off the supply within about 30 ms.

What does the earth wire do?

The metal casing of many appliances (kettle, washing machine, toaster, refrigerator) is connected to the earth wire (green and yellow in UK and European wiring), which leads to the ground through the building wiring. If a fault makes the live wire touch the casing, a large current flows through the earth wire (a path of very low resistance), which melts the fuse or trips the breaker, and the casing is disconnected from the supply. Without the earth, the casing would stay at the mains voltage, and the next person to touch it would receive a shock. Appliances with double insulation (a plastic casing) do not need an earth wire and carry a double-square symbol. Lightning rods and the earth connection of fuel tankers use the same idea.

Safety devices compared

Worked example: choosing a fuse

1. Kettle. A kettle is rated 2,300 W at 230 V. I = P ÷ V = 2,300 ÷ 230 = 10 A. A 13 A fuse is chosen (the next standard value above 10 A), because a 3 A or 5 A fuse would blow at once.
2. Television. A 100 W television: I = 100 ÷ 230 = 0.43 A, so a 3 A fuse is appropriate. A fuse that is too large would not protect the appliance.
3. Overloaded socket. An extension lead with three appliances of 2,300 W, 1,000 W and 700 W: total power 4,000 W, I = 4,000 ÷ 230 = 17 A. If the lead is rated at 13 A, it is overloaded and may overheat.
4. Fault current with the earth. A live wire touches a casing, and the earth path has a resistance of 0.5 Ω: I = 230 ÷ 0.5 = 460 A, which melts the 13 A fuse almost instantly. Without an earth, a person with a body resistance of 2,000 Ω touching the casing might get I = 230 ÷ 2,000 = 0.115 A, which is dangerous.

How did electrical safety develop?

Thomas Edison included fuses in his electric lighting system in the 1880s to protect wiring from overloads. The miniature circuit breaker was developed in the 1920s. The RCD (earth leakage circuit breaker) was invented by Gottfried Biegelmeier in the 1950s, and became mandatory for bathroom and outdoor sockets in many countries. Wiring regulations, such as the IET Wiring Regulations in the UK and the National Electrical Code in the United States, define safe practice.

Which mistakes are common?

Mistake 1: Replacing a blown fuse with a higher rating, or with a nail or foil

The fuse protects the wires. A higher rating allows dangerous overheating.

Mistake 2: Putting the fuse in the neutral wire

It must be in the live wire, so that the appliance is not live when the fuse blows.

Mistake 3: Using electrical appliances with wet hands or near water

Water reduces the body’s resistance and creates a path to the ground.

Mistake 4: Thinking that low voltage is always safe

Danger depends on current. High-voltage and mains sources are dangerous, but small batteries are generally safe because of the limited current.