Electrical safety
| Wire Name | Function | Voltage (approx.) |
|---|---|---|
| Live wire | Carries the alternating current from the supply to the appliance. It is at a high potential (voltage). | 230 \text{ V} |
| Neutral wire | Completes the circuit by providing a return path for the current to the supply. It is at zero potential. | 0 \text{ V} |
| Earth wire | A safety wire connected to the ground (earth). It carries no current during normal operation but provides a path to earth if a fault occurs. | 0 \text{ V} |
Why must the switch be on the live wire?
Building on the definition of voltage, the live wire is at high potential relative to the earth. If a switch were placed on the neutral wire, turning it 'off' would break the circuit and stop current flow, but the appliance would still remain connected to the high-voltage live wire.
If you then touched an internal part of the appliance (e.g., to change a bulb), your body could complete the circuit between the live wire and the earth (via the ground), resulting in an electric shock. Placing the switch on the live wire ensures that when it is open (off), the appliance is disconnected from the high voltage supply, making it safe.
These appliances use two layers of insulating material (or a non-conducting plastic casing) around the live parts. This ensures that even if the internal wiring fails and touches the outer casing, the casing remains non-conducting, preventing electric shock.
You must be able to state the specific hazards associated with mains electricity. These are distinct physical risks:
- Damaged insulation: If the plastic coating on wires is frayed or cut, the live conductor may be exposed. This can cause a short circuit or allow a person to touch a live part directly.
- Overheating cables: Cables have resistance. If too much current flows, heat energy (I^2Rt) increases, potentially melting the insulation and causing a fire.
- Damp conditions: Water (especially tap water with dissolved ions) is a conductor. Damp skin or wet surfaces significantly reduce electrical resistance, increasing the risk of severe electric shock if a live wire is touched.
- Excess current from overloading: Connecting too many high-power appliances to a single socket or extension lead causes the total current to exceed the cable's rating. This leads to overheating (see point 2) and fire.
A fuse is a safety device containing a thin wire that melts if the current exceeds a specific rating. It must be placed in the live wire. When it melts, it breaks the circuit, stopping the flow of current and preventing fire or shock.
Trip Switches (Circuit Breakers)
These are electromechanical devices that automatically switch off the circuit when they detect an excess current. Unlike fuses, they can be reset manually after the fault is fixed.
Choosing Appropriate Ratings
When selecting a fuse for an appliance, you must calculate the normal operating current using:
I = \frac{P}{V}
Where:
- I = Current in amperes (A)
- P = Power of the appliance in watts (W)
- V = Mains supply voltage (typically 230 \text{ V} in many syllabi, or 120 \text{ V} depending on region. Always use the value given in the question).
You then choose a fuse with a rating slightly higher than the calculated current to allow normal operation but blow if the current rises significantly.
Example: A heater has P = 2300 \text{ W} and operates at V = 230 \text{ V}.
I = 2300 / 230 = 10 \text{ A}.
A 13 \text{ A} fuse is appropriate. A 5 \text{ A} fuse would blow during normal use; a 30 \text{ A} fuse might not melt before the cable overheats.
Metal-Cased Appliances (Earthed)
If the live wire inside a metal-cased appliance comes loose and touches the casing, the casing becomes live.
- The earth wire provides a low-resistance path to the ground.
- This causes a very large current to flow, which melts the fuse (or trips the switch).
- The circuit is broken, and the casing is no longer live, preventing shock.
Double-Insulated Appliances
For appliances with plastic casings (e.g., hair dryers, lamps):
- They do not have an earth wire.
- However, they still require a fuse in the live wire.
- The fuse protects the internal wiring and the appliance itself from overheating due to excess current, even though there is no risk of shock from the casing.
The Correction: A switch must be in the live wire. If it is in the neutral wire, the appliance remains connected to the high-voltage live wire even when switched off. Touching internal components creates a path to earth through the body, causing electrocution.
The Correction: Double-insulated appliances do not need an earth wire, but they absolutely need a fuse. The fuse protects the circuit cabling and the appliance from fire due to excess current. The lack of an earth wire only removes the shock risk from the casing, not the fire risk from internal faults.
Why examiners accept this: Examiners look for the link between fault, path of current, and consequence.
Correct Phrasing Example: 'If the live wire touches the metal casing, the earth wire provides a low-resistance path to the ground. This causes a large current to flow, which melts the fuse and disconnects the appliance from the mains supply, preventing electric shock.'
Key Concept Connection: Note that you must mention the low-resistance path and the melting of the fuse. Without these, the explanation is incomplete.
Why examiners accept this: You must justify why the chosen fuse is better than others. Examiners want to see that you understand the fuse must carry normal current but blow on fault.
Correct Phrasing Example: 'A 13 \text{ A} fuse is appropriate because the normal operating current is 10 \text{ A}. A 5 \text{ A} fuse would melt during normal use, while a 30 \text{ A} fuse might not melt before the cable overheats and causes a fire.'
Key Concept Connection: Always compare the calculated current to the available standard fuse ratings (e.g., 3 \text{ A}, 5 \text{ A}, 13 \text{ A}).
- Damp conditions (water is a conductor) increasing the risk of electric shock. 2. Damaged insulation on cables due to moisture exposure.