4.4 Electrical safety (3)
Resources |
Revision Questions |
Physics
Login to see all questions
Click on a question to view the answer
1.
State the hazards of:
- (b) Overheating cables
- (c) Damp conditions
- (d) Excess current from overloading of plugs, extension leads, single and multiple sockets when using a mains supply
(b) Overheating Cables:
- Fire: Overheating cables can melt the insulation, leading to a fire. This is a major safety hazard.
- Damage to Appliances: The heat generated can damage appliances connected to the cable.
- Electrical Shock: The melted insulation can expose live wires, creating a risk of electric shock.
2.
A fault occurs in a household appliance, causing the metal casing to become live. Explain how the earth wire protects a person from electric shock. Include a diagram to illustrate your answer.
When a fault occurs in a household appliance, the live wire may come into contact with the metal casing. This makes the casing live, posing a significant risk to anyone who touches it. The earth wire provides a low-resistance path back to the power source.
Here's how it works:
- The live wire is connected to the appliance's live terminal.
- The earth wire is connected to the appliance's metal casing.
- If the live wire touches the casing, the casing becomes live.
- A large current flows from the live wire, through the earth wire, and back to the power source.
- This large current causes the circuit breaker or fuse to trip, disconnecting the power to the appliance.
This rapid interruption of the circuit prevents a dangerous voltage from remaining on the appliance casing, protecting the person from electric shock.
Diagram:
| Live Wire |
| Appliance Casing |
| Earth Wire |
The earth wire provides a low-resistance path, ensuring a large current flows quickly, tripping the safety device.
3.
A student states that "double insulation is sufficient to prevent electric shock because it simply stops people from touching the live parts." Evaluate this statement, explaining why it is not a complete solution and detailing the importance of earthing.
The student's statement is not entirely accurate. While double insulation does significantly reduce the risk of electric shock by preventing direct contact with live parts, it is not a complete solution. Here's a detailed evaluation:
- Limitations of Double Insulation: Double insulation can fail. Over time, the insulation can degrade due to wear and tear, heat, or chemical exposure. This can expose the live parts, creating a shock hazard.
- Importance of Earthing: Earthing provides a crucial secondary safety mechanism. Even if the double insulation fails and the casing becomes live, the earth wire offers a low-resistance path for the fault current to flow to the earth.
- How Earthing Works: When a fault occurs, a large current flows through the earth wire. This causes the circuit breaker or fuse to trip, disconnecting the power supply and preventing the casing from remaining live. This is a vital safety feature.
- Combined Protection: The best protection is achieved by using both double insulation and earthing. This provides a double layer of safety, mitigating the risk of electric shock even if one safety feature fails.
In conclusion, while double insulation is a valuable safety feature, it is not sufficient on its own. Earthing is essential for providing a reliable backup safety mechanism and protecting users from electric shock in the event of a fault.