CIE IGCSE Physics (0625) Electrical safety Study Notes - New Syllabus
CIE IGCSE Physics (0625) Electrical safety Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of Electrical safety
Key Concepts:
- Hazards of Mains Electricity
- Mains Circuit: Live, Neutral and Earth Wires
- Fuses and Trip Switches: Use and Operation
- Concepts of Earthing
Hazards of Mains Electricity
Hazards of Mains Electricity:
Electrical safety hazards arise when faults occur in household wiring, appliances, or user handling. Below are the four key hazards and their causes:
(a) Damaged Insulation:
The plastic coating (insulation) on wires prevents the live wire from touching the user or other components. If damaged:
- Exposed metal wires can cause electric shocks if touched.
- Can create a short circuit if live and neutral wires touch.
(b) Overheating Cables:
Cables overheat when too much current passes through thin or coiled wires. This can:
- Melt the plastic insulation.
- Cause burns, fire, or damage to connected devices.
(c) Damp Conditions:
Water is a good conductor of electricity. In damp conditions:
- Water can form a conducting path between a live wire and a person’s body.
- This increases the risk of electric shock or electrocution.
(d) Excess Current from Overloading:
Plug sockets and extension leads are designed to handle limited current (typically 13 A in the UK). If overloaded:
- It can exceed the safe current capacity, leading to overheating.
- May melt wires or cause fires in walls or plugs.
Example
A user uses a hairdryer with a frayed power cord. Part of the live wire is exposed and touches a metal table. Describe the Hazard Type.
▶️ Answer/Explanation
This is a case of damaged insulation.
If the user touches the metal table, current may pass through their body to the ground, causing a severe electric shock.
Example
In a small room, four high-wattage appliances (iron, heater, kettle, microwave) are plugged into the same extension lead.Describe the Hazard Type.
▶️ Answer/Explanation
This is a case of overloading a socket.
The total current exceeds the capacity of the extension lead. This leads to overheating, which may melt insulation and start a fire.
Mains Circuit: Live, Neutral and Earth Wires
Mains Circuit: Live, Neutral and Earth Wires
A standard AC mains electricity circuit uses three wires, each with a specific function and color code (based on international wiring standards):
Live Wire (Line Wire): Brown
- Carries the current to the appliance at high voltage (~230 V).
- This wire is always at high potential, which makes it dangerous.
Neutral Wire: Blue
- Completes the circuit by carrying current back to the source.
- It is close to 0 V, but still potentially dangerous.
Earth Wire: Green/Yellow stripes
- Safety wire that carries current to the ground if a fault occurs.
- Prevents metal casings of appliances from becoming live.
Why must a switch be connected to the live wire?
- The switch breaks the circuit by disconnecting the appliance from the live wire.
- This ensures no current enters the appliance once the switch is off.
If the switch were placed in the neutral wire:
- Appliance may appear “off” but still has live voltage inside.
- This creates a serious risk of electric shock if someone touches the internal parts.
Therefore: for complete safety, the switch must disconnect the live wire so that the whole appliance is at 0 V and safe to touch.
Fuses and Trip Switches: Use and Operation
Fuses and Trip Switches: Use and Operation
Fuses and trip switches are safety devices that protect people and appliances from excess current in an electrical circuit. They are usually connected in the live wire, so they disconnect the supply if something goes wrong.
Fuse:
- A thin wire inside a plug or fuse box that melts when the current is too high.
- Melting breaks the circuit, stopping current flow and protecting the appliance.
- Must be replaced after it blows.
Trip Switch (Circuit Breaker):
- A resettable switch that trips (turns off) when the current exceeds a safe limit.
- Uses electromagnetism or a bimetallic strip to detect excess current and break the circuit.
- Can be easily reset by pressing a button — no need for replacement.
Choosing Appropriate Fuse Ratings
A fuse rating must be just above the normal operating current of the appliance, so that it will blow only if something goes wrong.
Steps to choose a fuse:
- Use the formula \( I = \frac{P}{V} \) to calculate the operating current.
- Select the next highest standard fuse rating (usually 3 A, 5 A, or 13 A).
Example:
A kettle has a power rating of 2300 W and operates at 230 V.What will be the fuse rating for this kettle
▶️ Answer/Explanation
Calculate current: \( I = \frac{P}{V} = \frac{2300}{230} = 10 \text{ A} \)
Choose a fuse: next highest rating is 13 A.
\(\boxed{\text{Use a 13 A fuse}}\)
Concepts of Earthing
Why Appliance Casings Must Be Non-Conducting or Earthed
Metal (Conducting) Casings:
- If a fault causes a live wire to touch the metal case, the case becomes live.
- This poses a serious risk of electric shock to anyone touching it.
- So, metal-cased appliances must be earthed — connected to the earth wire.
- If the case becomes live, current flows to earth, causing the fuse to blow and breaking the circuit.
Plastic (Non-conducting) Casings – Double Insulation:
- Some appliances are made entirely from insulating materials like plastic.
- There is no metal casing, so the user cannot touch anything live even if there’s a fault.
- These appliances are marked with the double-insulation symbol and do not need an earth wire.
How the Fuse Works with or without an Earth Wire
With an Earth Wire (Earthed Appliance):
- If the live wire touches the metal case, a large current flows through the earth wire.
- This surge causes the fuse to melt, breaking the circuit and cutting off the current.
- This protects the user and the wiring.
Without an Earth Wire (Double-Insulated Appliance):
- Even though there’s no earth wire, the appliance is safe due to its insulating case.
- If a fault causes excess current (e.g. short circuit), the fuse still blows to protect the cabling and the circuit.
- But the casing itself never becomes live, so user safety is ensured without needing an earth wire.
Example:
Compare the safety design of a metal toaster and a plastic phone charger in terms of earthing, insulation, and fuse protection.
▶️ Answer/Explanation
Toaster (Earthed Appliance):
The toaster has a metal casing, which can conduct electricity if a fault occurs.
It is fitted with an earth wire. If the live wire touches the metal case, current flows through the earth wire.
This large current causes the fuse to blow, disconnecting the appliance and preventing electric shock.
The earth wire ensures that the casing never remains live.
Phone Charger (Double-Insulated Appliance):
The charger is made entirely from plastic, so no part of the casing can become live.
It does not require an earth wire due to double insulation.
If a fault causes excess current, the fuse in the plug still blows to protect the wiring and stop overheating.
There is no risk of electric shock from the casing, even if the charger malfunctions.
Conclusion:
A metal-cased appliance must be earthed to prevent shock in case of a fault.
A plastic-cased appliance is safe without earthing due to insulation, but the fuse still protects against overcurrent.