A student notices that the plastic insulation on a mains cable has been damaged, exposing the live wire. Explain why this damaged insulation is dangerous and describe how a fuse protects the user if a short circuit occurs as a result.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A household electric kettle is plugged into the mains supply. The mains supply in the UK is an alternating current supply at 230 V and 50 Hz. The cable connecting the kettle to the wall socket has a section of damaged insulation where the plastic covering has cracked and split, leaving the live wire exposed.
Model answer (4 marks)
Damaged insulation exposes the live conductor, so a person touching it can become part of the circuit and receive an electric shock.
If the exposed live wire touches another conductor or the earth, a short circuit is created and a large surge of current flows.
The surge heats the thin metal filament in the fuse; the filament melts and the fuse blows.
When the fuse blows it breaks the circuit, disconnecting the appliance from the mains and protecting the user.
If the exposed live wire touches another conductor or the earth, a short circuit is created and a large surge of current flows.
The surge heats the thin metal filament in the fuse; the filament melts and the fuse blows.
When the fuse blows it breaks the circuit, disconnecting the appliance from the mains and protecting the user.
Examiner tips
- Use the exact wording ‘exposed live wire’, ‘short circuit’, ‘large surge of current’, ‘fuse blows’, ‘disconnects the circuit’
- Show the causal chain: damaged insulation → shock or short → high current → fuse melts → protection
Common mistakes
- Confusing the fuse with a circuit breaker, or saying the fuse simply ‘reduces’ current instead of ‘blows’
- Omitting that the fuse protects the user by disconnecting the appliance
Mark scheme (4 marks)
- Damaged insulation means a person who touches the exposed live wire can receive an electric shock / current can flow through the person to earth
- Damaged insulation / exposed live wire touching another conductor can create a short circuit, causing a large/surge of current to flow
- The large current causes the thin metal wire/filament inside the fuse to overheat and melt/blow
- When the fuse melts it breaks/cuts off the circuit, disconnecting the appliance from the mains supply and protecting the user
Key terms in this question
damaged insulation · live wire · short circuit · fuse
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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