Explain how an earthed metal case is protected
Trace a live-to-case fault through protective earth and the supply return, without using the body as the intended path.
Core and Extended; Physics 0625, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked.
Before you begin
- Identify protective earth.
- Explain fuse or trip interruption.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
During ideal normal operation, where does the load current return?
What you will learn
- Trace a live-to-case fault through protective earth and the supply return, without using the body as the intended path.
- Explain earthing by a complete fault-return path and protective disconnection, with normal and fault states separated.
A conducting case needs a protective design
An accessible metal case can become live if an internal live conductor contacts it. In a specified earthed appliance, protective earth connects the case to a low-resistance fault-return path. The case is separate from the live–neutral load circuit during normal operation.
The fault path returns to the source through the installation’s protective arrangements; charge does not vanish into soil. A sufficiently large fault current can operate the live overcurrent protection, and appropriate residual-current protection can respond to imbalance. The exact protective arrangement and operation time matter.
Trace fault and disconnection
In the simplified correctly connected case, a live-to-case fault gives a path from live through the case and earth connection back toward the source. The low path resistance permits a large fault current and supports prompt protective disconnection. An earth wire is therefore part of a system, not a magic shield against every shock.
If a required earth connection is broken, that intended path is lost and the case can remain hazardous after a fault. The absence of normal load current in earth does not make earth optional. These diagrams are for written reasoning; practical testing and repair of mains protective connections belong to competent people.
Pause and explain
A required earth wire is broken but the appliance still operates. Is that evidence its fault protection is intact?
Worked example
Describe why a sound protective-earth connection helps in a live-to-metal-case fault.
Show the worked solution
- The case is connected to a low-resistance fault-return path.
- The fault current can operate appropriate protection.
- Opening live removes the supply to the faulted appliance.
Answer Protective earth supports a fault path and protective disconnection.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Draw a block fault path for live contacting the metal case of a specified earthed appliance.
Give me a hint
Keep normal neutral return separate from the new protective path.
Compare my reasoning
- Mark live contacting the case.
- Connect the case through earth to the protective source-return path.
- Show live protection opening when its conditions are met.
Live → case → protective fault-return path → source, with appropriate live disconnection.
Look for these in your work
- Complete the fault loop.
- Do not show charge disappearing into earth.
Explain why a low-resistance earth path can help a fuse operate in the supplied live-to-case fault model.
Give me a hint
Connect path resistance, fault current and fuse heating.
Compare my reasoning
- The fault has the supplied mains driving p.d.
- Low fault-path resistance permits a large current.
- Sufficient current heats and melts the live fuse.
The earth fault path supports a fault current sufficient for the specified protective operation.
Look for these in your work
- Preserve the causal sequence.
- Avoid a universal instantaneous guarantee.
A required earth connection is removed because no current was observed in it during normal operation. Critique the reasoning.
Give me a hint
A protective component can be inactive in the normal state.
Compare my reasoning
- Earth has a fault role rather than a normal load role.
- Removing it loses the intended live-to-case return path.
- Working through live and neutral does not restore that protection.
The removal defeats a required fault-protection path; normal zero earth current is expected.
Look for these in your work
- Distinguish normal and fault states.
- Explain the lost protection.
Common mistakes
- Earth current returns to the source.
- The protective system needs a sound connection.
- Do not guarantee zero shock or instant operation.
What can you explain now?
Why is a low-resistance protective path useful in the stated earthed-case model?
Compare with the explanation
It supports fault current and protective disconnection.
A suitable path lets the protective system respond to the live-to-case fault.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain earthing by a complete fault-return path and protective disconnection, with normal and fault states separated.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.