Electrical hazards and circuit protection
Key idea: Excessive current heats cables and can cause fire. A fuse melts or a circuit breaker opens to break the live connection; switching neutral alone leaves the appliance connected to live.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Recall the operating conductors and protective earth path.
- Connect each changed current path to shock or heating.
- Trace the three possible fault paths separately.
- Compare combined current with the extension’s rating.
- Find the current factor and trace the contact path.
- Compare how fuses and circuit breakers open a circuit.
- Identify both devices and explain why the fault needs correction.
- Calculate operating current before comparing fuse ratings.
- Calculate current and choose the smallest suitable offered fuse.
- Compare a broken live connection with stopped operating current.
- Explain why zero current and a potential difference can coexist.
- Find both branch currents and the shared cable’s total.
On this page
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ContinueMains electricity heats a kettle safely because its current flows along a planned path. The same supply can give a deadly shock if a person becomes part of the path, or start a fire if a cable carries more current than it was designed for. Fuses and circuit breakers cut the current when it grows too large, and choosing a fuse uses I = P/V from Electrical power, energy and cost. Learn from the diagrams and the values given in a question; never open, rewire or test a mains plug, socket or appliance. EMA’s electrical-safety guidance also addresses wet conditions, damaged outlets and overloading.
The mains connections
Live connects to the alternating supply potential and neutral completes the normal circuit. The earth wire is a protective connection and normally carries no current. Singapore’s mains supply is 230 V A.C.; if a question gives another value, use it.
The colours and plug wiring are in Live, neutral and mains plugs.
Three electrical hazards
| Situation | Complete explanation | Danger |
|---|---|---|
| Damaged insulation | Live may be exposed. A person who touches it while connected to earth or another conductor can complete a circuit through their body. Bare live and neutral conductors touching can create a low-resistance short circuit. | Electric shock; fire |
| Overheating cables | A cable carries the combined current of the appliances it supplies. If that current exceeds what the cable is designed to carry, heating can damage its insulation or ignite nearby material. | Fire; exposed conductors |
| Damp conditions | Water containing dissolved impurities conducts, and wet skin usually has lower resistance. For the same applied p.d., a lower body-path resistance allows a larger current. | Greater shock risk |
Worked example 1
One fault, two paths
Problem
A live conductor’s insulation is cracked. Describe what can happen if a person touches it, if it touches neutral, and if it touches an unearthed metal casing.
Show full solution
A person touches it
Method
Trace the path through the body.Reason
A person connected to earth forms part of a return path to the supply.Working
Answer
Current through the body causes a shock.
It touches neutral
Method
Trace the path through the wiring.Reason
The fault makes a low-resistance circuit through the supply wiring.Working
Answer
A large current can cause rapid heating.
It touches an unearthed metal casing
Working
Answer
The casing becomes live, even though no large fault current flows. Anyone who touches it while connected to earth completes a path and gets a shock.
In each case, explain the danger by naming the connection and the complete path the current takes, not by saying that electricity leaks out.
Check your understanding 1
Five appliances share an extension cable. Its insulation is intact and nobody touches a conductor. What hazard should you investigate, and where does the current flow?
Show answer
Investigate overloading and fire risk. The extension carries the combined operating current of all five appliances. Compare that current with the extension’s stated rating: the appliance count alone cannot give the combined current. Excessive current can overheat the cable, damage its insulation and ignite nearby material.
Check your understanding 2
In a model of contact with exposed live, wet skin halves the resistance of the body path while the p.d. stays unchanged. Calculate the factor by which current changes. Does intact insulation elsewhere remove this conducting path?
Show answer
From I = V/R, halving the resistance doubles the current through the body. An intact supply cable does not remove the risk from the exposed live contact described: the body still completes a conducting path. The greater current increases the shock risk.
Fuses, circuit breakers and fuse ratings
Correctly specified overcurrent protection in the live wire opens the circuit when excessive current flows for long enough, limiting harmful heating in the wiring.
| Device | How it opens the circuit | After it operates |
|---|---|---|
| Fuse | A thin fuse element is connected in series. Excessive current heats it until it melts and breaks the path. | Correct the cause, then replace it with the specified fuse. |
| Circuit breaker | Excessive current operates a mechanism that opens a switch. | Correct the cause before resetting it. |
Check your understanding 3
After an excessive current has been interrupted, device A has a broken metal element and device B has an open switch. Identify each device. A learner proposes closing B’s switch immediately, without investigating the circuit. Explain why that is unsafe.
Show answer
A is a fuse: its element has melted and broken the circuit. B is a circuit breaker: its mechanism has opened the switch. Closing B without correcting the fault could restore the excessive current and its heating hazard. Correct the cause before resetting a breaker; a blown fuse needs replacement with the specified fuse after the cause is corrected.
A fuse rating is the current the fuse is designed to carry. It is not an exact current at which the fuse melts at once: both fuses and circuit breakers respond to how large the extra current is and how long it flows.
To choose a fuse from a list of ratings, calculate the normal current and choose the smallest rating above it. The fuse then carries the normal current without melting but still melts quickly if a fault makes the current much larger. For a real appliance, use the fuse its maker specifies; a calculation is not an instruction to change a fuse.
Worked example 2
Choose a fuse for a heater
Problem
A resistive heater is rated 1.84 kW on a 230 V supply. The question offers 3 A, 5 A and 13 A fuses. Choose from that list.
Show full solution
Find the normal current
Method
Convert to watts and use I = P/V.Reason
The fuse must carry the normal current without melting.Working
Answer
I = 1840/230 = 8.0 A
Rule out low ratings
Method
Compare each rating with 8.0 A.Reason
A fuse rated below the normal current would melt in normal use.Working
Answer
3 A and 5 A are unsuitable
Choose
Working
Answer
Choose 13 A, the smallest offered rating above 8.0 A. This does not mean that a 13 A fuse melts as soon as current reaches 13 A.
Check your understanding 4
A toaster’s resistive element is rated 805 W on 230 V. Calculate its operating current, then choose a fuse from 3 A, 5 A and 13 A.
Show answer
I = 805/230 = 3.5 A. Reject 3 A because it is below the normal current. Choose 5 A, the smallest offered rating above 3.5 A. The 13 A option is unnecessarily high for this simplified choice. Do not treat either rating as an instant melting threshold.
A plug fuse cannot protect against every electric shock. A dangerous current through a body can be much smaller than the current needed to operate a plug fuse. The fuse protects against excessive circuit current; it does not detect every dangerous contact.
Why switches and protection belong in live
In the simple appliance circuit shown, opening the switch, fuse or breaker in live breaks the connection from the appliance to live. Normal current stops.
Opening neutral also breaks the normal circuit, so the appliance stops operating. However, the appliance still has a connection to live. A person who touches an internal live-connected part, or a casing made live by a fault, may complete another return path and receive a shock.
The distinction is stopped operating current versus a broken live connection. Neither a diagram nor an off switch can tell you whether real equipment is safe to open.
Check your understanding 5
In a circuit model, a switch opens only neutral. An ideal ammeter shows zero operating current, but an ideal voltmeter between the live-connected terminal and earth still shows 230 V. A learner says the readings contradict each other. Explain why they can both be correct.
Show answer
The open neutral path prevents operating current from completing its circuit, so the ammeter reads zero. The terminal is still connected to live, so its p.d. relative to earth can remain 230 V. A p.d. can exist without operating current through the appliance. Contact that completes a different return path could therefore cause a shock despite the zero ammeter reading.
Check your understanding 6 Check the shared cable separately
Two resistive heaters operate from 230 V: one is 1380 W, the other 2300 W. Each has a 13 A plug fuse; their shared extension is rated 13 A. Calculate both currents and their sum. Is the extension overloaded?
Show the steps Optional working
Show answer
The appliance currents are 1380/230 = 6.0 A and 2300/230 = 10 A. The shared cable carries 6.0 + 10 = 16 A, above its stated 13 A rating. Each individual current is below its own plug fuse rating, but that does not keep the combined current below the extension’s rating. The claim confuses protection in separate appliance paths with the load in the shared path. The extension is overloaded because it carries 16 A. A fuse’s operating time depends on both the current and its duration.
Common mistakes
- Saying five plugs always overload an extension cable. Compare the combined current with the cable’s rating.
- Saying a 13 A fuse melts as soon as the current reaches 13 A. It depends on how large the extra current is and how long it flows.
- Choosing the largest fuse “to be safe”. Choose the smallest rating above the normal current.
- Saying a fuse stops any electric shock. A current far too small to melt a plug fuse can still kill.
- Saying an appliance that has stopped working is disconnected from live. Opening neutral stops the current but leaves the live connection.
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Syllabus and review details
- SEC G2 Science Physics component 2027 · 2027
Physics section, PDF pages 8–23
Last reviewed: