Mains Electricity and Circuit Wiring Safety - Worksheets, Questions and Revision

15 original exam-style questions - 4 pages of questions with a full mark scheme - free printable PDF.

Download PDF Jump to mark scheme (page 5)Mains Electricity and Circuit Wiring Safety revision guide

This topic is chapter 5 of IGCSE Physics Practice Book.

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3.5 Mains Electricity and Circuit Wiring Safety

EDEXCEL 4PH1 · Calculator allowed · about 55 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

State the nominal rms voltage supplied by the UK mains electricity supply that household appliances use.

(Total for Question 1 is 1 mark)

State the standard frequency of the UK mains electricity supply and include the unit.

(Total for Question 2 is 1 mark)

State the purpose of the neutral wire in a domestic mains circuit and what potential it is close to during normal operation.

(Total for Question 3 is 1 mark)

State the role of the live wire in a mains appliance and the danger it poses if touched while the appliance is switched on.

(Total for Question 4 is 1 mark)

Describe two differences between direct current as supplied by a cell or battery and alternating current supplied by the UK mains, naming each type in your answer.

(Total for Question 5 is 2 marks)

State the colour coding of the three wires found in a standard three-core mains cable used in the UK, giving the colour and the name of each wire: live, neutral, earth. Include the earth colour exactly as used in UK wiring.

(Total for Question 6 is 3 marks)

Describe the purpose of the earth wire in a mains appliance and how it reduces the risk of electric shock to a user.

(Total for Question 7 is 3 marks)

The diagram shows a simple three-pin plug and a length of three-core cable for a domestic appliance. Label the plug pins and the cable cores with the correct names and colours. In your answer label: Live pin and wire, Neutral pin and wire, Earth pin and wire. Also indicate where the fuse is placed in a correctly wired plug.

(Total for Question 8 is 6 marks)

Explain how a fuse protects an appliance and a user from danger in the event of a fault that allows excessive current to flow.

(Total for Question 9 is 2 marks)

Describe briefly how a circuit breaker provides protection compared with a fuse, and give one advantage of a circuit breaker over a fuse in domestic use.

(Total for Question 10 is 2 marks)

Explain how double insulation protects a user from electric shock and state an example of an item that might use double insulation instead of an earth wire.

(Total for Question 11 is 3 marks)

A hairdryer is labelled 900 W for power and is designed to be used on UK mains 230 V. Calculate the current the hairdryer draws from the mains and hence suggest a suitable fuse rating for the plug. Show your working and state the unit.

(Total for Question 12 is 2 marks)

A kettle marked 2300 W is used on 230 V mains. Calculate the current it draws and determine the smallest standard fuse rating from the common UK plug ratings 3 A, 5 A and 13 A that should be used. Show the equation, substitution and evaluation. State the unit for the current.

(Total for Question 13 is 3 marks)

A toaster has power rating 1500 W and is used on 230 V mains. Calculate the current drawn by the toaster. Then explain why a 3 A fuse would be unsuitable in the plug for this toaster, giving the numerical reason. Show the equation, substitution and evaluate the current; state the unit.

(Total for Question 14 is 3 marks)

A small charger has a label showing 12 V output and 2 A maximum on the low-voltage side. The charger is powered from 230 V mains via an internal transformer and internal fuse in the live input. Explain why the charger still needs an internal fuse on the mains side even though the low-voltage side limit is 2 A, and state what the fuse on the mains side protects against. Give two distinct reasons.

(Total for Question 15 is 4 marks)

Mark scheme · 3.5 Mains Electricity and Circuit Wiring Safety

Question 1

B1 230 V (or 230 volts) cao
Answer: 230 V

Question 2

B1 50 Hz (50 hertz) cao
Answer: 50 Hz

Question 3

B1 completes the circuit back to the supply and is close to earth potential (near 0 V) during normal operation
Answer: Neutral completes the circuit back to the supply and is close to earth potential (near 0 V) in normal use.

Question 4

B1 carries the alternating voltage from the supply to the appliance and can give a dangerous electric shock if touched while live
Answer: Live carries the supply voltage to the appliance and can give a dangerous shock if touched when the appliance is switched on.

Question 5

B1 DC: current flows in one direction only (from a cell or battery)
B1 AC: current changes direction periodically, e.g. 50 times per second for UK mains
Answer: DC flows in one direction only; AC reverses direction periodically (50 Hz in UK mains).

Question 6

B1 live is brown
B1 neutral is blue
B1 earth is green-and-yellow (green with yellow stripe) cao
Answer: Live = brown; Neutral = blue; Earth = green-and-yellow

Question 7

B1 provides a low resistance path to earth for fault current if the live conductor touches a metal case
B1 this allows large fault current to flow so protective devices operate (fuse blows or circuit breaker trips)
B1 metal case is kept at near earth potential, reducing risk of electric shock to a user who touches the case
Answer: Earth gives a low-resistance path for fault current so a fuse or breaker operates and the metal case stays near earth potential, reducing shock risk.

Question 8

B1 live pin labelled and live wire colour given as brown
B1 neutral pin labelled and neutral wire colour given as blue
B1 earth pin labelled and earth wire colour given as green-and-yellow
B1 correct position of the fuse inside the live conductor in the plug is shown or stated
B1 correct identification of the live, neutral and earth pin positions (live on the right when looking at the pins, neutral on the left, earth top)
B1 shows that the cable outer sheath is clamped and that individual cores are connected to the correct pins
Answer: Live = brown to live pin (right pin), Neutral = blue to neutral pin (left pin), Earth = green-and-yellow to top pin; fuse is in the live conductor inside the plug; outer sheath clamped.

Question 9

B1 wire in the fuse melts when current exceeds its rating, breaking the circuit
B1 this stops current flowing so prevents overheating or fire and reduces risk of electric shock
Answer: If current exceeds the fuse rating its thin wire melts and breaks the circuit, stopping current flow and preventing overheating, fire or a live casing that could give a shock.

Question 10

B1 circuit breaker trips to open the circuit when current is too high, often using an electromagnet or bimetallic strip
B1 advantage: it can be reset rather than replaced, so quicker and more convenient after a fault
Answer: A circuit breaker trips to open the circuit when excess current flows, often using electromagnetism or a bimetal strip; it can be reset after a fault instead of replacing it like a fuse.

Question 11

B1 double insulation has two layers of insulation or reinforced insulation so live parts cannot touch external metal parts
B1 prevents any fault current reaching the outside casing, so user cannot receive a shock even if internal wiring fails
B1 gives a correct example, e.g. a hairdryer with a plastic casing or many handheld power tools and battery chargers
Answer: Double insulation uses two layers or reinforced insulation so live parts cannot reach an external metal case; this prevents fault current reaching a user. Example: a hairdryer with a plastic outer case or many handheld power tools.

Question 12

M1 uses I = P / V with P = 900 W and V = 230 V
A1 I = 900 / 230 = 3.913... A, therefore suitable fuse rating 5 A cao
Answer: I = P / V = 900 / 230 = 3.91 A (about 3.9 A), so use a 5 A fuse.

Question 13

M1 quotes the equation I = P / V and substitutes values I = 2300 / 230
A1 evaluates current = 10 A (2300 / 230 = 10.0 A) cao
B1 states smallest suitable fuse is 13 A, since 3 A and 5 A are too small and 13 A is the next standard rating above 10 A
Answer: I = P / V = 2300 / 230 = 10.0 A, so use a 13 A fuse (3 A and 5 A would blow).

Question 14

M1 quotes I = P / V and substitutes I = 1500 / 230
A1 evaluates current = 6.5217... A, cao, states unit A
B1 explains 3 A fuse is unsuitable because 3 A < 6.52 A so it would blow under normal operation; suggests a 13 A fuse is appropriate
Answer: I = P / V = 1500 / 230 = 6.52 A (about 6.5 A). A 3 A fuse is unsuitable because 3 A is less than the operating current (6.52 A) so it would blow; use 13 A.

Question 15

B1 internal fuse on mains side protects the charger and wiring from excessive current drawn from the mains due to a fault in the charger, preventing overheating and fire
B1 it protects the mains supply and user by disconnecting the high-voltage side if a fault causes a short or overload, limiting damage and reducing shock/fire risk
B1 the low-voltage 2 A rating does not limit mains-side current directly because transformation changes voltage and current; a mains fuse ensures faults on either side cannot cause unsafe mains currents
B1 explicit statement that the fuse protects against internal short circuits, earth faults or component failures that would otherwise cause excessive heating or fire risk
Answer: The mains-side internal fuse stops excessive current from the 230 V supply if the charger develops a fault, protecting the charger and cable from overheating and preventing fire, and it disconnects the mains to reduce shock risk. The 2 A low-voltage limit does not prevent mains-side faults, so a mains fuse is needed to protect against internal shorts, earth faults and overloads.

Mains Electricity and Circuit Wiring Safety - Worksheets, Questions and Revision

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3.5 Mains Electricity and Circuit Wiring Safety

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