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Cambridge IGCSE0625

Electrical safety

Physics 0625 Chapter Notes

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Electrical safety
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Electrical safety notes

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In this chapter

  1. 1Electrical hazards
  2. 2Mains wires and the switch
  3. 3How a fuse protects the circuit
  4. 4Choosing a fuse or trip switch rating
  5. 5Trip switches
  6. 6Earthing a metal casing
  7. 7Double-insulated appliances

1. Electrical hazards

The four hazards to know come from damaged insulation, overheating cables, damp conditions and excess current from overloading plugs, extension leads and sockets.

Damaged insulation can leave a live wire exposed. A person who touches it gets an electric shock. The hazard is electrocution.

Many appliances on one socket or extension lead give a large total current. The cable can overheat and cause a fire.

Examiner insight

“Some answers lacked detail, with answers such as “the wire is damaged”.” “Some candidates thought that the damaged insulation would stop the laptop from charging. This was identified as a possible fault rather than a hazard.”

Worked example 11 mark

Multiple sockets can be connected to the mains electricity supply so that a lot of appliances can use the same socket. Why can this be a problem?

A The current in each appliance may not be enough for it to work correctly · B The potential difference (p.d.) across each appliance may not be enough for it to work correctly · C The total current may be large and cause overheating or a fire · D The total potential difference (p.d.) may be large and cause overheating or a fire

  1. 1

    C: the total current may be large and cause overheating or a fire. [1]

2. Mains wires and the switch

A mains circuit has a live wire (line wire), a neutral wire and an earth wire.

A switch must be connected to the live wire. Then the appliance is disconnected from the mains voltage when the switch is off.

Examiner insight

“Some candidates thought that the switch should be in the Earth wire as this would reduce the possibility of electric shocks.” “Although the majority of candidates knew that the fuse was placed in the live wire, many candidates incorrectly thought that the switch was placed in the neutral wire.”

Worked example 12 marks

State and explain in which wire in the mains electricity supply the switch is connected.

  1. 1

    the switch is in the live (line) wire [1]

  2. 2

    the device is disconnected from the mains voltage when switched off [1]

3. How a fuse protects the circuit

A fuse is connected in series with the appliance it protects, in the live wire.

If the current is too large, the fuse melts and breaks the circuit. This disconnects the live wire from the appliance. A fuse does not control the current: it prevents excess current.

A fuse protects the wiring from overheating. It does not protect a person who touches the live wire from an electric shock.

Examiner insight

“The most common errors were to use ‘voltage’ or ‘electricity’ instead of ‘current’.” “Candidates tended to say that the fuse controlled the current rather than the fuse prevented excess current.”

Worked example 12 marks

The live wire connects to a fuse which protects the circuit. Explain how a fuse protects the circuit.

  1. 1

    there is a large current [1]

  2. 2

    the fuse melts and breaks the circuit (any two of the three points score) [1]

4. Choosing a fuse or trip switch rating

Find the normal current from the power and the p.d. Choose the nearest rating above that current.

If the rating is below the normal current, the fuse melts when the appliance is used normally. A rating equal to the normal current is not suitable either: the rating has to exceed the current.

A trip switch is chosen in the same way: for an electric shower that uses 50 A, the suitable rating is 60 A.

I = P/V

Examiner insight

“Candidates needed to realise that choosing a fuse is not a matter of selecting the nearest value. It has to be the nearest value above the expected current.” “A common error was rounding down the answer from (i), giving 11 A for the value of fuse to be used.”

Worked example 11 mark

The current in the heater is normally 11.5 A. State a suitable value for the fuse. Choose one of these values: 5 A, 10 A, 13 A or 30 A.

  1. 1

    13 A, the smallest rating above 11.5 A [1]

5. Trip switches

A trip switch trips when the current is too high. It breaks the circuit and stops the current. This protects, for example, the person using the appliance.

Unlike a fuse, it can be reset after it has tripped. It also has a faster response and is more sensitive. Like a fuse, it disconnects the live wire from the appliance.

Examiner insight

“Many candidates did not recognise that the role of either a circuit-breaker or a fuse is to disconnect the live wire from the appliance and so did not select the correct answer.” “Stronger candidates referred to the protection against electric shock or overheating or to the fact that (unlike fuses) circuit breakers could be easily re-set.”

Worked example 12 marks

The lawnmower circuit contains a trip switch. Explain the purpose of the trip switch.

  1. 1

    it protects a named part from the effect of high current, e.g. the person [1]

  2. 2

    it stops the current or breaks the circuit when the current is too high [1]

6. Earthing a metal casing

The outer casing of an appliance must be either non-conducting (double-insulated) or earthed.

The live wire may become loose and touch a metal casing. The earth wire connects the casing to the ground. If the casing becomes live, the earth wire conducts current to the ground. This helps prevent a person from receiving an electric shock.

The earth wire needs a low resistance.

Examiner insight

“Few candidates explained that the earth wire carries current to earth if the casing becomes live.” “Expressions such as “electricity is directed to earth” or “the earth wire prevents electric shock” were not precise enough for credit.”

Worked example 11 mark

The metal cases of electrical appliances are connected to an earth wire. Which statement is not correct?

A The live wire may become loose and touch the metal case · B If the metal case becomes live, the earth wire conducts current to the ground · C The earth wire needs to have a high resistance · D Earthing metal cases helps prevent a person from receiving an electric shock

  1. 1

    C: the earth wire needs a low resistance, not a high one. [1]

7. Double-insulated appliances

A double-insulated appliance has a non-conducting outer casing, so it does not need an earth wire.

A fuse without an earth wire protects the circuit and the cabling for a double-insulated appliance.

Examiner insight

“The majority of candidates had the misconception that a double-insulated appliance requires an earth wire.” “This question proved challenging for many candidates and only the strongest had a secure concept of the role of a fuse to protect the cables between it and the appliance from overheating.”

Worked example 11 mark

Which statement is correct for a hair dryer with a double-insulated outer casing?

A It does not need a fuse or an earth wire · B It needs an earth wire but not a fuse · C It needs a fuse but not an earth wire · D It needs a fuse and an earth wire

  1. 1

    C: it needs a fuse but not an earth wire. [1]

Recap

  • Hazards: damaged insulation, overheating cables, damp conditions, overloaded sockets.
  • A mains circuit has a live, a neutral and an earth wire.
  • The switch goes in the live wire, so the appliance is disconnected from the mains when off.
  • A fuse melts and breaks the circuit when the current is too large.
  • Choose a fuse rating just above the normal current: I = P ÷ V.
  • A trip switch breaks the circuit when the current is too high and can be reset.
  • A casing must be non-conducting (double-insulated) or earthed.
  • A fuse without an earth wire protects the circuit and the cabling of a double-insulated appliance.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. A kettle cable has damaged insulation and the live wire is exposed. State the hazard.1 mark
  2. Name the three wires in a mains cable.3 marks
  3. Six appliances are plugged into one extension lead. Explain why this is dangerous.2 marks
  4. State two advantages of a trip switch compared with a fuse.2 marks
  5. A lamp switch is connected in the neutral wire. Explain why this is not safe.2 marks
  6. A student writes: ‘The 13 A fuse in the plug keeps the current in the kettle at 13 A.’ State what is wrong with this statement and describe what a fuse does.3 marks
  7. The metal casing of a toaster is earthed. The live wire comes loose and touches the casing. Explain how the earth wire protects the user.3 marks
  8. A heater has a power of 4.6 kW on a 230 V supply. A trip switch is rated 15 A, 20 A, 25 A or 30 A. Choose the most suitable trip switch rating.3 marks
  9. An extension lead with four sockets is used in a workshop on a 230 V mains supply. The cable of the extension lead overheats if the current in it is greater than 13 A. A 1.84 kW heater, a 920 W drill and a 460 W pump are plugged into the lead and all three are switched on. (a) Calculate the total current in the cable of the extension lead. (b) The plug of the extension lead holds a 13 A fuse. State and explain what happens. (c) A worker replaces the fuse with a 30 A fuse so that all three appliances work together. Explain why this is dangerous.7 marks
  10. A patio heater on a 230 V mains supply has a metal casing and two heating elements. Element X has a power of 920 W and element Y has a power of 1380 W. Each element has its own switch, so the heater can run with X only, with Y only or with both. (a) Calculate the largest normal current in the cable of the heater. (b) Fuses of 5 A, 10 A, 13 A and 30 A are available. Choose the fuse for the plug and explain why each of the other three is unsuitable. (c) A new model of the heater has a double-insulated casing. State the wire that the new model does not need, and state what its fuse protects.9 marks

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