1. P.1 Book 4 Section 3.2 Mains electricity and household wiring Electricity: friend or foe? Alternating current and mains circuit Safety designs in domestic electricity Check-point 3 Household wiring Check-point 4 3.2Mains electricity and household wiring

2. P.2 Book 4 Section 3.2 Mains electricity and household wiring Electricity: friend or foe? What would you do if you were one of the Tung Choi Street residents, or a victim in the fire accident?

3. P.3 Book 4 Section 3.2 Mains electricity and household wiring Electricity: friend or foe? Electricity is essential to modern life. BUT improper use of it can be very dangerous.

4. P.4 Book 4 Section 3.2 Mains electricity and household wiring 1 Alternating current and mains circuit Two ways to move the hoop: (i) move it in one direction continuously (ii) wiggle it back and forth Recall the hula hoop analogy. Electrons gain energy in both cases.  almost an instant and concurrent conversion of energy

5. P.5 Book 4 Section 3.2 Mains electricity and household wiring 1 Alternating current and mains circuit Current driven by battery: one-way  direct current or d.c. Symbol: Current driven by mains socket: two-way  alternating current or a.c. Symbol: Mains electricity supply in HK: Voltage = 220 V a.c. frequency = 50 Hz

6. P.6 Book 4 Section 3.2 Mains electricity and household wiring 1 Alternating current and mains circuit Cables consist of three insulated colour- coded wires: earth wire (green and yellow) live wire (brown) neutral wire (blue)

7. P.7 Book 4 Section 3.2 Mains electricity and household wiring 1 Alternating current and mains circuit Electric potential at live wire changes from +ve to –ve continuously and alternatively. A.c. power supply is applied across live wire and neutral wire. BUT that of neutral wire stays at 0 V ( ∵ connected to the earth).

8. P.8 Book 4 Section 3.2 Mains electricity and household wiring 1 Alternating current and mains circuit Electric potential at live wire = +ve Current: live wire  appliance  neutral wire 3.1 Alternating current Simulation Electric potential at live wire = –ve Current: neutral wire  appliance  live wire

9. P.9 Book 4 Section 3.2 Mains electricity and household wiring  melts when overheated by large current  breaks the circuit  prevents cable from overheating and cause a fire 2 Safety designs in domestic electricity a Fuse Fuse: a short & thin wire fitted in live wire

10. P.10 Book 4 Section 3.2 Mains electricity and household wiring a Fuse Which fuse is suitable? Fuse value: min. current to blow the fuse Suitable fuse for an appliance: fuse value slightly larger than normal current Normal current can be found by mains voltage and power rating 3.2 Fuse Video Example 7  I = —— PVPV

11. P.11 Book 4 Section 3.2 Mains electricity and household wiring current through the microwave oven Example 7 Which fuse is suitable? A microwave oven is rated at ‘220 V, 750 W’. Which fuse should be used, 5-A or 15-A? By P = VI, I = PVPV 750 220 = = 3.41 A ∴ A 5-A fuse should be used.

12. P.12 Book 4 Section 3.2 Mains electricity and household wiring a Fuse Position of the fuse and the switch Example 8

13. P.13 Book 4 Section 3.2 Mains electricity and household wiring Example 8 Position of the fuse and the switch Why should we place both the fuse and the switch in the live wire instead of the neutral wire? If the switch/fuse is placed at live wire, once it is opened/broken, the appliance has zero potential.  safe to touch

14. P.14 Book 4 Section 3.2 Mains electricity and household wiring Example 8 Position of the fuse and the switch If the switch/fuse is placed at neutral wire, even if it is opened/broken, the appliance is still live.  dangerous to touch ( ∵ appliance and the person has a p.d.) ∴ Both the switch & fuse should be placed in the live wire, not the neutral wire.

15. P.15 Book 4 Section 3.2 Mains electricity and household wiring a Fuse How many lamps can be connected? Example 9

16. P.16 Book 4 Section 3.2 Mains electricity and household wiring Example 9 How many lamps can be connected? An electric lamp is rated at ‘220 V, 60 W’. (a) Rated current through the lamp = ? By P = IV, rated current through the lamp I = PVPV 60 220 = = 0.273 A (b) How many lamps can be connected in parallel to the 220-V mains (with a 5-A fuse)? 5 A 0.273 A = 18.3  18 lamps can be connected.

17. P.17 Book 4 Section 3.2 Mains electricity and household wiring 2 Safety designs in domestic electricity b Earth wire as a safety wire Earth wire is a safety wire:  connects metal body of appliance to the earth  prevents appliance from becoming ‘live’ If live wire touches the metal body:  large current flows to the earth  blows the fuse in live wire

18. P.18 Book 4 Section 3.2 Mains electricity and household wiring 2 Safety designs in domestic electricity b Earth wire as a safety wire If earth wire is removed and the live wire touches the metal body due to a fault:  large current may flow through the user  user experiences electric shock

19. P.19 Book 4 Section 3.2 Mains electricity and household wiring 2 Safety designs in domestic electricity c Double insulation Appliance with insulating plastic case does not need an earth wire.  double insulation 1. covering of wires 2. plastic case Symbol:

20. P.20 Book 4 Section 3.2 Mains electricity and household wiring 2 Safety designs in domestic electricity d Three-pin plug and socket Three-pin plug and its corresponding socket: live (L) neutral (N) earth (E)

21. P.21 Book 4 Section 3.2 Mains electricity and household wiring 2 Safety designs in domestic electricity d Three-pin plug and socket Earth pin is slightly longer:  to open ‘shutters’ of live and neutral holes (closed prevent accidental insertion)  to ensure that earth wire is connected before live wire 3.2 The 3D model of plug and socket Simulation

22. P.22 Book 4 Section 3.2 Mains electricity and household wiring Check-point 3 – Q1 Match the pins, the holes, and the wires correctly. EarthLiveNeutral Pin:___________ ___________ ___________ Hole: ___________ ___________ ___________ Wire: ___________ ___________ ___________ Colour: ___________ ___________ ___________ X Z Y A CB K I J Yellow/green Brown Blue

23. P.23 Book 4 Section 3.2 Mains electricity and household wiring = 0.455 A Check-point 3 – Q1 A lamp has 5 light bulbs rated at ‘220 V, 100 W’ connected in parallel. Which fuse should be used in the plug of the lamp? (Only 1-A, 3-A, 5-A and 10-A are available.) Current through each bulb = PVPV 100 220 = Total current through cable = 5  0.455 = 2.275 A Fuse value should be slightly higher than the total current through the appliance. ∴ A _______ fuse should be used. 3-A

24. P.24 Book 4 Section 3.2 Mains electricity and household wiring 3 Household wiring Wiring in the home provides circuits for electrical appliances.  connected to consumer unit (‘fuse box’) Mains supply cable contains two wires: live and neutral

25. P.25 Book 4 Section 3.2 Mains electricity and household wiring 3 Household wiring  The live and the neutral wires branch out into parallel circuits. Circuits in parallel  faults in one circuit will not affect the others.

26. P.26 Book 4 Section 3.2 Mains electricity and household wiring 3 Household wiring In consumer unit, each circuit is fitted with a circuit breaker which  has automatic switch that cuts off the current flow if it exceeds a certain value.  can be reset without replacement after the fault has been fixed. 3.3 Household wiring Parallel branches Simulation Example 10 3.3 The electrician's 'live' probe Video

27. P.27 Book 4 Section 3.2 Mains electricity and household wiring Example 10 Parallel branches Why do we wire up several branch circuits, instead of using one big circuit, to supply electricity to all appliances?

28. P.28 Book 4 Section 3.2 Mains electricity and household wiring Example 10 Parallel branches If only one big main circuit is used: 1.very thick wires are needed but… cables for carrying a large current are very expensive 2.only one circuit breaker at high current rating will be fitted but… circuit may not be broken for a faulty appliance running on a low current

29. P.29 Book 4 Section 3.2 Mains electricity and household wiring 3 Household wiring a Lighting circuit Two-way switch: either of the switches can turn on/off the lamp. The lamp is controlled by two switches: Choice of cables Example 11

30. P.30 Book 4 Section 3.2 Mains electricity and household wiring Example 11 Choice of cables A household electrical wiring circuit: (a) Name the meter M. Kilowatt-hour meter

31. P.31 Book 4 Section 3.2 Mains electricity and household wiring Example 11 Choice of cables (b) Which cable, with current rating 2-A, 6-A, 17-A and 63-A, should be used at X, Y and Z ?  63-A cable at X (5 + 15 + 30 = 50)  17-A cable at Y ∵ Max current flowing in each parallel branch = 5  3 = 1.67 A  2-A cable at Z Current cannot exceed fuse value of circuit breaker.

32. P.32 Book 4 Section 3.2 Mains electricity and household wiring 3 Household wiring b Ring main Advantages of a ring main Mains sockets in the home are connected to a circuit.  ring main Live, neutral and earth wires each branch into two paths and form a large ‘ring’.  loops around the room Example 12

33. P.33 Book 4 Section 3.2 Mains electricity and household wiring Example 12 Advantages of a ring main Advantages of using a ring main circuit? Current flows in 2 separate sets of cables Also, all sockets are connected in parallel  a fault in any socket will not affect the others  thinner and cheaper cables can be used

34. P.34 Book 4 Section 3.2 Mains electricity and household wiring 3 Household wiring Electric kettle Example 13

35. P.35 Book 4 Section 3.2 Mains electricity and household wiring Example 13 Electric kettle An electric kettle rated at 2000 W can boil 2 kg of water at 25  C in 6 min when connected to 220-V mains. (Specific heat capacity of water = 4200 J kg –1  C –1 ) (a) (i) Energy absorbed by the water to boil = ? E = mc  T= 2  4200  (100 – 25) = 6.3  10 5 J (ii) Energy provided by the kettle = ? E = Pt = 2000  6  60 = 7.2  10 5 J

36. P.36 Book 4 Section 3.2 Mains electricity and household wiring Example 13 Electric kettle (b) Account for the difference in the answers of (a)(i) and (ii). Energy is lost to the surroundings and absorbed by the kettle.  The energy absorbed by the water is less than the energy provided by the kettle.

37. P.37 Book 4 Section 3.2 Mains electricity and household wiring Example 13 Electric kettle (c) The kettle is designed to operate at 220 V or 110 V with the same power output of 2000 W. (i) Where should S be connected when the kettle is connected to the 110-V mains and the 220- V mains? 110-V mains: S should be connected to P 220-V mains: S should be connected to Q

38. P.38 Book 4 Section 3.2 Mains electricity and household wiring Example 13 Electric kettle (c) (ii) What happens if S is connected wrongly? At 110-V and S connected to Q :  longer time for the water to boil At 220-V and S connected to P :  large current flows in the circuit  fuse broken

39. P.39 Book 4 Section 3.2 Mains electricity and household wiring Example 13 Electric kettle (c) (iii) Find the resistance of R 1 and R 2. By P =, V 2 R R1 =R1 = V 2 P = 6.05  R 1 + R 2 = V 2 P = 220 2 2000 = 24.2  R 2 = 24.2 – 6.05 = 18.15  110 2 2000 =

40. P.40 Book 4 Section 3.2 Mains electricity and household wiring A The failure of one appliance connected to a socket does not affect the others. B All appliances connected to sockets can function at the same voltage of 220 V. C Unlimited sockets can be installed. D Thinner wires can be used. Check-point 4 – Q1 Which of the following is not an advantage of a ring main?

41. P.41 Book 4 Section 3.2 Mains electricity and household wiring Check-point 4 – Q2 Appliances connected to sockets are joined in A series. B parallel.

42. P.42 Book 4 Section 3.2 Mains electricity and household wiring Check-point 4 – Q3 A fuse inside a consumer unit is rated at 30 A. Which cables (rated at 10 A, 15 A and 30 A) can be used in the ring main connecting to that fuse? 15-A and 30-A cables

43. P.43 Book 4 Section 3.2 Mains electricity and household wiring The End