Building Services Engineering 5th Edition Handbook

Electrical installations 315
9. State the function of power factor correction in alternating current circuits.
10. Calculate the apparent power, in kilovolt-amperes, of an electric motor which is connected
to a 415 V AC three-phase supply and has a current flow of 17.5 A.
11. Calculate the resistance of a3kWimmersion heater on a 240 V AC circuit.
12. What current, in milli-amperes, would flow to earth during an insulation resistance test when
a 500 V DC emf is applied between the line and protective conductors and the resistance is
found to be 1.75 M?
13. Show by sample calculation why smaller cables can be used for long-distance power
transmission when very high voltages are used.
14. A 33 kV supply to a factory carries 250 A per phase or line. Calculate the usable electrical
power in the factory if the average power factor is 0.68.
15. Find the maximum length of 6 mm 2 cable that can be used if the maximum current-carrying
capacity is to be utilized on a 240 V circuit.
16. A building site is to have the following electrical equipment in use each day:
(a) concrete mixer, 5 kW, 4 h, 415 V;
(b) sump pump, 1.5 kW, 6 h, 240 V;
(c) 20 lamps, 150 W each, 4 h, 240 V;
(d) 5 flood lamps, 300 W each, 4 h, 240 V;
(e) 6 hand tools, 750 W each, 5 h, 110 V.
The power factor of the machinery is 0.8. Site work takes place 5 days per week for 28
weeks. Electricity costs 6 p per unit. Find
(i)
(ii)
the total kilovolt-amperes and the line current of the required temporary incoming
supply system, and
the cost of the electricity used on site during the contract.
17. Sketch and describe the safety precautions taken to avoid contact with both overhead and
underground electricity cables during site construction work.
18. Sketch a suitable arrangement of temporary wiring, control and safety equipment on a site
where the following items are employed: tower crane, sump pump, five flood lamps, security
lighting and circuits on each of three floors for hand lamps and tools.
19. List the site programme for the main contractor in the installation and operation of temporary
site electrical services.
20. Sketch and describe the characteristics of rewirable and cartridge fuses and residual current
devices.
21. Show how an underground electrical service cable enters a building. Sketch the arrangement
of electricity distribution within a typical residence.
22. List the cable and conduit systems used for electricity distribution and state their applications.
23. Briefly describe the methods of testing electrical installations.
24. State the requirements of telecommunications installations.
25. Sketch and describe a lightning conductor installation for a city centre office block.
26. Design a lightning conductor system for a 60 m high building on clay shale using 4.5 m
earth rods. The down conductor is to be a copper rod 10 mm in diameter.
27. Why is power factor in electrical systems an issue for concern? More than one correct
answer.
1. It is not of any concern.
2. Low-power factor means electrical energy is used inefficiently.
3. High-power factor means electrical energy is wasted.