Building Services Engineering 5th Edition Handbook

314 Electrical installations
Joint or junction box
Lamp
Single fluorescent lamp
Time switch
Switched socket outlet
Intake and control point
Main switch
kWh meter
Consumer’s earthing terminal
Electricity appliance
Heater
Telephone call point
Earth
Single-pole switch
13.13 Drawing symbols for electrical installations.
Questions
1. Explain how electricity is generated and transmitted to the final user.
2. List the sources of energy used for the generation of electricity and state their immediate
and long-term benefits.
3. Explain, with the aid of sketches, the meaning of the terms ‘single-phase’ and ‘three-phase’
electricity supplies and show how they are used within buildings.
4. What does ‘balancing the phases’ mean?
5. Calculate the electrical resistance per metre length at 20 ◦ C of a copper conductor of a
10 mm 2 cross-sectional area.
6. Find the electrical resistance of a copper conductor 1.5 mm 2 in cross-sectional area if its
total length is 25 m and its temperature is 20 ◦ C.
7. A 28 m copper conductor 4 mm 2 in cross-sectional area is covered with thermal insulation,
which causes the cable temperature to rise to 45 ◦ C. Calculate the percentage increase in
electrical resistance compared with its value at a cable temperature of 20 ◦ C.
8. Sketch the methods of connection used for measurements of current, voltage drop and
power consumption in an electrical resistance heater on an alternating current circuit.