Building Services Engineering 5th Edition Handbook

294 Electrical installations
current flows alternately in the forward and backward directions along the line wire. The overall
effect of three driving coils in the motor is a balance in the quantity and direction of the current
taken from the line conductors. There is no net return current in the neutral wire from such a
balanced load. Single-phase electrical loads, which are not in balance, produce a net current in
the neutral conductor.
The casings of all electrical appliances are connected to earth by a protective conductor, the
earth wire, connected to the earthed incoming service cable of the electricity supply authorities
or an earth electrode in the ground outside the building. Gas and water service pipes are bonded
to the earth by a protective conductor.
Circuit design
The resistance R ohms () of an electrical conductor depends on its specific resistance ρm,
its length l m and its cross-sectional area A m 2 . The specific resistance of annealed copper is
0.0172 µm (µ, micro stands for 10 −6 ) at 20 ◦ C.
R = ρ l A
EXAMPLE 13.1
Calculate the electrical resistance per metre length at 20 ◦ C of a copper conductor of
2.5 mm 2 cross-sectional area.
R = 0.0172
10 6 m × 1m
2.5 mm 2 × 106 mm 2
1m 2
= 0.0069
The resistance of a cable increases with increase in temperature and the temperature coefficient
of resistance (α) of copper is 0.00428 / ◦ Cat0 ◦ C. If the resistance of the conductor is
R 0 at 0 ◦ C, then its resistance at another temperature R t can be found from:
R t = R 0 (1 + αt)
where t is the conductor temperature ( ◦ C).
EXAMPLE 13.2
Find the resistance of a 2.5 mm 2 copper conductor at 40 ◦ C.
R 0 is not known but the resistance of this conductor at 20 ◦ C was found in Example 13.1
and t can represent the increase in temperature above this value. A graph of resistance versus