Building Services Engineering 5th Edition Handbook

298 Electrical installations
power in watts = volts × amps
= 240 V × 1.5 A
= 360 W
From Ohm’s law:
I = V R
and so,
R = V I
= 240
1.5
= 160
EXAMPLE 13.4
PVC insulation on a conductor carrying 415 V has an electrical resistance to earth of
500 M. What leakage current could flow through the PVC when the cable is laid on an
earthed metal support? (1 M = 10 6 .)
The difference between line and earth is 240 V. From Ohm’s law
I = V R
240 V
=
500 × 10 6
= 0.48 × 10 −6 A
= 0.48 µA
EXAMPLE 13.5
Compare the currents carried by an overhead line at 400 and 33 kV for the transmission
of 10 MW of power for unity power factor, cos φ = 1.
For a three-phase system, using:
watts = volts × amps × √ 3 × cos φ
current in amps = watts
volts × 1 √
3
× 1
cos φ