Page 252 - Handbook of Electrical Engineering

236 HANDBOOK OF ELECTRICAL ENGINEERING

e) Disconnection time:
The standard recommends two nominal disconnection times 0.4 and 5.0 seconds. The time of
0.4 seconds is based on a nominal phase-to-neutral voltage of approximately 240 Vac, where as
the time of 5.0 seconds is invariant of voltage.

Where the distribution circuit feeds a stationary item of equipment, not socket outlets and not
portable equipment, the disconnection time may be taken as 5.0 seconds. This applies to motors.
The nominal time of 0.4 seconds is intended for circuits supplying socket outlets, regularly
moved portable equipment and Class 1 hand-held equipment. For voltages (V ph ) different from
240 Vac, the disconnection time (t dis ) of 0.4 seconds becomes approximately related as,

600

t dis 1.149 log 10 seconds
V ph
with a lower limit of 0.1 second.
The maximum value of Z loop can be determined from the following information,

• The network phase-to-neutral voltage V ph .
• The operating current that causes the supply protective device to disconnect the consumer in the
specified time t dis . This can be found from the manufacturer’s data.

9.4.3.6.1 Worked example
A 37 kW 415 V induction motor is protected by fuses at the motor control centre. The route length
of the motor feeder cable is 200 metres. The supply frequency is 50 Hz. The MCC is fed by one
250 kVA, 4.5% impedance, transformer. Assume an X/R ratio of the transformer of 10.0. The motor
running efficiency at full-load is 92% and its power factor is 0.85. The starting to running current
ratio is 7.0, and the starting power factor is 0.45. The cable is routed in air that has an ambient
◦
◦
temperature of 40 C. The conductor maximum temperature is 90 C. The insulation material is EPR
and the armouring is galvanised steel wire braid. Assume the cable data in Tables 9.23 and 9.11
for 3-core cables is applicable. The motor fuse data are shown in Figure 8.4 for 100 A, 125 A and
160 A fuses. The permissible volt-drops in the cable for running and starting are 3.0% and 15.0%
respectively.
Find the most appropriate cable and fuses for the motor. Determine whether or not an earth
leakage current relay should be used at the motor control centre. Assume a TN earthing arrangement.

Replace the steel wire braid armour with round steel wires (GSWA) and reduce the metallic
return path impedance Z mr to 0.1 ohm, and compare the effect on the hazardous shock voltage.
Then replace the fuses with moulded case circuit breakers.

Solution:

a) Find the source impedance for a line-to-ground fault of negligible impedance. Refer all calculations
to the nominal supply voltage of 415 V. The source impedance is that of the single transformer
feeding the MCC.