224 HANDBOOK OF ELECTRICAL ENGINEERING
giving the form of equation found in the reference literature,
Isc=AK
√
tamps (9.7)Or
A=Isc√
t
Kmm^2If the time durationtis taken to be 5 seconds then the lowest acceptable cross-sectional areaA
for the cable for the various insulating materials is:-
PVC,A= 0. 01944 Iscmm^2
XLPE and EPR,A= 0. 01564 Iscmm^2
Bare copper,A= 0. 02051 Iscmm^2Equation (9.7) can be used for plotting the time-current characteristic of the cable when this
needs to be coordinated with those of the protective relays and fuses in the circuit.
In which case the equation is transposed as,t=A^2 K^2
Isc^2When plotted on log-log paper the equation has the form,
log 10 t=log 10 (A^2 K^2 )−2log 10 IscOr of the formy=a−bx
Where y=log 10 t, a=log 10 (A^2 K^2 )
b=−2andx=log 10 Isc
which is a straight line having a slope of−2.
The straight line is usually plotted over the time range of 0.1 to 5.0 seconds, to correspond
with the operating times of the protective devices.
When cables are to be sized for a particular project with regard to their short-circuit perfor-
mance it is necessary to consider the let-through current of the protective device in the circuit e.g.,
fuse, circuit breaker. It is also necessary to determine whether the consumer has ‘fixed’ equipment
such as a motor, or ‘temporary’ equipment such as a portable tool plugged into a socket, because this
establishes the minimum time duration. This aspect is described in more detail when the earth-loop
impedance is being considered, see sub-section 9.4.3.6.
Low voltage three-phase power systems often have their star-points earthed (grounded) by a
very low impedance conductor, usually the TT method of IEC60364, sub-section 13.3.3. Therefore
all the cables and consumer equipments are subject to a very high prospective short-circuit current
when a line-to-ground fault occurs. Low voltage induction motors used in the oil industry are usually
started direct-on-line. They have starting currents that can be as high as 7.5 times their full-load