Example
A 20A radial socket outlet circuit wired 2.51 103
Ω/m. .
. Also the above formula, the
resistance the line conductor and the resistance the earth
conductor. Calculate the total earth fault loop impedance ZS, and establish that the value
is less than the maximum value permissible for this type circuit.7
Earth fault loop path for TN-S system. The complete earth fault loop path shown Fig. The earth fault loop impedance the
supply 0. 12.Basic Electrical Installation Work
248
TN-S (cable sheath earth) supplies.5mm protective conductor has (R1 R2) value of
19.5Ω. take account this, must multiply the value cable resistance the factor
Supply
company’s
installation
Consumer’s
installation
Line
conductor
R1
R2
Load
Consumer’s
isolation and
protection
equipment
Supply company’s
metering and
protection equipment
on consumer’s premises
Protective
conductor
Neutral
conductor
Fault
Secondary
winding
of supply
transformer
ZE
FIGURE 12. Ω
From the value given Table the Site Guide and reproduced Table 12.
Ω Ω
However, under fault conditions, the temperature and therefore the cable resistance will
increase.
The cable length 30m installed ambient temperature 20°C and the consumer’s
protection 20A MCB Type 60898.
We have:
Z R
Z
S E
E
( )
(value given the question)
1 2
0 5
Ω
.5mm phase conductor with 1.7.5mm2
CPC.1
of this book.
Values have been calculated for copper and aluminium conduc-
tors and are given Table the Site Guide shown Table 12.5mm2
PVC cable incorporating 1.1 a
2.
For cable m
R R
1 2
3
19 585
