Handbook of Electrical Engineering

CABLES, WIRES AND CABLE INSTALLATION PRACTICES 241

For a 16 mm^2 cable,

Za+Zc= 0. 2960 +j 0. 0268 + 0. 72 = 1. 016 +j 0. 0268

Zloopc=Za+Zc+Zsec+ 0. 1

= 1. 016 +j 0. 0268 + 0. 00308 +j 0. 0308 + 0. 1

= 1. 119 +j 0. 0576

Magnitude ofZloopc= 1 .1206 ohms

The resultingZloopccalculated from the circuit for each cable is given below,

For the 16 mm^2 the shock voltage is,

Vshock=

0. 72 + 0. 1

1. 1206

(

415

√

3

)

= 175 .3 volts

If a 50 mm^2 cable and a fuse rating of 125 amps are chosen as recommended in g) then the circuit

earth loop impedance is still too high by a ratio of about 1.65:1. Hence an earth leakage circuit

breaker should still be used for this motor circuit. The hazardous shock voltage is still too high.

j) Now replace the fuses with moulded case circuit breakers and show whether or not the situation
is improved. From Figure 7.9 it can be seen that a typical MCCB for motor application operates
in its inverse region for times equal to 5.0 seconds.
Repeating d) but for MCCBs gives the following limits forZloopf,

Table 9.35. Earth loop impedance results for the worked example with steel

wire armouring

Nominal conductor

area (mm^2 )

Za+Zc Zloopc

magnitude

(ohms)

Vshock

16 1. 016 +j 0. 0268 1.1206 175.3

25 0. 6872 +j 0. 0250 0.7922 181.5

35 0. 5948 +j 0. 0242 0.6999 191.7

50 0. 4998 +j 0. 0236 0.6053 197.9

70 0. 4288 +j 0. 0224 0.5344 206.2

Table 9.36. Limiting values of earth loop impedance when MCCB

is used

MCCB rating

(amps)

Lowest operating

current at

5.0 sec (amps)

Maximum earth loop

impedance at 240 V/phase

Zloopf(ohms)

100 370 0.6476

125 470 0.5098

160 720 0.3328

200 900 0.2662