Page 257 - Handbook of Electrical Engineering

CABLES, WIRES AND CABLE INSTALLATION PRACTICES 241

2
For a 16 mm cable,
Z a + Z c = 0.2960 + j0.0268 + 0.72 = 1.016 + j0.0268

Z loopc = Z a + Z c + Z sec + 0.1
= 1.016 + j0.0268 + 0.00308 + j0.0308 + 0.1
= 1.119 + j0.0576

Magnitude of Z loopc = 1.1206 ohms
The resulting Z loopc calculated from the circuit for each cable is given below,
2
For the 16 mm the shock voltage is,
0.72 + 0.1 415
V shock = √ = 175.3 volts
1.1206 3

2
If a 50 mm 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 for Z loopf ,

Table 9.35. Earth loop impedance results for the worked example with steel
wire armouring
Nominal conductor Z a + Z c Z loopc V shock
2
area (mm ) magnitude
(ohms)
16 1.016 + j0.0268 1.1206 175.3
25 0.6872 + j0.0250 0.7922 181.5
35 0.5948 + j0.0242 0.6999 191.7
50 0.4998 + j0.0236 0.6053 197.9
70 0.4288 + j0.0224 0.5344 206.2

Table 9.36. Limiting values of earth loop impedance when MCCB
is used
MCCB rating Lowest operating Maximum earth loop
(amps) current at impedance at 240 V/phase
5.0 sec (amps) Z loopf (ohms)
100 370 0.6476
125 470 0.5098
160 720 0.3328
200 900 0.2662