Question 1. An 11 kV motor is fed by cables from a transformer via switchgear, having a phase impedance of 0.3 + j0.3 ohm. The earth return path to the transformer neutral has a resistance of 0.42 ohm. Determine a suitable value of neutral resistance if the voltage rise at the motor in the event of an earth fault at the motor is not to exceed 430 V. Neglect transformer winding resistance.
Question 2. Using TABLE 1, determine the resistance of a conductor and the volt drop per ampere metre at 70°C for a 95 mm2 pvc/swa/pvc copper cable. The temperature coefficient of resistance is 0.004 per °C.
|
Copper Conductor Size
|
Resistance/metre of Conductor
(microhms per metre)
|
Resistance/metre of Steel Armour
(microhms per metre)
|
|
6 mm2
|
3080
|
4600
|
|
10 mm2
|
1830
|
3700
|
|
16 mm2
|
1150
|
3200
|
|
25 mm2
|
727
|
2400
|
|
35 mm2
|
524
|
2100
|
|
50 mm2
|
387
|
1990
|
|
70 mm2
|
268
|
1400
|
|
95 mm2
|
193
|
1200
|
|
120 mm2
|
153
|
1100
|
|
150 mm2
|
124
|
740
|
|
185 mm2
|
99
|
680
|
|
240 mm2
|
75
|
600
|
|
300 mm2
|
60
|
540
|
TABLE 1 Resistances of Copper Conductor and Steel Wire Armour for 3-core copper PVC Insulated Cables at 20°C.
Question 3. A load, 100 m from a 415 V Supply Authority point is connected by a 35 mm2 pvc/swa/pvc copper cable. The current rating is 119 A and the volt drop per ampere metre is 1.1 mV. If the load current is 90 A, determine whether the volt drop at the load is within the 17th Edition of the IEE Wiring Regulations 5% limit. Using TABLE 1, calculate the earth loop impedance, minimum fault current (neglect the supply/earth impedance) and select an appropriate fuse for the circuit with reference to B.S. 88 part 2 (see FIGURE 3, Lesson 4 - 4).