Calculate the earth loop impedance minimum fault current

Question

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).