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. 3. 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 6 mm2 10 mm2 16 mm2 25 mm2 35 mm2 50 mm2 70 mm2 95 mm2 120 mm2 150 mm2 185 mm2 240 mm2 300 mm2 Resistance/metre of Conductor (microhms per metre) 3080 1830 1150 727 524 387 268 193 153 124 99 75 60 Resistance/metre of Steel Armour (microhms per metre) 4600 3700 3200 2400 2100 1990 1400 1200 1100 740 680 600 540 TABLE 1 Resistances of Copper Conductor and Steel Wire Armour for 3-core copper PVC Insulated Cables at 20°C. Teesside University Open Learning (Engineering) © Teesside University 2011 4 4. 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).