1. 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. TABLE 1 Resistances of Copper Conductor and Steel Wire Armour for 3-core copper PVC Insulated Cables at 20°C.
2. 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 from Figure 3 Below.
3. Calculate the primary setting current and TMS for an inverse-time overcurrent relay in a system where the maximum load is 400 A, the maximum fault current is 7000 A and CT ratio is 600/1, if the relay has to operate in 0.7 s. Assume settings are 50% to 200% in 25% steps. (110% of max. load has to be catered for.) Use the BS curve of an inverse – time overcurrent relay, FIGURE 1 for reference.
4. For the radial power system shown in FIGURE 2, the outgoing circuit at C is fused and will operate in 0.1 s for the fault level at C. The fault level at B is 6000 A. If the required discrimination time between the relay at B and the fuse at C is 0.3 s, calculate a suitable TMS setting for the relay at B. Assume CT settings are from 50% to 200% in 25% steps. 110% of maximum load is to be catered for.
5. FIGURE 3 shows the use of unit protection where differential relays are protecting transformers. For each of the circuit breaker configurations: (i) CB A open and CB B closed (ii) CB A closed and CB B open (all other CBs closed) (a) To a base of 10 MVA determine initially the fault levels at the 11 kV and 3.3 kV busbars for a fault at the transformer, T1, output as shown. (b) Determine the current settings and TMSs for the relays shown if the operating time of the directional overcurrent relays on the 3.3 kV side of the transformers is 0.5 s. (Use a 0.4 s discrimination interval.) State with which of the CB configurations the directional overcurrent relays (if any) will not operate.