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.
