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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
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).
If I put a short in across the terminals a and b, does that mean the Isc = 0 since the capacitor would eventually charge and thus become an open circuit If so, how to do you find Vt and Rt
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the discrete-time systems, where y[n] and x[n] are, respectively the output and the imput sequences, determine whether or not the system is (1) linear, (2) causal, (3) stable, and (4) shift-invariant: y[n] = x[n+3];
We start with two initially uncharged capacitors C1 = 5μF and C2 = 20μF connected in series. Then, a 20-Vsource is connected to the series combination, as shown in Figure. Find the voltages v1 and v2 after the source is applied.
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