November 2010 Paper 2 Exam Question 1 and Question 2 (ANSWERS)

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PAPER 2 QUESTION 1 OF NOVEMBER 2010
THREE PHASE AC CIRCUITS: PROSPECTIVE SHORT CIRCUIT CURRENT

1. Calculate the PSCC for a three phase fault if the following information is given:

*Transformer capacity 200 kVA at 400/250 volt
*Transformer protection 200 amp per phase
*70 mm2 4 core copper cable + ECC of 100 meters
*Z% of the transformer is 5% ohm

2. Calculate the touch voltage at the load if the ECC is the same size as one of
the phase conductors

3. What is the total loop impedance for the circuit?

ANSWERS

1. 2.26 kA

2. 12.748 V

3. 0.1037418 Ohm

Hint For Question 3:
Z(Total) = Z(Transformer) + Z(Phase Conductor) + Z(PEN or PE Conductor)


PAPER 2 QUESTION 2 OF NOVEMBER 2010 VOLTAGE DROP CALCULATION

A 1 kW single phase electrical motor is used as part of a lift system to hoist and deliver a 4 tons of bricks per hour at a building site.
If the supply voltage is 230 V at the basement of the lift with the dumping floor level 65 m higher, calculate the following:

2.1 The percentage volt drop just before the load is dumped (7 Marks)**

NOTE: Take a motor power factor of 0,9 with a hoist speed of 100 r/min through the reduction gearbox when fully loaded. Use a 3 core PVC armoured copper cable of 4 mm2 for the entire installation

2.2 The resistance of the ECC (1 Mark)**

2.3 The touch voltage at the motor if a 5 amp circuit breaker is used (2 Marks)**

ANSWERS
2.1 1.50%

2.2 0.3575 Ohm

2.3 3.575V


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