Applied Electricity Paper 2 , May/June 2010  
Questions: 1 2 3 4 5 6 7 8 Main
General Comments

Question 3

(a) State one method of damping for each of the following instruments:
(i) impedance;
(ii) current.

(b) List the electrical quantities that can be measured with the moving - coil instrument.

(c) 25mA20A

Figure 4 shows a moving-coil instrument which is to be used as an ammeter. Calculate the value of resistor R to enable the instrument to read up to 20A


The expected answers were:
(a) (i) air/air dash-pot
(ii) eddy current
(b) (i) d.c. current only
{ii)d.c. voltage only
(c) 25mA = 0.025 A
Current through shunt, I = 20 - 0.025 = 19.975 A
Voltage across instrument, V = 0.025 x 10 = 0.25 V
.'. Shunt resistor, R = V /1
The question was on Measuring Instruments. The question part (b) was less popular with the candidates. Many candidates could not recall the method in which the damping is anchored in each instrument. However, a very large number got the 1(c} and calculated the shunt value.

(ii) Phase voltage = 415N3 = 239.6 V
Current in Red phase = kW x 1000/phase voltage
= 15 x 1000/239.6 = 62.60 A
Current in the Yellow phase = 10 x 1000/239.6 = 41.74 A
Current in Blue phase= 5 x 1000/239.6 = 20.87 A

The question was on Electrical Energy Supply and Electrical Wiring. It was quite unpopular. The performance was poor. The explanation was not got by most of the candidates as they could not differentiate between line and phase voltages. Most of the candidates did not attempt part (b) and this was attributed to inadequate preparation as pointed out by the Chief Examiner.

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