Question 4

The expected answers were:

• The two types of controlling device in measuring instruments are:
• Gravity control device
• Spring control device
•   (i) The resistance Rsh to read up to 10 A is

Ish = I – I­m = 10 – 0.003 = 9.997 A
Vsh = RshIsh = Vm = ImRm
Rsh = ImRm/ Ish
Rsh =
Rsh = 3 x 10-3 = 3 mΩ.
                 (ii) The resistance Rseries  to  read up to 100 V is obtained by using:
                             VT = Vm + Vs = ImRm + IsRs = 100 V
                                                IsRs = 100 - ImRm
                                                Rs =
                                                 = Rseries
                                                Rseries = 33 kΩ
The question was on Measuring Instruments. It was very popular with the candidates. Many candidates attempted this question but more than half of those who attempted could not list control devices.
It was reported that the calculation aspect was well-done by almost all the candidates. However, few candidates did not subtract the current through the meter and voltage drop across the meter from the determination of shunt and multiplier values respectively.