Measurements

Explanation of question 11

A moving coil instrument has a sensitivity of 20,000/V and a resistance of 100. The resistance required to enable the instrument to read up to 250A is

1. 25 in parallel with the instrument
2. 25 in series with the instrument
3. 20 in parallel with the instrument
4. 20 in series with the instrument

The answer is :- 1Sensitivity = 1/fsd. Therefore fsd = 1/20000 Amps = 50A. To increase the fsd we must add a shunt in parallel.

Current through shunt = 200A. If the current through the shunt is four times that through the meter, then it's resistance must be four times lower (or one quarter) ie 25