Electrical and electronics measurements miscellaneous Difficult Questions and Answers

Electrical and electronics measurements miscellaneous

The power factor in a single phase circuit is cos φ = P/VI where P power read by an wattmeter, V = voltage read by a voltmeter and I = current read by an ammeter. All the three instruments have a limiting error of ± 1%. The power factor of the circuits can be known if only one meter i.e. an electrodyna-mometers type power factor having an limiting error ± 1% used. The preferred method is to use a single instrument like a power factor meter rather than using 3 instruments because the error with using wattmeter, voltmeter and ammeter is

The voltage of a circuit is measured by a voltmeter having an input impedance comparable with the output impedance of the circuit thereby causing error in voltage measurement. This error may be called

1. Gross error
1. Random error
1. Error caused by misuse of instrument
1. Error caused by loading effect

In the center zero analog ammeter having a range of – 10 A to + 10 A, there is a detectable change of the pointer from its zero position on either side of the scale only if the current reaches a value of 1 A (on either side). The ammeter has a

1. resolution of 1 A
1. dead zone of 1 A
1. dead zone of 2 A
1. sensitivity of 1 A

A d.c. circuit can be represented by an internal voltage source of 50 V with an output resistance of 100 kΩ. In order to achieve 99 percent accuracy for voltage measurement across its terminals, the voltage measuring device should have a resistance of

The power in a 3-phase, 3-wire load is measured using two 100W full-scale wattmeters W₁ and W₂. W₁ is of a accuracy class ± 1% and reads 100 W. W₂ is of accuracy class ± 0.5% and reads – 50 W. Then the uncertainty in the computation to total power is

∴ W₁ + W₂ =		50 ±	1.5		× 100 = ± 3%
			50

∴ W₁ + W₂ =		50 ±	1.5		× 100 = ± 3%
			50