Page 823 - Basic Electrical Engineering

Example 11.4 A voltmeter of range 0–300 V, and accuracy of 0.1 per cent on
full scale deflection has been used to measure 15 V. Calculate the error in
measurement as a result of using an instrument of higher voltage range to

measure a comparatively low voltage.

Solution:

The magnitude of error on full scale deflection, i.e., 300 V is

The maximum error or the limiting error is 0.3 V.

Now on 15 V the limiting error is to be taken as 0.3 V.

Therefore, percentage error = ×100 = 2 per cent.

It is to be noted that the error is much higher now. That is why one should not
use a high-range instrument to measure a low value.

Example 11.5 A voltmeter of the range 0–300 V has a guaranteed or limiting
error mentioned by the manufacturer as 1.0 per cent. Calculate the limiting
error (relative error) when the same instrument is used to measure lower

voltages.

Solution:

Limiting error for full-scale deflection is 1 per cent magnitude of limiting
error for

full scale deflection