Question

A current of [2 + \(\sqrt{2}\) sin (314t + 30) + 2\(\sqrt{2}\) cos (952t +45)] is measured with a thermocouple type, 5A full scale, class 1 meter. The meter reading would lie in the range?

(a) 5 A ± 1 %

(b) (2 + 3\(\sqrt{2}\)) A ± 1%

(c) 3 A ± 1.7 %

(d) 2 A ± 2.5 %

The question was posed to me during an online exam.

My query is from Advanced Problems on Indicating Instruments topic in section Extension of Instrument Ranges of Electrical Measurements

Answer 1

Correct answer is (c) 3 A ± 1.7 %

The best I can explain: I = [2 + \(\sqrt{2}\) sin (314t +30°) + 2\(\sqrt{2}\) cos (952t + 45°)]

Thermocouple measure the rms value of current.

Irms = \([2^2 + (\frac{\sqrt{2}}{\sqrt{2}})^2 + (\frac{2\sqrt{2}}{\sqrt{2}})^2]^{1/2} = \sqrt{9}\) = 3 A ± 1.7%.