CDC 3E052 Electrical Power Production Journeyman Practice Exam

When referring to RMS value, what do most voltmeters measure?

Instantaneous value. Average value. Effective value. Peak value.

Correct Answer:

Effective value.

Explanation:

RMS value is the effective value of an AC signal—the voltage that would produce the same average power (heating effect) in a resistor as a corresponding DC voltage. Most voltmeters are designed to display this effective, or RMS, value, not an instantaneous snapshot, not the simple average, and not the peak amplitude. For a sine wave, the RMS value equals the peak value divided by the square root of two, and true RMS meters read the actual RMS for any waveform, while older meters may approximate it only for sine waves. The instantaneous value is a momentary reading, the average value is not the same as the heating-equivalent value, and the peak value is just the maximum amplitude—RMS relates to power equivalence.

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