Energy Audit Instruments Questions and Answers

Answer: A

Correct answer: the power analyser

A power analyser measures voltage, current, real and reactive power, power factor and harmonics simultaneously, and logs them over time - which is what an audit needs, since loads vary and a single reading proves nothing. A multimeter gives one quantity at one instant, and the insulation tester and tachometer answer quite different questions.

Answer: B

The split core encircles the conductor and behaves as a current transformer, or carries a Hall element for direct current. Being able to measure a live feeder without shutting the plant down is what makes it the auditor's first instrument.

Answer: C

An averaging instrument is calibrated on the assumption of a sine wave and reads well below the truth on the peaky current a switched-mode supply or a variable speed drive draws. The error can be tens of per cent, which falsifies the whole load estimate.

Answer: D

Its scaling assumes the rectified mean of a sinusoid, and a waveform with narrow tall peaks has a much smaller mean for the same rms value. Believing such a reading understates the load and hides the very inefficiency being looked for.

Answer: A

Harmonic currents circulate without doing useful work, heat the transformer and the neutral, and derate the cables, all of which is invisible on an ordinary meter. Quantifying them is what justifies a filter or a change of drive.

Answer: B

Correct answer: raises the current for the same useful power

\[ I = \frac{P}{\sqrt{3}\,V\cos\phi}, \] so a low power factor means more current for the same real power. That extra current heats the cables and the transformer, consumes capacity that could carry useful load, and attracts a penalty on most tariffs. It does not reduce the voltage at the load appreciably, nor cause harmonics - though harmonics do reduce the overall power factor.

Answer: C

Areas lit to three times the recommended level are common and represent energy spent for nothing, and the meter is what turns an impression into a number that can be compared with the standard. Lamp power and ballast loss need an electrical measurement.

Answer: D

The quantity that matters is the illumination on the surface where the work is done, and the sensor must weight light arriving obliquely as the surface does. Measuring near the lamp reports its output rather than the useful result.

Answer: A

A polished metal surface of low emissivity radiates far less than a matt one at the same temperature, so an uncorrected reading is badly low. Setting the emissivity, or sticking a patch of matt tape on the surface, is what makes the reading trustworthy.

Answer: B

Emissivity of the order of 0.1 means only a tenth of the expected radiation reaches the instrument, and the reading can be a hundred degrees low on a steam line. A matt sticker or a contact probe resolves it in seconds.