Energy Efficiency in Lighting Questions and Answers

Answer: B

Correct answer: 22 per cent

\[ \frac{46 - 36}{46} = \frac{10}{46} = 21.7\ \mathrm{per\ cent}. \] Taking the loss against the lamp wattage instead gives 28 per cent, which is offered here. This is why an installation must be counted at circuit watts rather than at the figures marked on the tubes.

Answer: C

The area is 80 square metres, so 24 000 lumens would suffice at the working plane, and dividing by the product of the two factors, 0.4, gives 60 000 from the lamps. The two factors together double the requirement, which is why they must never be omitted.

Answer: D

Sixty thousand divided by four thousand is fifteen, which would be arranged as three rows of five for uniformity. The arrangement matters as much as the count, since the standard sets a uniformity as well as an average.

Answer: A

Correct answer: over-lighting, and an opportunity to remove or dim luminaires

Two and a half times the required illuminance is energy spent on light nobody needs, and it can be recovered by removing lamps, dimming, or reducing the maintained level at the next relamping. Note that a design allows some margin for depreciation, but not a margin of this size.

Answer: B

Correct answer: 80 per cent of that room's lighting energy

\[ \frac{10-2}{10} = 80\ \mathrm{per\ cent}. \] Eight of the ten hours are lit with nobody present, and the sensor removes them. This is why stores, washrooms and meeting rooms are where sensors pay, and why fitting one in a continuously occupied space saves nothing.

Answer: C

Two hundred tubes removed at 46 watts is 9.2 kilowatts, and over 2500 hours that is 23 000 units. The remaining fittings should be checked to confirm the illuminance still meets the standard and the uniformity is acceptable.

Answer: D

Correct answer: 70 per cent of its original light

Thirty per cent of the output is absorbed by the dirt, so seventy per cent emerges - while the power drawn is unchanged. The effective efficacy of the installation has therefore fallen by nearly a third, which is what a cleaning programme recovers for the cost of the labour.

Answer: A

Correct answer: 1-Q, 2-R, 3-P

Three quantities and three units. Luminous flux is measured in lumens, the total light emitted. Illuminance is measured in lux, the flux arriving per square metre. And efficacy is lumens per watt, the flux obtained per unit of electrical power - the figure of merit for a lamp.

Answer: B

Correct answer: 1-Q, 2-R, 3-P

Three measures acting on three different terms of the same product. A sensor reduces the hours of operation. An electronic ballast reduces the circuit watts per lamp. And light decoration raises the utilisation factor, so less flux is needed for the same illuminance. Energy is hours times watts, and the third measure reduces how many lamps are required at all.

Answer: C

Correct answer: 3, 1, 2, 4

Recording what is installed and how long it runs comes first, since the energy cannot be computed without it. The illuminance is then measured, compared against the standard for the task, and the opportunities identified from the gap between what is provided and what is needed.