Energy Meters Questions and Answers

Answer: A

Rated current is forced through the few thick turns of the series magnet by a source of only a few volts, because the impedance of that circuit is very low. The phase between the two sources is adjustable so that any power factor may be simulated.

Answer: B

Error is always referred to the true value, so the denominator is the energy actually supplied. A positive result means the meter registers more than the consumer has used, which is why the tolerance is specified in both directions.

Answer: C

If the disc turns faster than it should, the register advances too far for the energy passed and the consumer is overcharged. National regulations set limits on the error in both directions, typically a few per cent for a domestic meter.

Answer: D

It tests the whole chain including the gear train and register, whereas counting disc revolutions tests only the driving element. A long run is needed for a useful result, so it is used for a final check rather than for adjustment.

Answer: A

Creeping is a torque that exists in the absence of load current, so the test applies voltage alone and watches the disc. Standards allow the disc no more than one complete revolution in a stated period under this condition.

Answer: B

Blondel's theorem applies to energy just as to power, so a three-wire system needs two elements. Both act on the same spindle, or on two discs on one spindle, so that the register adds their contributions automatically.

Answer: C

One fewer than the number of wires gives three, and each element then measures the energy of its own phase with the neutral as reference. This handles any degree of unbalance, which a four-wire system is very likely to have.

Answer: D

If one element is more effective than the other, the meter reads correctly only when the phases are equally loaded. The elements are therefore balanced during manufacture by testing each alone at the same power and equalising their torques.

Answer: A

Demand is defined over a block of time, usually fifteen or thirty minutes, because the plant supplying it has thermal inertia. A momentary peak is of no consequence to the supply, so the indicator integrates over the block and retains the largest value reached.

Answer: B

Generators, transformers and cables must be rated for the greatest load a consumer imposes, and that capital cost is incurred whether the consumer uses it for one hour or for a thousand. The demand charge recovers it, while the energy charge recovers the running cost.