Discussion
A meter of constant 800 rev/kWh makes 41 revolutions in 60 seconds while a wattmeter reads 3 kW. The error is about
Answer: C
Correct answer: plus 2.5 per cent
The meter records \( 41/800 = 0.05125 \) kWh in one minute, which is a power of \[ 0.05125 \times 60 = 3.075\ \mathrm{kW}. \] Against the true 3 kW that is \[ \frac{3.075-3}{3} = +2.5\%. \] The sign matters commercially: the meter is running fast and would over-charge the consumer.
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