The mean error of an electricity meter

Questions concerning the accuracy of electricity meters in actual service have reference to the error of registration over a period, rather than the speed error at any particular value of the power in the circuit. There will be a certain load at which the speed error is the same as the error of registration over the period in question, and the purpose of the author is to investigate a method of ascertaining this load. From a consideration of the sources of error in electricity meters it appears that, in meters in which a constant frictional torque is the only disturbing factor, the error of registration over a period is the speed error at the average load over that period. In meters having in addition a disturbing torque proportional to the square of the speed, the error over a period will be slightly slower than the error at the average speed. A discussion of the effect of varying power factor leads to the result that the error of registration due to this cause is equal to that determined by a speed test at an average power factor deduced from the readings of an energy meter and a meter integrating the wattless component of the power in the circuit.