Note: This article deals only with the aspects related to energy meters.
Recently I read a request for life-time calibration.
Actually electronic energy meters are adjusted for lifetime. While the old ferraris meters had the possibility for a re-adjustment, electronic meters are locked when leaving the factory. There is at least for CLOU energy meters no possibility for a re-adjustment. This is an additional feature to prevent tamper or fraud.
The meters are initial calibrated when leaving the factory.
How long is this calibration valid?
This is not a manufacturer decision. The calibration validity is fixed by the national metrological institute of the country where the meters are used. Power supply companies or utilities can set up their own more restrict rules.
The measurement instrument directive (MID) requires a first calibration done at the manufacturer site (module D). Common practice is to do a re-check on sample base after 8 years with portable meter test equipment. If the sample lot is inside of the accuracy class and no other negative observations, the meters are allowed to stay in the grid for another 4 years. Then the installed batch is sampled again.
Why is it necessary to recalibrate the meters?
Each electrical measurement instrument has an annual drift. Means, it changes the accuracy slightly. So it will reach somewhen the accuracy limits. Interesting fact is that the meter can move either to positive or negative direction even when produced with components from the same batch.
Good thing: The annual drift is under reference conditions linear. You can build up a history and make a prediction for the future.
For active power CLOU energy meters class 1 have an annual drift of 60 ppm to 100 ppm (0.01 %). So, after 20 years in the field we will have an accuracy change of ±0.2 %. It’s essential that the evaluation of the initial calibration is considering this drift.
All CLOU energy meters class 1 have an initial error of less than ±0.4%.
Keep an eye on the maximum initial error when you are sourcing energy meters.
The above explanations are simplified. If you have specific questions please put a comment in the box below.
Thank you for reading.
Editor's note: This article was originally published in October 2019 and has been updated for comprehensiveness.