There are several ways to predict the reliability of an energy meter. The International Electrotechnical Commission (IEC) provides a series of standards related to reliability and dependability tests or evaluations.
IEC 62059-11 - general concepts
IEC 62059-21 - collection of meter dependability data from the field
IEC 62059-31-1 - accelerated reliability testing
IEC 62059-41 - reliability prediction
The method described in IEC 62059-41 is a white box method. The calculation of the failure rate this way is done with a theoretical approach. Several accredited testing laboratories offer to review and certify the evaluation performed by the manufacturer.
The manufacturer calculates the failure rate according to this method, as he is the only party able to do this. The testing laboratory will review the calculation and method. Calculations that meet the requirements will be awarded with a certificate.
The method for reporting and calculating the failure rate is up to the manufacturer. Most important is that it meets the requirements of IEC62059-41.
pictures are taken from running tests in the environmental section of our ISO17025 accredited type test laboratory
Beside of theoretical analysis another approach is to test the reliability and functionality of the
meters inside a climatic chamber for a duration of 8...12 weeks. The climatic chamber follows a certain profile for temperature and humidity while the meters
inside are energized with a profile varying from nominal load to maximum load.
Before and after this test the meter is verified for functionality and accuracy. Still this test can not cover all aspects of the annual drift.
We deliver the sources for doing these tests and can take care about integration into the climatic chamber. The test procedure itself has to be defined together with the meter manufacturer. We can make suggestions based on best practice.
When you are generating power by solar or wind, you might produce more energy than you actually need. This additional power is feed to the power grid and you as customer get those produced units as a credit for later use. Many utilities worldwide have already implemented such incentives coming together with a NEM (net energy metering) meter, such as our CL710K20.
Do you have already experience with NEM ? Drop us a message and share your opinion with us. Thank you for taking a look.
Note: This article deals only with the aspects related to energy meters.
Recently we saw in a tender 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. Example Europe: 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. 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.