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The Critical Change Value

The critical change value indicates the maximum amount of change allowed in the meter’s energy registers during disturbance tests without any current flowing in the meter’s current circuits.

Whenever you see that the test evaluation has to be done according to the acceptance criteria B in the edition 2 of IEC 62052-11, you need to check the critical values at least for the active- and reactive SUM registers.

You can only do this evaluation when the register resolution is high enough. Preferable you will read the registers by PC via the optical interface. Meters without interface must have a possibility to switch the registers temporary to a high resolution mode.

Aristo Trilog slide rule

You can do the calculation manually, with your slide rule or you use the calculator below.

Critical change value calculator
elements m
nominal voltage Un V
maximum current Imax A
Formula x = 10-6 * m * Un * Imax
maximum allowed change

What about the elements?
This question comes regularly up. To keep it simple:
Single phase meters: one element
Three-phase three wire meters: two elements
Three-phase four wire meters: three elements

Thank you for reading and “Happy Testing!”

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