Accuracy classes for current transformers explained
What do specifications like class 0,5 or 0,2s mean for a current transformer? An overview of the common accuracy classes and their areas of application.
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What does the accuracy class mean?
The accuracy class indicates the maximum permissible percentage deviation of the measured from the actual current at rated load, defined according to the relevant IEC/VDE standards (including IEC 61869, VDE 0414). The lower the class number, the more accurate the measurement.
Overview of common measuring classes
- Class 0,1: Precision measurement
- Class 0,2 / 0,2s: very accurate measurement
- Class 0,5 / 0,5s: accurate measurement
- Class 1: general operational measurement
- Class 3: coarse measurement
The "s" marking: extended measuring range
Classes with the suffix "s" (e.g. 0,2s or 0,5s) maintain the specified accuracy even at very low load, well below the rated current. This is particularly relevant for calibratable billing measurements or for strongly fluctuating currents – for example when measuring PV feed-in currents, see our article on current transformers for photovoltaic systems.
Protection current transformers: own class classification
For protection purposes, such as controlling overcurrent protection devices, own classes such as 5P or 10P apply. Here, the focus is not on measurement accuracy in rated operation, but on defined behavior in the event of a fault, e.g. in the case of a short circuit. These protection classes are not to be confused with the measuring classes mentioned above.
Which class for which application?
- Calibratable billing measurement: Class 0,2s or 0,5s, see also our billing current transformers
- General operational and consumption measurement: Class 1
- Protection functions: 5P/10P classes via separate protection current transformers
Questions about the appropriate accuracy class?
Tell us your application – we will advise you on the appropriate class and the appropriate current transformer type.
Frequently asked questions
The "s" denotes an extended accuracy class that maintains the specified measuring accuracy even at very low loads below the rated current – relevant, for example, in billing measurement of feed-in currents that can fluctuate strongly.
No. Measuring classes such as 0.2 or 0.5 describe the accuracy in normal measuring operation. Protection classes such as 5P or 10P, on the other hand, describe the behaviour in the event of a fault, for example during a short circuit, and are used with separate protection current transformers.