Current transformer accuracy classes: 0.2, 0.5, 1 and 0.2S, 0.5S explained
The accuracy class of a current transformer states the permissible ratio error in percent at rated current. A class 0.5 transformer may deviate from the ideal ratio by at most ±0.5 % at 100 % and 120 % of rated primary current and shift the current by at most ±30 minutes of arc. The basis is the standard IEC 61869-2.
At partial load the limits widen: at 20 % and 5 % of rated current classes 0.2, 0.5 and 1 allow larger errors, below 5 % the standard specifies nothing. The S classes 0.2S and 0.5S are specified down to 1 % of rated current and reach their full class value from 20 % – they are intended for metering points whose load is often far below rated current.
All limits apply at rated frequency and a burden between 25 % and 100 % of the rated output (classes 0.1 to 1). CELSA manufactures metering current transformers in classes 0.5, 1 and 3, and calibrated plug-on current transformers (ALO-E) in 0.2S, 0.2, 0.5S and 0.5.
Error limits to IEC 61869-2 at a glance
| Class | 1 % In | 5 % In | 20 % In | 100–120 % In | Phase displacement at In |
|---|---|---|---|---|---|
| 0.2S | ±0.75 % | ±0.35 % | ±0.2 % | ±0.2 % | ±10′ |
| 0.2 | – | ±0.75 % | ±0.35 % | ±0.2 % | ±10′ |
| 0.5S | ±1.5 % | ±0.75 % | ±0.5 % | ±0.5 % | ±30′ |
| 0.5 | – | ±1.5 % | ±0.75 % | ±0.5 % | ±30′ |
| 1 | – | ±3 % | ±1.5 % | ±1 % | ±60′ |
| 3 | – | – | – | ±3 % (50–120 % In) | not specified |
In = rated primary current. “–” means the standard specifies no limit at this point. Between two reference points the value of the lower one applies. Valid at rated frequency and 25–100 % of rated burden (class 3: 50–100 %).
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FAQ
What does accuracy class 0.5 mean for a current transformer?
At 100 % and 120 % of rated primary current the ratio error may be at most ±0.5 % and the phase displacement at most ±30 minutes of arc. At 20 % of rated current ±0.75 % is allowed, at 5 % ±1.5 %. This applies to a burden between 25 % and 100 % of the rated output.
What is the difference between class 0.5 and 0.5S?
The S class is specified down to 1 % of rated current (±1.5 %) and already holds the full class value of ±0.5 % from 20 %; at 5 % ±0.75 % is allowed. Class 0.5 is only specified from 5 % and allows ±1.5 % there. Where the load is often far below rated current, a 0.5S transformer is clearly more accurate at partial load.
Which accuracy class do I need for billing?
Billing third parties requires calibrated (conformity-assessed) transformers; the class required – usually 0.5, 0.5S, 0.2 or 0.2S – is set by the grid or metering point operator. CELSA supplies the calibrated ALO-E plug-on current transformers in all four classes.
Which class is sufficient for energy monitoring and display?
For operational metering and energy monitoring without billing, class 1 is usually sufficient, class 0.5 for load comparisons at partial load. Class 3 is intended for display and supervision only; it is specified only between 50 % and 120 % of rated current and has no phase displacement limit.
Does the accuracy class apply at any burden?
No. For classes 0.1 to 1 the class applies at a burden between 25 % and 100 % of the rated output, for class 3 between 50 % and 100 %. If the burden is higher the transformer saturates earlier and reads low; if it is much lower the class is no longer guaranteed either.
What does FS 5 on the rating plate mean?
FS is the instrument security factor of a metering transformer. With FS 5 the composite error at 5 times rated current is at least 10 % – the transformer saturates and so limits the secondary current that reaches the instruments during a short circuit. Most CELSA metering transformers have FS 5, some versions FS 10.
Does a class 0.2S transformer make the whole measurement more accurate?
Only up to the accuracy of the connected device: the errors of transformer and meter or instrument add up. A class 1 instrument on a 0.2S transformer measures no better than class 1. Transformer and device should therefore match.