Definition
The accuracy class of a measuring device indicates the maximum permissible deviation between the measured value and the actual value, expressed as a percentage of full scale. The lower the class, the higher the accuracy. For example: class 0.5 → ±0.5% maximum error at the nominal value.
Usually, current transformers (CTs) used in electrical installations are in class 0.5.
This means 0.5% error at their nominal value. This accuracy class is applicable from 100 to 120% of In.
For an X/5 current transformer, the accuracy class is guaranteed from 100 to 120% of the nominal rating (which is why it is better to “overload” a CT).
Example: a 100/5 current transformer in class 0.5 with a primary current of 100 A will have an error of ± 0.5 A. At 20% of the nominal value,either 20 A, the error will also be ± 0.5 A and in this case we arrive at an error of 2.5%.
As a reminder; definition of nominal value
This is the reference value given by the manufacturer for which a device, component or system is designed to function correctly.
It is somewhat like its “ideal” value in normal operation, either a standard value intended for normal and safe use.
Important
The current transformer is usually connected to an energy meter or a measuring station, which has its own accuracy class. The accuracy classes are added together.
In certain energy management applications (such as consumption distribution or under-billing), it is important to limit these measurement errors. There are therefore current transformers in class 0.5s or class 0.2s. The ‘S’ requires that the current transformer has an accuracy class of 20% to 120% of the nominal value.
Percentage error depending on the current transformer load
| Class | 1% In | 5% In | 20% In | 100% In | 120% In |
|---|---|---|---|---|---|
| 0.2s | ± 0,75 % | ± 0,35 % | ± 0,2 % | ± 0,2 % | ± 0,2 % |
| 0.5s | ± 1,5 % | ± 0,75 % | ± 0,5 % | ± 0,5 % | ± 0,5 % |
| 0.5 | - | - | ± 2,5 % | ± 0,5 % | ± 0,5 % |
| - | - | ± 5 % | ± 1 % | ± 1 % |