The rated accuracy limit factor (Fn) is the ratio of the rated accuracy limit primary current to the rated primary current. For example, a protective current transformer of type 5P10 has the accuracy class 5P and the accuracy limit factor 10. For protective current transformers, the accuracy class is designed by the highest permissible percentage composite error at the rated accuracy limit primary current prescribed for the accuracy class concerned, followed by the letter "P" (meaning protection).
| Accuracy class | Current error at rated primary current (%) | Phase displacement at rated primary current | Composite error at rated accuracy limit primary current (%) | |
|---|---|---|---|---|
| minutes | centiradians | |||
| 5P | ±1 | ±60 | ±1.8 | 5 |
| 10P | ±3 | – | – | 10 |
The accuracy classes 5P and 10P are both suitable for non-directional overcurrent protection. The 5P class provides a better accuracy. This should be noted also if there are accuracy requirements for the metering functions (current metering, power metering, and so on) of the protection relay.
The CT accuracy primary limit current describes the highest fault current magnitude at which the CT fulfils the specified accuracy. Beyond this level, the secondary current of the CT is distorted and it might have severe effects on the performance of the protection relay.
In practise, the actual accuracy limit factor (Fa) differs from the rated accuracy limit factor (Fn) and is proportional to the ratio of the rated CT burden and the actual CT burden.
The actual accuracy limit factor is calculated using the formula:
| Fn | the accuracy limit factor with the nominal external burden S n |
| Sin | the internal secondary burden of the CT |
| S | the actual external burden |