Accuracy Limiting Factor a very important term for Electrical Design Engineers but unfortunately, not much attention is paid to this factor.

This term is associated with Protection Current Transformer.

Actually Protection CT unlike Measuring CT must remain stable – it must not saturate, for currents that are many multiples of its rated current. This level of saturation in protection CTs is denoted by a term called ALF

Let us take a typical example of a CT Class 5P10,
Here 5P indicates the Accuracy Class
and
10 indicates the ALF. Typical ALFs are 5, 10, 15, 20 & 30.

A 5P10 CT means this is a protection CT with a composite error of +/- 5% and this error will be maintained upto 10 times the rated primary current of the CT. If the primary current is more than 10 times the rated primary current, then this CT will saturate and will not reproduce secondary current linearly with the primary current.