Accuracy Limit Factor (ALF) of Current Transformer

Accuracy Limit Factor (ALF) is the ratio of the rated accuracy limit primary current to the rated primary current. This factor is critical for ensuring accurate relay operation during fault conditions, safeguarding the integrity of the protective system.

ALF is used for protection class CTs. For metering class CTs there is similar term called Instrument security factor (ISF).

In simple terms, it is the number times rated current that the stated accuracy is good up to. For example, if a CT has rating of 5P20, it will have 5% accuracy upto 20 ALF. Meaning the CT will maintain 5% accuracy up to 20 times rated current with rated burden applied. In practice, it means that the CT won't saturate at upto 20 times rated current with rated burden applied. 

Equivalent circuit of CT:

I_P = Primary current

I_S = Secondary current

V_S = Secondary voltage

Z_E = Exciting impedance

I_E = Exciting current

R_S = Secondary resistance

X_L = Leakage reactance

Z_B = Burden impedance 

Effect of burden on ALF: ALF is specified for a certain burden. If actual burden is different from rated burden, ALF will change as below:

In practise, the actual accuracy limit factor differs from the rated accuracy limit factor (Fn) and is proportional to the ratio of the rated CT burden and the actual CT burden.

ALF at actual burden = ALF at rated burden x (Rated burden / Actual burden)

* Internal secondary winding resistance of CT to be added for calculating burden.

For example, for a CT with rated burden 20VA and ALF 5, if actual burden is 10VA, ALF will be =5 x (20/10) =10. I.e. accuracy will be maintained upto 10 times rated current.