Instrument Security Factor or ISF of Current Transformer

Instrument Security Factor or ISF of Current Transformer

 

Instrument security factor is the ratio of instrument limit primary current to the rated primary current. Instrument limit current of a metering current transformer is the maximum value of primary current beyond which current transformer core becomes saturated. 

 Security or Safety of the measuring unit is better, if ISF is low. If we go through the example below it would be clear to us.

 Suppose one current transformer has rating 100/1 A and ISF is 1.5 and another current transformer has same rating with ISF 2. 

That means, in first CT, the metering core would be saturated at 1.5 × 100 or 150 A, whereas is second CT, core will be saturated at 2 × 100 or 200 A.

That means whatever may be the primary current of both CTs, secondary current will not increase further after 150 and 200 A of primary current of the CTs respectively.

Hence maximum secondary current of the CTs would be 1.5 and 2.0 A.