Metering 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. Instrument security factor of current transformer is the significant factor for choosing the metering instruments which to be connected to the secondary of the current transformer. 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 current transformer, the metering core would be saturated at 1.5 × 100 or 150 A, whereas is second current transformer, core will be saturated at 2 × 100 or 200 A. That means whatever may be the primary current of both current transformers, secondary current will not increase further after 150 & 200 A of primary current of the current transformers respectively. Hence maximum secondary current of the current transformers would be 1.5 & 2.0 A.

Another significance of ISF is during huge electrical fault, the short circuit current, flows through primary of the current transformer does not affect destructively, the measuring instrument attached to it as because, the secondary current of the current transformer will not rise above the value of rated secondary current multiplied by ISF.

Therefore the points i was trying to make was that, even if the primary current increases to any value higher than the rated primary current, the secondary equipments like the protective Relays and the Instruments should not damage. But rather the protective relay should be able to operate to trip off the source of supply.

A current transformer need not and will not reach its saturation point when the primary current increases above the rated current. Any current transformer will maintain its declared accuracy until its Instrument Security Factor (for metering current transformers) and Accuracy Limiting Factor (for protection current transformers). Even after these limits are reached, the current transformer may not saturate. Only its errors will increase. It may saturate anywhere beyond its ISF or ALF. It may or may not be exactly at the ISF or ALF, in many cases much beyond.

Secondly, even in metering current transformers, when the primary current increases above the rated current, the metering equipment need not damage, for any metering equipment will be designed to have a short time withstand current rating at least up to the ISF multiple of the current transformer secondary current, if not more. Meters have been in operation for centuries, in industries and other electrical installations under such conditions.