1. Typically the c.t. is specified by means of an accuracy factor and an overcurrent factor up to which the c.t. remains accurate with the maximum specified burden connected to its secondary, eg:
5P10 : 5VA (Accuracy Limit, Overcurrent Factor : Maximum Burden(at c.t. secondary level))
In the example of a 5P10, 5VA c.t., this will transform primary current within the accuracy limit of 5% when a burden of 5VA (at rated current) is connected to the c.t. secondary, for a primary current up to 10 times its rating.
2. Knee Point Voltage: The Knee point voltage of the current transformers should be at least twice the relay setting voltage. The knee point voltage is expressed as the voltage at fundamental frequency applied to the secondary circuit of the current transformer which when increased in magnitude by 10% causes the magnetising current to increase by 50%.
5P10 : 5VA (Accuracy Limit, Overcurrent Factor : Maximum Burden(at c.t. secondary level))
In the example of a 5P10, 5VA c.t., this will transform primary current within the accuracy limit of 5% when a burden of 5VA (at rated current) is connected to the c.t. secondary, for a primary current up to 10 times its rating.
2. Knee Point Voltage: The Knee point voltage of the current transformers should be at least twice the relay setting voltage. The knee point voltage is expressed as the voltage at fundamental frequency applied to the secondary circuit of the current transformer which when increased in magnitude by 10% causes the magnetising current to increase by 50%.
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