FAQ01558 of Measuring / Motor Protective Relays FAQ

The operating current for the timing element is usually set to approximately 150% of the current value for the contract power. The tap value is obtained from the following formula:

ITAP: The setting tap value (A) for the OCR timing element
I1: The current value for the contract power (equals contract power (kW)/√3 x 6.6 (kV))
ICT: Primary rated current of the current transformer (secondary current is 5 A)

Because the tap value in the K2CA Series can be set in relatively fine increments of 3, 3.5, 4, 5.4, 5, or 6 A, it is quite easy to make the 150% setting to provide optimal overload protection.

The following table shows the relationship between the contract power and the setting tap. The α value in this table is the proportion of the current for the contract power and the OCR operating current, indicating the tap values near the 150% point. The tap value can be easily changed by turning a rotary switch. There is no need to insert a spare tap to prevent an open circuit on the CT2 side as in previous systems.

Contract Power vs. K2CA Timing Element Current Setting