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# FAQ01558 of Measuring / Motor Protective Relays FAQ

FAQ No. FAQ01558

Question

Please describe the current setting for the timing element in the K2CA Over-Current Relay(OCR).

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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

 Contractpower(kW) CT1 primary current (A) 5 10 15 20 30 40 Tap(A) α(%) Tap(A) α(%) Tap(A) α(%) Tap(A) α(%) Tap(A) α(%) Tap(A) α(%) 50 6 136 3 136 3.5 159 100 6 138 3.5 121 3 138 4 138 4.5 155 3.5 161 5 172 200 6 137 3.5 120 3 137 4 137 4.5 154 3.5 160 5 171 300 6 137 4 122 4.5 137 5 153 500 750 1,000 1,500
 Contractpower(kW) CT1 primary current (A) 50 75 100 150 200 300 Tap(A) α(%) Tap(A) α(%) Tap(A) α(%) Tap(A) α(%) Tap(A) α(%) Tap(A) α(%) 50 100 200 3 171 300 3.5 134 3 172 4 153 4.5 172 500 6 137 3.5 120 3 137 4 137 4.5 154 3.5 160 5 172 750 6 137 4 122 3 137 4.5 137 3.5 160 5 152 1,000 6 137 3.5 120 3 137 4 137 4.5 154 3.5 160 5 171 1,500 6 137 4 122 3 137 4.5 137 3.5 160 5 153

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