Burden for PS Class CTs

Please note that Class PS CTs & Class 5 P 10 CTs are not the same. A Class PS CT is not specified the same way as a Class 5 P 10 CT.

Now, answering Mr. Venu:

Your Class PS CT Name Plate will have a specification called "Knee Point Voltage or Vkp". Please note it down.

Vkp = K If (Rct + 2 Rl + RB)

where

Vkp = Knee Point Voltage of the CT, in Volts

K = Constant (normally taken as 2)

If = Maximum Fault Current at the CT Location, as reflected at the Secondary side of the CT, in Amperes

Rct = CT Secondary Winding Resistance, in Ohms

Rl = Lead Resistance of the Connecting Leads from the CT Secondary to the Burden, in Ohms

RB = Resistive Burden of the Connecetd Load, in Ohms

You will know all other parameters. Then you can calculate the Load Burden that can be connected to the CT Secondary. This you will get in Ohms. Now, multiply this resistive burden by the square of the CT rated current. You will get the burden in VA.

Hope this helps.

This (electricalexpert65's answer) is good answer. The formula is here rewritten for the better clarity:

Vkp = K * If * (Rct + 2 Rl + RB)

Further addition:

PS CTs are specified by

- Turn ratio

- Minimum knee point voltage (Vkp)

- Maximum exciting current at Vkp or at its fraction.

- Maximum resistance of secondary winding.

- MS