Enter the fault current (amps), the CT ratio, and the secondary winding resistance (ohms) into the calculator to determine the Knee Point Voltage. 

Knee Point Voltage Formula

The following formula is used to calculate the Knee Point Voltage.

Vkp = 2 * IF / CT * (RCT + RL + RB)
  • Where Vkp is the Knee Point Voltage (volts)
  • IF is the fault current (amps) 
  • CT is the CT ratio 
  • RCT is the secondary winding resistance (ohms) 
  • RL is the lead resistance (ohms)
  • RB is the relay burden (ohms)

How to Calculate Knee Point Voltage?

The following example problems outline how to calculate Knee Point Voltage.

Example Problem #1

  1. First, determine the fault current (amps). In this example, the fault current (amps) is determined to be 510 .
  2. Next, determine the CT ratio. For this problem, the CT ratio is measured to be 5 .
  3. Next, determine the secondary winding resistance (ohms). In this case, the secondary winding resistance (ohms) is found to be 456.
  4. Next, determine the lead resistance. This is 100 ohms.
  5. Next, determine the relay burden. This is found to be 10 ohms.
  6. Finally, calculate the Knee Point Voltage using the formula above: 

Vkp = 2 * IF / CT * (RCT + RL + RB)

Inserting the values from above and solving the equation yields: 

Vkp = 2 * 510 / 5 * (456+ 100 + 10) = 115,464 (volts)

Example Problem #2

Using the same method as above, determine the variables required by the equation. For this example problem, these are:

fault current (amps) = 49

CT ratio = 50

secondary winding resistance (ohms) = 14

lead resistance (ohms) = 3

relay burden (ohms) = 2

Enter these given values into the calculator or above yields: 

Vkp = 2 * 49 / 50 * (14 + 3 + 2) = 37.24 (volts)