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**

- First, determine the fault current (amps). In this example, the fault current (amps) is determined to be 510 .
- Next, determine the CT ratio. For this problem, the CT ratio is measured to be 5 .
- Next, determine the secondary winding resistance (ohms). In this case, the secondary winding resistance (ohms) is found to be 456.
- Next, determine the lead resistance. This is 100 ohms.
- Next, determine the relay burden. This is found to be 10 ohms.
- 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)