1. What is CT Secondary Resistance?

CT Secondary Resistance (R_CT) is the internal resistance of the current transformer's secondary winding. It is a critical parameter in protection and metering applications that affects the accuracy and performance of current transformers.

2. How Does the Calculator Work?

The calculator uses the CT secondary resistance formula:

Where:

• \( R_{CT} \) — CT secondary resistance (Ω)
• \( V_s \) — Secondary voltage (V)
• \( I_s \) — Secondary current (A)
• \( R_L \) — Lead resistance (Ω)

Explanation: The formula calculates the internal resistance of the CT secondary winding by subtracting the external lead resistance from the total measured resistance.

3. Importance of R_CT Calculation

Details: Accurate R_CT calculation is essential for proper CT sizing, burden calculation, protection relay settings, and ensuring measurement accuracy in electrical systems.

4. Using the Calculator

Tips: Enter secondary voltage in volts, secondary current in amperes, and lead resistance in ohms. All values must be positive numbers with secondary voltage and current greater than zero.

5. Frequently Asked Questions (FAQ)

Q1: Why is lead resistance subtracted in the calculation?
A: Lead resistance is part of the external circuit and not the CT's internal winding resistance. Subtracting it gives the true internal resistance of the CT secondary.

Q2: What are typical R_CT values for current transformers?
A: Typical values range from 0.1 to 10 ohms depending on CT ratio, construction, and accuracy class. Higher ratio CTs generally have lower secondary resistance.

Q3: How is secondary voltage measured for this calculation?
A: Secondary voltage is measured with rated secondary current flowing through the circuit, typically using a precision voltmeter across the CT secondary terminals.

Q4: What factors affect CT secondary resistance?
A: Factors include winding material, number of turns, core size, temperature, and CT construction. Resistance increases with temperature and winding length.

Q5: Why is R_CT important in protection applications?
A: R_CT affects the CT's saturation characteristics, burden capability, and overall performance in fault conditions, making it critical for reliable protection system operation.