Question

A current transformer has ratio 500/5. If the burden is 2 VA at rated secondary current, the corresponding burden impedance is

• A. \(0.08\ \Omega\)
• B. \(0.4\ \Omega\)
• C. \(0.8\ \Omega\)
• D. \(2.0\ \Omega\)

Hint:

Always use the rated secondary current (the second number in the CT ratio, which is almost always 5 A or 1 A) to calculate secondary burden parameters. Using the formula \(Z = \frac{\text{VA}}{I_s^2}\) gives \(\frac{2}{25} = 0.08\ \Omega\).

Answer 1

The Correct Option is A

Concept: The electrical burden of an instrument transformer refers to the total connected load across its secondary terminal output ports, expressed in Volt-Amperes (VA). For a Current Transformer (CT), this burden power is determined by the secondary winding current flowing through the secondary load circuit impedance: \[ \text{Burden (VA)} = (I_s)^2 \cdot Z_b \] where:
• \(I_s\) is the rated secondary current flowing in the circuit.
• \(Z_b\) is the total burden impedance across the secondary side.

Step 1: Identify the secondary operating parameters. From the given current transformer nominal rating ratio: \[ \text{Ratio} = \frac{500}{5} = \frac{\text{Primary Rated Current}}{\text{Secondary Rated Current}} \] Thus, the rated secondary current is: \[ I_s = 5\text{ A} \]

Step 2: Calculate the burden impedance (\(Z_b\)). The given secondary burden rating value is 2 VA. Using the formula: \[ \text{VA Burden} = I_s^2 \cdot Z_b \] Substitute the known values into the equation: \[ 2 = (5)^2 \cdot Z_b \] \[ 2 = 25 \cdot Z_b \] Isolating the variable \(Z_b\): \[ Z_b = \frac{2}{25}\ \Omega \] To express this as a decimal value, multiply both the numerator and the denominator by 4: \[ Z_b = \frac{2 \times 4}{25 \times 4} = \frac{8}{100} = 0.08\ \Omega \] This calculated value corresponds to option (A).