Understanding Circuit Breaker Making Current

The making current (also known as the peak making current) of a circuit breaker is the maximum asymmetrical current that the breaker can safely make (close) under short-circuit conditions. It's an important parameter for ensuring the breaker's mechanical and electrical integrity during the initial moments of a fault.

Calculating Rated Breaking Current

First, we need to calculate the RMS symmetrical breaking current ($I_b$) using the circuit breaker's rated breaking capacity ($S_{rated}$) and rated voltage ($V_{rated}$). The formula for a 3-phase system is:

$$I_b = \frac{S_{rated}}{\sqrt{3} \times V_{rated}}$$

Given:

• Rated Breaking Capacity ($S_{rated}$) = 3000 MVA = $3000 \times 10^6$ VA
• Rated Voltage ($V_{rated}$) = 66 kV = $66 \times 10^3$ V

Substitute the values into the formula:

$$I_b = \frac{3000 \times 10^6 \text{ VA}}{\sqrt{3} \times (66 \times 10^3) \text{ V}}$$

$$I_b = \frac{3000 \times 10^3}{\sqrt{3} \times 66} \text{ A}$$

$$I_b \approx \frac{3000000}{1.732 \times 66} \text{ A}$$

$$I_b \approx \frac{3000000}{114.312} \text{ A}$$

$$I_b \approx 26243.7 \text{ A}$$

Converting this to kiloamperes (kA):

$$I_b \approx 26.24 \text{ kA}$$

Calculating Making Current

The making current ($I_m$) is typically calculated as a multiple of the RMS symmetrical breaking current. This multiple, known as the making current factor (K), accounts for the DC offset that occurs during a short circuit. For standard industrial applications, a common factor is K = 2.55 (based on IEC standards for a typical X/R ratio).

The formula is:

$$I_m = K \times I_b$$

Using the calculated breaking current and the factor K = 2.55:

$$I_m = 2.55 \times 26.2437 \text{ kA}$$

$$I_m \approx 66.921 \text{ kA}$$

Final Result

Therefore, the making current for the circuit breaker is approximately 66.92 kA.