How to calculate short circuit current of circuit breaker

Calculating the short-circuit current of a circuit breaker involves several steps and requires knowledge of the circuit's electrical parameters. Here's a step-by-step guide to help you calculate the short-circuit current:

Step 1: Identify the circuit's electrical parameters

• Voltage (V) of the circuit
• Resistance (R) of the circuit
• Inductance (L) of the circuit
• Capacitance (C) of the circuit (if present)
• Short-circuit impedance (Zsc) of the circuit (if known)

Step 2: Determine the fault current

• Calculate the fault current (If) using the following formula:

If = V / Zsc

where V is the voltage of the circuit and Zsc is the short-circuit impedance of the circuit.

Step 3: Calculate the short-circuit current

• Calculate the short-circuit current (Isc) using the following formula:

Isc = If / (1 + (R / Xc))

where R is the resistance of the circuit, Xc is the capacitive reactance of the circuit (if present), and If is the fault current calculated in Step 2.

Step 4: Consider the effects of inductance

• If the circuit has inductance (L), you need to consider its effect on the short-circuit current. Calculate the inductive reactance (XL) using the following formula:

XL = 2πfL

where f is the frequency of the circuit (typically 50 or 60 Hz) and L is the inductance of the circuit.

• Calculate the total impedance (Z) of the circuit using the following formula:

Z = √(R^2 + (XL - Xc)^2)

• Calculate the short-circuit current (Isc) using the following formula:

Isc = If / (1 + (R / Z))

Step 5: Consider the effects of circuit breaker's characteristics

• The circuit breaker's characteristics, such as its interrupting rating and arcing time, can affect the short-circuit current. You may need to consult the circuit breaker's datasheet or manufacturer's documentation to determine its interrupting rating and arcing time.

Example calculation

Suppose we have a 3-phase, 480V, 60Hz circuit with the following parameters:

• Resistance (R) = 0.1 ohms
• Inductance (L) = 0.05 henries
• Capacitance (C) = 0.01 microfarads
• Short-circuit impedance (Zsc) = 0.05 ohms

Using the formulas above, we can calculate the short-circuit current as follows:

1. Calculate the fault current (If):

If = V / Zsc = 480V / 0.05 ohms = 9600A

2. Calculate the short-circuit current (Isc):

Isc = If / (1 + (R / Xc)) = 9600A / (1 + (0.1 ohms / (2π * 60Hz * 0.01 microfarads))) ≈ 9600A

3. Consider the effects of inductance:

XL = 2πfL = 2π * 60Hz * 0.05 henries ≈ 3.14 ohms

Z = √(R^2 + (XL - Xc)^2) ≈ √(0.1^2 + (3.14 - 0.02)^2) ≈ 3.16 ohms

Isc = If / (1 + (R / Z)) ≈ 9600A / (1 + (0.1 ohms / 3.16 ohms)) ≈ 9600A

In this example, the short-circuit current is approximately 9600A.

Important notes

• This calculation assumes a simple, linear circuit with no non-linear loads or harmonics.
• The actual short-circuit current may be higher or lower due to various factors, such as the circuit's topology, the presence of non-linear loads, and the accuracy of the circuit breaker's characteristics.
• It is essential to consult the circuit breaker's datasheet or manufacturer's documentation to determine its interrupting rating and arcing time, as these values can affect the short-circuit current calculation.