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📘 Circuit Breaker Ratings & Short Circuit Analysis

Circuit breaker must safely interrupt fault current without damage. Breaker rating depends on maximum short circuit level.

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🔹 1️⃣ Short Circuit Current

For 3-phase fault:

I_sc = V / Z_th

Where: V = Prefault voltage Z_th = Thevenin impedance at fault location Short Circuit MVA:

SC MVA = (Base MVA) / (Z_th pu)

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🔹 2️⃣ Circuit Breaker Ratings

✔ Breaking Capacity

Maximum RMS current breaker can interrupt safely.

Breaking Capacity = √3 × V × I_sc

(Unit: MVA)

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✔ Making Capacity

Breaker must withstand first peak current. Peak current ≈ 2.55 × I_sc

Making Capacity = 2.55 × I_sc

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✔ Short Time Rating

Breaker must withstand fault current for short duration (1 sec typically).

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🔹 3️⃣ Numerical Example

Given: Base MVA = 100 MVA Z_th = 0.2 pu Voltage = 11 kV Step 1: SC MVA = 100 / 0.2

SC MVA = 500 MVA

Step 2: Fault current: I_sc = (SC MVA) / (√3 × V) = 500 × 10⁶ / (1.732 × 11 × 10³) ≈ 26.2 kA Step 3: Making Capacity: = 2.55 × 26.2

≈ 66.8 kA

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🎯 Important GATE Points

• Breaking capacity expressed in MVA
• Making capacity ≈ 2.55 × RMS current
• Breaker rating must exceed system fault level
• Higher system MVA → stronger breaker required

Protection = Fast Isolation of Fault Energy