📘 Air Blast Circuit Breaker – Numerical Problems with Detailed Solutions
These problems help in understanding breaking capacity, making capacity, and breaker current rating calculations. Important for GATE and PSU exams.
🔹 Problem 1: Breaking Capacity & Making Capacity
A 3-phase 132 kV system has a symmetrical short circuit current of 25 kA. Calculate:
- 1) Breaking Capacity (in MVA)
- 2) Making Capacity
✅ Step 1: Breaking Capacity Formula
Breaking Capacity = √3 × V × Isc
Given:
- V = 132 kV
- Isc = 25 kA
Step 2: Substitution
= 1.732 × 132 × 25 = 5715.6 MVA
Breaking Capacity ≈ 5716 MVA
Step 3: Making Capacity
Making capacity = 2.55 × Symmetrical breaking current
= 2.55 × 25 = 63.75 kA
Making Capacity = 63.75 kA
🔹 Problem 2: Required Breaker Current Rating
A power station has a fault level of 4000 MVA at 220 kV bus. Find the required breaker current rating.
Step 1: Short Circuit Current Formula
Isc = MVA / (√3 × V)
Step 2: Substitution
= 4000 / (1.732 × 220) = 4000 / 381 = 10.5 kA
Required Breaker Current Rating ≈ 10.5 kA
🎯 Exam Tips
- Always convert kV and kA properly
- Making Capacity = 2.55 × Breaking Current
- Breaking Capacity = √3 × V × Isc
- Fault Level (MVA) determines breaker selection
Correct Breaker Rating = Safe & Stable Power System
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