Circuit breakers breaking capacity is the rated RMS value of current that a CB can break at the rated voltage.
- Breaking capacity (B.C) = 1.732 * V * I * 10-6
Where 1.732 = √3 represents a multiplier for three phase circuits
B.C is always expressed in terms of MVA.
Where M = Mega (A prefix representing 106)
V = Rated service voltage
A = Short-circuit current
Calculations – Solved Examples
Example 1: Calculate the breaking capacity required to break 200 A short circuit current at rated service voltage of 11 kV in a 3 phase system.
Solution: B.C = 1.732 * V * I * 10-6
= 1.732 * 11 kV * 200 * 10-6 = 3.814 MVA
Example 2: Repeat the above example problem for I = 50 A, V = 33 kV.
Solution: B.C = 1.732 * 33 kV * 50 A * 10-6 = 2.85 MVA
Frequently asked questions
Question 1: What is Icu in MCCB and other breakers?
Answer: Icu or Icn is rated short-circuit breaking capacity or ultimate breaking capacity. It is maximum fault current that a CB should be able to interrupt.
Question 2: What do you mean by Ics?
Answer: Ics means service breaking capacity.
Ics is the rated short-circuit breaking capacity.
Question 3: Why we need Icu and Ics and how Icu is different from Ics?
Answer: Icu is maximum fault current that could prevail in case of extreme menacing faults. It can be very high such as of the order 5000 A in case of CB or even 5,00,000 A in case of MCCB. Practically there exist very small chances for occurrence of such faults. For making practical design another term Ics is used. Ics is actually a multiple of k times Icn. Simply saying Ics is a percentage of Icu.
Mathematically: Ics = k * Icu
Where k represents a percentage number eg 10%, 20%, 25%, 50%, 75%, or 100%.
Question 4: What do you mean by Ics = 100 % Icu or Ics = 100 % Icn?
Answer: The European industrial standards involve the use of k factor as 100%. So as to equate both equations: Ics = Icu
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