What is the difference between a 'B' curve and 'C' curve MCB, and when would you use each?

• A. B-curve trips at 3–5x rated current; C-curve trips at 5–10x; B for more sensitive circuits, C for general circuits with surge loads
• B. B-curve trips at 1–2x rated current; C-curve trips at 2–3x; B for outdoor circuits
• C. B-curve is for DC circuits; C-curve is for AC circuits
• D. B-curve never trips on surge; C-curve does

Answer: (A)

The main idea is how quickly an MCB reacts to a short‑term, high current through its magnetic trip. The curve tells you the instantaneous trip threshold, measured as a multiple of the breaker’s rated current.

B curve and C curve differ in how sensitive they are to a brief surge. A B curve trips when the current reaches about 3 to 5 times the breaker’s rated current (In). A C curve is a bit less sensitive, tripping at about 5 to 10 times In. This means the B curve will shut off more quickly on a short circuit, while the C curve allows a bit more leeway for brief surges or inrush currents before tripping.

Use the B curve on circuits that are more sensitive to nuisance trips and don’t have large inrush—think typical domestic lighting and general outlets with straightforward, low-inrush loads. Use the C curve on general circuits that can experience surge currents or moderate inrush, such as power supplies with capacitive input or other loads that briefly spike current on startup.

The other statements don’t fit standard practice: the thresholds listed aren’t 1–2x or 2–3x, and B or C curves apply to AC circuits (not DC specifically). Both can respond to surge conditions through the magnetic trip, so it’s not true that B never trips on surge.