The letter printed on an MCB's faceplate is its tripping curve — the multiple of rated current at which the magnetic trip operates instantaneously. Choosing the wrong curve causes either nuisance tripping on healthy circuits or, worse, a cable left without effective short-circuit protection.
B curve devices trip at 3–5 times rated current. They suit long cable runs and mainly resistive loads — lighting and socket circuits in residential boards, where switch-on inrush is low and fault currents at the far end of the run can be modest.
C curve, tripping at 5–10 times rated current, is the general-purpose choice for residential and commercial distribution. It rides through the inrush of LED drivers, fluorescent lighting, small motors and power supplies without tripping, which is why most of our distributor partners stock C curve as their standard.
D curve devices hold to 10–20 times rated current to absorb the heavy inrush of transformers, large motors and welding equipment. On a lighting circuit, though, a D curve breaker may not trip fast enough to protect the cable — so match the curve to the load, and check the cable's fault-current rating before you specify. Send us your board schedule and we will confirm the curve and rating for every way.