Circuit breaker (MCB)

A circuit breaker is an automatic protective switch. It carries normal current but disconnects quickly on a fault — and unlike a fuse, it can be reset.

It has two trip mechanisms. A thermal element (bimetal) handles sustained overloads with a time delay. A magnetic element (a coil) handles short circuits, tripping almost instantly on the large fault current.

That's why an instant trip on reset usually means a short, while a trip after running a while means an overload — the two mechanisms tell you which.

Protecting final circuits and sub-mains in switchboards. Breakers come in 'curves' (characteristics) suited to different loads — some ride through motor inrush, others trip more readily.

Never fit a larger breaker to stop nuisance tripping — that defeats the protection. Find the overload or fault instead.

Read the trip style first: instant trip on reset means a short circuit or earth fault; a trip after running for minutes means an overload. The two mechanisms are telling you which fault to chase. A breaker that won't reset with its circuit disconnected has a worn mechanism.

A breaker that's discoloured or warm at one terminal is being cooked by a loose connection — which also makes it trip early, because the heat reaches the bimetal. And nuisance trips at switch-on are usually curve mismatch: motor, transformer or a wall of LED drivers inrushing past a B-curve's instantaneous trip.

Isolated: continuity through the closed breaker — effectively zero ohms; anything measurable is burnt internals or a bad termination. Under load — that's a live board, licensed work with due care: millivolt drop across the device, and feel for heat — a warm breaker at modest load has a problem, and it's usually the termination rather than the breaker.

Proving trip performance properly takes injection test gear. The field version: does it hold its rated load, and does the tripping stop once you find and fix the real fault. A breaker that has cleared a serious short deserves suspicion — they're allowed to retire after a hard fault.

Curves: B trips instantly at roughly 3–5× rating, C at 5–10×, D at 10–20× (typical figures — check the device data). Inrush loads on a B curve trip at switch-on with nothing wrong downstream; the fix is the right curve, never a bigger rating.

Upsizing a breaker to stop trips removes the cable's protection — the breaker protects the cable, not the appliance. And a loose line-side termination heats the bimetal enough to trip well under rated current: check terminal torque before condemning the breaker.