MCB (Miniature Circuit Breaker)
Breaker
A resettable mains circuit breaker that trips on overload or short circuit, protecting house/building wiring. Replaces rewireable fuses; just switch it back on after a trip.
Schematic symbol
Switch (SPST)
How mcb (miniature circuit breaker) appears in a circuit diagram.
Types & variants
Key specs
Rated current (In)
AContinuous current, e.g. 6, 10, 16, 20, 32A.
Trip curve
B/C/DMagnetic trip multiple for short-circuit response.
Breaking capacity (Icn)
kAMax fault current it can interrupt, e.g. 6kA/10kA.
Poles
-1P (live), 2P (live+neutral), 3P/4P for three-phase.
Laws that govern it
- Joule's law of heatingH = I² × R × t
The thermal (bimetal) element trips on accumulated I²Rt — slow overloads — which is why an MCB tolerates a brief motor inrush.
- Ampère's circuital law∮ B · dl = μ₀ × I(enc)
The magnetic element trips on the field of the fault current itself, giving the instantaneous trip that the B/C/D curve defines.
Breaking a fault means quenching an arc: the splitter plates stretch and cool it until the gap can withstand the returning voltage.
Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.
How it compares
- MCB vs RCCB
Side by side with RCCB / RCD / GFCI — specs, variants and which one the job calls for.
- MCB vs MCCB
Side by side with MCCB — specs, variants and which one the job calls for.
- Fuse vs MCB
Side by side with Fuse (glass/blade/SMD/thermal) — specs, variants and which one the job calls for.
Markings
Front print like 'C16' = Type C, 16A; plus voltage (230/400V) and kA rating.
Standard values
6, 10, 16, 20, 25, 32, 40, 63A; Type B/C/D; 6kA or 10kA breaking.
How to choose
Size In to the cable's safe current (not the load) and coordinate with wire gauge. Type B for general/lighting, C for motors/AC, D for heavy inrush. Breaking capacity >= prospective fault current. Pair with an RCD/RCBO for shock protection.
Pinout & package
DIN-rail mount; line in (top), load out (bottom). MUST be installed by a qualified electrician - this is mains.
Example circuits
- Final-circuit protection in a consumer unit/DB
- Dedicated breaker for an AC or geyser (Type C)
- RCBO = MCB + earth-leakage in one
Common failures
Nuisance tripping (under-rated or faulty), fails to trip (dangerous - replace), worn contacts, loose terminals causing heat.
How to test
Toggle on/off; use the test button on RCBOs. Proper trip testing needs a loop/RCD tester by an electrician. Discoloured terminals = overheating.
Substitutes
Same In, trip curve and >= breaking capacity, matching poles and brand DIN range. Do not up-size current to stop tripping.