Two releases inside one breaker
A miniature circuit breaker (MCB) contains two completely different mechanisms, and the marking on the front describes both of them.
- The thermal release is a bimetallic strip. It answers sustained overload and takes minutes to hours — the same timescale on which a cable heats up.
- The magnetic release is a coil. It answers a short circuit and disconnects within milliseconds.
The number — 6, 10, 16, 20, 32 — is the rated current and belongs to the thermal side. The letter in front of it belongs to the magnetic side: it says at what multiple of the rated current the breaker snaps open instantly.
The curves compared
Trip bands for the magnetic release, as multiples of rated current, with the actual amps for a 16 A device:
- B curve — 3 to 5 times rated current. On a B16 that is roughly 48 to 80 A.
- C curve — 5 to 10 times. On a C16, 80 to 160 A.
- D curve — 10 to 20 times. On a D16, 160 to 320 A.
The thermal side is identical across all three: at 1.13 times rated current the breaker must not trip, and at 1.45 times it must trip within an hour. So a B16 carries 16 A indefinitely, but not 18 A.
Rated currents in housing are typically 6, 10, 16, 20, 25, 32 and 40 A, with a breaking capacity of 6 kA.
Which letter to use
- B is the default for homes. Lighting, sockets, ordinary appliances. It disconnects fastest and keeps the required disconnection time even on long circuits.
- C is for inrush. Motors, transformers, banks of LED drivers and switch-mode supplies. These draw a brief multiple of their running current at switch-on, which a B curve reads as a short circuit.
- D is for severe inrush. Welding sets, large machines, some pumps. It rarely belongs in a house.
Why not fit C everywhere?
Because protection would quietly stop working. For a fault to be cleared in time, the prospective short-circuit current must reliably exceed the magnetic threshold. On a long, thin circuit the loop impedance is high and the fault current small. A C16 may need up to 160 A to snap open; if the circuit can only deliver 90 A, the magnetic release stays put and only the thermal one acts — seconds instead of milliseconds.
An RCD does not fill this gap: it sees leakage to earth, not a line-to-neutral short. So use C where there is a concrete reason, and check the circuit length when you do.
What about the RCBO?
An RCBO carries the same letter and number as an MCB — it is an MCB with residual-current protection built in. Everything above applies to it unchanged.
When a breaker keeps tripping
Find the cause first; never fit a bigger breaker. The cable in the wall has not changed.
- Tripping after a while under load is overload — the circuit carries too much and needs a circuit of its own.
- Tripping the instant one appliance starts is inrush — this is where a C curve may be the correct answer.
- Tripping with nothing obvious running points at a fault in an appliance or the cable. Work through it as in nuisance tripping.
The rule that outranks the rest
The breaker protects the cable, not the appliance. Rating and curve only make sense together with the cable cross-section: a 1.5 mm² cable stays on a B16 however much the load would like more.
What PlanMyPanel does
The generator uses B curves for household circuits and proposes C only where the wizard identified a load with real inrush. Rating and cross-section are always checked as a pair — if one does not suit the other, the project shows it rather than rounding silently.