Switchboards and breakers

A circuit breaker protects the wire from overload and short circuit. Its rating (current in amps) is chosen for the wire's cross-section, not the appliance. Typical domestic ratings: 10 A for lighting, 16 A for sockets, 25–32 A for heavy circuits.

The letter before the number is the time-current curve. C is the most common for home and office and tolerates normal inrush. B is for circuits without big inrush surges, D for motors and heavy equipment with a large starting current.

The key rule: the breaker protects the wire. You cannot put a 40 A breaker on a circuit of 1.5 mm² wire — the wire will overheat before the protection trips. Wire size first, breaker rating second.

An RCD (residual current device) watches for current leakage and cuts the circuit if current "escapes" the wire — through a person or a wet wall, say. The domestic leakage rating is 30 mA, and 10 mA for a bathroom or nursery. An RCD does not protect against overload, only leakage, so it's paired with a breaker.

An RCBO is an RCD and breaker in one body: it protects against leakage and against overload and short circuit. It saves board space and simplifies assembly.

In practice: wet and "person-dangerous" circuits (bathroom, kitchen, outdoors, sockets) must have leakage protection. Lighting is often left under a common RCD. The more critical the circuit, the closer to a dedicated RCBO.

A board is built from the general to the specific. At the input, a main breaker (or isolator) for the whole flat, then a common RCD, then separate breakers per group: lighting, room sockets, kitchen, bathroom, air conditioner, water heater.

Split the loads: don't hang the whole flat on one circuit. Lighting apart from sockets, heavy appliances on their own circuit. That way a fault drops one group, not the whole home, and the fault is easier to find.

Leave a margin: 20–30% of free modules in the board for future circuits. Adding space later costs more than fitting a board a couple of rows larger from the start.