MCB, RCD and RCBO: what does each one protect against, and what do B, C and D curves mean?

A miniature circuit breaker protects the cable, not the person. It has a bimetal element for overloads and a magnetic trip for short circuits. It is designed not to trip at its rated current: under the IEC standard for household breakers it must hold 1.13 times rated current and must trip within an hour at 1.45 times. A B16 will carry 16 A all day.

A person touching 230 V with a body resistance of 1 to 2 kΩ passes roughly 0.1 to 0.2 A. That is a hundred times too small for the breaker to notice, yet currents from about 30 mA upward through the chest can cause ventricular fibrillation if they last long enough.

The residual current device works on a different principle. Line and neutral both pass through a small current transformer. In a healthy circuit the current going out equals the current coming back, and the two fields cancel. If some current returns by another path, through a person or a damaged cable to earth, the imbalance induces a signal that trips the mechanism.

A 30 mA general-type device must trip at no more than 30 mA and is required to do so within 300 ms at rated residual current and within 40 ms at five times that. It does not limit the shock current, only its duration. It gives no protection if you touch line and neutral at the same time, because then the currents still balance, and on its own it does not protect against overload. An RCBO combines both functions in one device.

The letter is the magnetic trip band. A B curve trips instantaneously between 3 and 5 times rated current, a C curve between 5 and 10 times, and a D curve between 10 and 20 times. The thermal overload behaviour is the same for all three. B suits resistive loads and general sockets and lighting; C is for loads with moderate inrush such as motors and some lighting; D is for high inrush such as transformers and welders.

The letter has a safety consequence. For a fault to earth to trip a C20 instantly the current must reach 200 A, so the earth fault loop impedance has to be below about 230 / 200 = 1.15 Ω, against 2.3 Ω for a B20. Swapping a B breaker for a C to cure nuisance tripping is only acceptable if the circuit's loop impedance has been measured and is low enough.

Residual current devices come in types as well as ratings. Type AC detects only sinusoidal residual current. Type A also responds to pulsating DC, which is what faults in appliances with rectifiers produce, and many national rules now require it as a minimum. Type B additionally detects smooth DC and is called for with some vehicle chargers and drives; the equipment instructions state what is needed.

On sensitivity, 30 mA is for protecting people. Ratings of 100 mA and 300 mA are for fire protection and for upstream devices, where a time-delayed (selective) type lets the downstream 30 mA device trip first.

Two practical notes. Every appliance with a mains filter leaks a little current to earth in normal operation, from a fraction of a milliamp up to a few milliamps. Put many of them on one 30 mA device and the standing leakage brings it close to its trip band, which begins at half the rated residual current, so it trips for no apparent reason. Spreading circuits over several devices, or using one RCBO per circuit, solves that.

Use the test button at the interval the manufacturer states; it proves the mechanism still moves. Changes inside the distribution board itself, including swapping breaker types, should be done and tested by a qualified electrician.