UK Circuit Breaker Types — MCB, RCD, RCBO

The letter indicates the magnetic trip characteristic — how much overcurrent causes instant tripping. Type B trips at 3-5 times rated current (best for domestic circuits — lighting, sockets). Type C trips at 5-10 times (used where moderate inrush currents occur — fluorescent lighting, small motors). Type D trips at 10-20 times (used for high inrush loads — welding equipment, transformers, X-ray machines). For domestic installations, Type B is standard for virtually all circuits.

An RCD (Residual Current Device) provides earth leakage protection for a group of circuits but no overcurrent protection. An RCBO (Residual Current Breaker with Overcurrent) combines RCD and MCB protection in a single device — it provides both earth leakage AND overcurrent protection per circuit. RCBOs prevent nuisance tripping affecting multiple circuits, as each circuit has independent protection.

Type AC detects sinusoidal AC earth faults only — it is the basic type but not suitable for all modern loads. Type A also detects pulsating DC faults — this is the minimum requirement for most domestic circuits under BS 7671 18th Edition. Type F adds protection for high-frequency faults from variable speed drives and inverters (e.g. washing machines, air conditioning units with VFDs).

Common causes: a genuine earth fault on an appliance (test by unplugging devices one at a time), moisture in outdoor sockets or junction boxes, deteriorating cable insulation, faulty RCD (they should be tested regularly with the test button), or cumulative leakage from multiple appliances on one RCD circuit. RCBO boards solve the last issue by giving each circuit independent RCD protection.