RCBO
/ ɑː siː biː əʊ /
Also known as: Residual Current Breaker with Overcurrent protection, combined RCD-MCB
Definition
An RCBO (Residual Current Breaker with Overcurrent protection) is a single device combining an RCD (30mA earth fault / shock protection) and an MCB (overcurrent and short-circuit protection) in one unit the same size as a standard MCB. Each circuit gets its own RCBO, providing independent protection - a fault on one circuit trips only that RCBO, leaving all other circuits live. This is the primary advantage over the older arrangement of one shared RCD covering multiple circuits, where a fault on any one circuit (or even a nuisance trip from an appliance) kills the whole group. 18th Edition BS 7671 strongly promotes individual RCBO protection for new installations.
In practice
Modern consumer units are increasingly supplied factory-fitted with RCBOs on every way (circuit position). When an electrician upgrades or replaces a consumer unit, moving from the old split-load arrangement (one RCD for half the circuits) to individual RCBOs on every circuit is now standard practice under the 18th Edition. The consumer unit will have a double-pole main switch but no main RCD - each RCBO independently handles both protection functions for its circuit.
A fault that would previously have been difficult to locate (tripping half the consumer unit and requiring systematic disconnection of loads to find the offending circuit) is immediately identified with individual RCBOs - the tripped RCBO is the faulty circuit. The test button on each RCBO should be pressed every 6 months to verify the RCD mechanism operates. An RCBO that fails the test button check must be replaced - the MCB part may still function but the earth fault protection has failed.
See also