When to use an RCBO?
In order to reduce the number of injuries caused by electric shock, the 17th Edition Wiring Regulations state that all circuits must be protected from earth leakage. In most situations, this is achieved by the use of an RCD (residual current device) which protects a bank of circuits simultaneously.
Whilst this is a cost effective and still popular solution it has a considerable drawback as an earth leakage fault on one of the circuits will knock out the power to all the other circuits on that RCD. This may not be a serious issue unless there are critical circuits which may be affected. This nuisance tripping could become a serious issue if the other circuits distribute power to your fridge freezer, security system, fire alarm or medical equipment. Such circuits are often referred to as 'mission critical circuits' though what constitutes 'mission critical' of course depends on the home/business owner.
An RCBO, which does the job of both an RCD and MCB in one unit, thus prevents this type of nuisance tripping and should be used on mission critical circuits. A circuit which is protected by an RCBO provides all the protection of an RCD but is completely independent of other circuits. This is by far the best option in terms of safety and reliability but it does come at some additional cost.