The term RCD (residual current device) applies to several different types of devices, all designed to disconnect the electrical supply in the event of an earth fault. Essentially, an RCD provides electric shock protection.
In a domestic setting, RCDs were initially a recommendation in the regulations, then a requirement for certain circuits and today, in the latest 18th edition of BS7671, they are required for virtually all circuits within a property.
There are so many situations where RCD protection is required – cables buried within walls, socket outlets, circuits supplying items in a bathroom, cables passing through a bathroom and now all luminaires or light fittings – within a domestic property that this level of protection being provided is now a basic assumption whenever electrical work is carried out.
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In some cases, this will involve adding RCD protection to the circuit being worked on; in other cases, it will be more beneficial to replace the consumer unit and bring the rest of the installation (all other circuits) up to current requirements.
Under the 18th edition of the regulations, the dual-RCD configuration may not comply, and an RCBO-protected consumer unit is a better option to ensure compliance.
Nuisance tripping is caused when an RCD disconnects the supply to one or more circuits due to a fault on a separate circuit. In a dual-RCD configured consumer unit, water ingress in a floodlight connected to the downstairs lighting circuit would cause all circuits protected by the same RCD to be disconnected.
This will typically include the upstairs sockets and possibly kitchen sockets, electric shower, cooker, garage, and smoke alarms, depending on the configuration of the consumer unit.
In an RCBO-only configured consumer unit, the only circuit to be disconnected would be the affected one – in the above example, the downstairs lights. This makes for a safer situation (it is possible to use lamps plugged into sockets, for example, if it happens at night) as well as convenience and makes it easier to identify the cause of the tripping.
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