The short answer
A main-switch consumer unit has a large isolating switch at the top (or to one side) that cuts all circuits when operated, but provides no residual-current protection at the incomer position. The RCD protection in this configuration comes from devices further into the board — either shared RCDs protecting groups of circuits (the split-load arrangement) or individual RCBOs per circuit. An RCD-incomer (RCCB) consumer unit has a combined RCD/main switch at the top, so all circuits benefit from the RCD at the incomer position as a first layer of protection. However, an incomer RCD covering the whole board is generally less favoured under modern BS 7671 practice because a single RCD trip cuts all circuits at once. The current preferred domestic arrangement is a main switch with RCBOs per circuit, giving per-circuit isolation without a single point of whole-board trip.
The device at the top of a consumer unit — the main incomer — determines the first layer of protection and how the board as a whole behaves when a fault occurs. Here is what distinguishes the main-switch and RCD-incomer configurations, and how they relate to the boards fitted in UK homes today.
Incomer configurations
- Main switch onlyIsolates all circuits; no RCD at incomer
- RCD incomer (RCCB)Isolates all circuits and provides earth-leakage protection
- Split-load boardMain switch + two RCDs inside; MCBs per circuit
- RCBO boardMain switch + RCBO per circuit; no shared RCDs
- Preferred new-work configMain switch + RCBOs per circuit (18th Edition)
What the main switch does (and does not do)
The main switch (sometimes called the main incomer or main isolator) is the device at the top of the consumer unit that disconnects the whole board from the supply when operated. It allows the electrician — or the homeowner in an emergency — to cut all circuits in the property simultaneously.
What a main switch alone does not do:
- It provides no residual-current (earth leakage) protection at the incomer position.
- It does not protect individual circuits from overload or short circuit — that is the job of the MCBs or RCBOs in each circuit position.
In a main-switch consumer unit, the RCD protection must come from somewhere else in the board design: either shared RCDs covering groups of circuits (the split-load or dual-RCD board) or individual RCBOs per circuit. This is the configuration in most modern UK domestic consumer units.
What an RCD incomer does
An RCD incomer (sometimes written as RCCB incomer — RCCB being the technical term for a standalone residual current circuit breaker) combines the isolation function of the main switch with residual-current protection at the incomer position. All circuits in the board benefit from the incomer RCD as a first layer of earth leakage protection.
This sounds like a comprehensive arrangement, but it has a significant practical drawback: a single earth leakage fault anywhere in the installation trips the whole-board RCD, cutting all circuits simultaneously. In a domestic property this means the whole house — all lighting, all sockets, the boiler, the fridge — goes off in response to a single fault on any circuit.
For this reason, a whole-board RCD incomer is not the preferred configuration under current BS 7671 design guidance for new domestic installations. The regulation does not prohibit it, but the guidance favours arrangements that minimise the number of circuits affected by any single fault trip.
| Configuration | Incomer device | Fault behaviour | Typical use |
|---|---|---|---|
| Main switch only | Main switch (no RCD) | Individual circuit MCB trips on overload/short | Base for split-load or RCBO boards |
| Main switch + split-load (dual-RCD) | Main switch + 2 shared RCDs | Half the board trips on earth fault in that group | Very common; 17th Ed. default |
| RCD incomer (RCCB at top) | RCD/RCCB main switch | Whole board trips on any earth fault | Older boards; less common in new work |
| Main switch + RCBO per circuit | Main switch | Only the faulted circuit trips | Preferred for new domestic installs |
| RCD incomer + individual RCBOs | RCD main switch + RCBOs | RCBO trips first; incomer RCD as backup | High-resilience commercial setups |
Indicative configuration comparison. Sources: IET BS 7671 18th Edition; Electrical Safety First best practice guidance.
Why the main switch plus RCBO board is the current preference
Under the 18th Edition of BS 7671, the preferred domestic consumer unit configuration is a main switch at the incomer plus individual RCBOs per circuit. This arrangement has several advantages over an RCD incomer board:
- Per-circuit fault isolation: an earth leakage fault, an overload, or a short circuit on any circuit trips only that circuit's RCBO — the rest of the board continues to supply power.
- No false-trip cascade: an RCD incomer board with MCBs behind it means any earth leakage fault trips the entire board, even if the fault is minor or temporary (a washing machine motor surge, for example). RCBOs limit this nuisance-tripping effect to the affected circuit.
- Simpler fault identification: the tripped RCBO is the faulted circuit's device — there is no question of which RCD group is involved.
The RCD-incomer configuration is more commonly found in older domestic boards (pre-17th Edition) and in some simpler outbuilding sub-boards where the whole board being tripped is less disruptive. When a board is being replaced, the main switch plus RCBO arrangement is now the standard recommendation.
Frequently asked questions
Can I tell from looking at my consumer unit which configuration I have?
Yes. If the large device at the top says only 'Main Switch' or 'Isolator', it is a main switch. If it also has a test button, it is an RCD or RCCB incomer. If the board has two wider devices in the middle of a row with smaller MCBs either side, it is a split-load (dual-RCD) board with a main switch. If every device in the row is the same width and has its own test button, it is an RCBO board.
Is an RCD incomer board safe?
Yes, it provides earth-leakage protection across all circuits. The limitation is that a single fault trips the whole board rather than just one circuit. It is not unsafe — it is less discriminating in its response to faults than an RCBO board. An older RCD incomer board in sound condition is not a safety risk in itself.
When should I consider changing from an RCD incomer to a different configuration?
If the whole-board trips are causing a practical problem — particularly if a critical load like a boiler, freezer or medical device is being affected by faults on other circuits — the argument for switching to a split-load or RCBO arrangement is a resilience case rather than a safety emergency. Any planned change should be assessed and carried out by a registered electrician.
Sources & further reading
- IET — BS 7671 18th Edition wiring regulations
- Electrical Safety First — consumer unit replacement best practice guide
- NAPIT — consumer unit technical guidance
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property and installation. They are guidance, not a quotation.