Glossary · Consumer units and protection
Key points
- The circuits are divided between two RCDs, so a fault trips only the circuits on that RCD.
- Each circuit still has its own circuit breaker for overload and short circuit protection.
- The IET gives the example of upstairs and downstairs lighting on separate RCDs, so you are not left in complete darkness.
- A high integrity unit with RCBOs goes further and limits a fault to one circuit.
- RCDs should be tested with the test button every six months, according to the IET.
How a dual RCD consumer unit works
A dual RCD consumer unit has a main switch and two RCDs. The circuits in the home are divided between the two RCDs. A circuit breaker on each circuit gives it its own protection against overload and short circuit.[1]NICEIC, Consumer units and fuse boxes: installation and replacement
The reason for two RCDs is practical. NICEIC says the split stops a single fault from disrupting power to all the connected circuits.[1]NICEIC, Consumer units and fuse boxes: installation and replacement The IET describes the usual arrangement as two RCDs, each protecting a number of circuits, to prevent danger or inconvenience to the user when a fault occurs. Its example is upstairs and downstairs lighting on separate RCDs. If one RCD operates, the other lighting circuit keeps working, so nobody is left in complete darkness.[2]IET, Consumer units: safe fuse-box upgrades for modern homes
How it differs from other layouts
NICEIC lists five common types of consumer unit. Three are relevant here.[1]NICEIC, Consumer units and fuse boxes: installation and replacement
| Layout | How RCD protection is arranged |
|---|---|
| RCD main switch | One RCD over the whole installation. NICEIC says this type is rarely installed these days |
| Dual RCD | Two RCDs, circuits divided between them, a circuit breaker on each circuit |
| High integrity | A circuit breaker or RCBO on each circuit, with no circuits combined under one RCD |
NICEIC also describes a high integrity split load unit. It mixes one or two RCDs with individual RCBOs, so circuits that benefit most get their own RCBO and the rest share an RCD.[1]NICEIC, Consumer units and fuse boxes: installation and replacement Because the word split-load is used loosely, check exactly what a quote describes. The RCD vs RCBO page explains the difference between a shared RCD and an RCBO.
Keeping the RCDs working
RCDs have a test button. The IET says to press it every six months, and that the device should switch off when pressed.[2]IET, Consumer units: safe fuse-box upgrades for modern homes If it does not switch off, contact a registered electrician. The IET FAQ suggests doing it when the clocks change, so it happens twice a year.[3]IET, BS 7671 FAQs: Consumer Units and Protective Devices The RCD trip test page has more.
Replacing a consumer unit is notifiable work under Part P of the Building Regulations in England and Wales, so it is for a registered electrician.[1]NICEIC, Consumer units and fuse boxes: installation and replacement See consumer unit upgrades for the firm’s service.
Example
Tom lives in a semi in Moortown with a dual RCD unit. His upstairs lighting and bedroom sockets are on one RCD. His downstairs lighting, kitchen and living room are on the other.
A fault in a bedroom lamp trips the first RCD. The upstairs goes off, but the downstairs stays on, so Tom can see to unplug the lamp and reset. With a single RCD the entire house would have gone off at once.
What people get wrong
People assume two RCDs means every circuit is protected separately. It does not. Each RCD covers a group. A fault on any circuit in that group switches off the whole group.
The second mistake is thinking a dual RCD board is out of date. NICEIC lists it as one of the five common types and describes it as keeping a single fault from cutting power everywhere.[1]NICEIC, Consumer units and fuse boxes: installation and replacement Whether to move to RCBOs depends on the home and is for a registered electrician to advise on.
The bottom line
A dual RCD consumer unit splits a home’s circuits between two RCDs, with a circuit breaker on each circuit. It keeps a single fault from switching off the whole house. It does not isolate faults to one circuit the way RCBOs do. Test the RCDs every six months and leave any work on the unit to a registered electrician.
Common questions
What is the difference between a dual RCD and a split-load board?
Split-load is often used for the dual RCD layout. NICEIC uses the name high integrity split load for a different mix of RCBOs and shared RCDs, so check what the quote actually lists.
Why have two RCDs instead of one?
With one RCD, any fault switches off every circuit. The IET says the usual arrangement is two RCDs, each protecting a number of circuits, to prevent danger or inconvenience if one operates.
Does a dual RCD board have circuit breakers too?
Yes. NICEIC says a dual RCD unit has circuit breakers installed to give each circuit its own overcurrent protection. The RCDs add shock protection over groups of circuits.
Is a dual RCD unit as good as RCBOs?
It gives the same shock protection, but a fault switches off the whole RCD group, not one circuit. NICEIC says the high integrity layout limits a single fault to the disconnection of a single circuit.
Sources
We cite the original source for every fact: legislation, GOV.UK, HSE, the IET, registration bodies and manufacturers' own documents. Each one shows the date we last checked it.
- NICEIC. Consumer units and fuse boxes: installation and replacement. Checked 11 November 2026. ↩
- IET. Consumer units: safe fuse-box upgrades for modern homes. Checked 11 November 2026. ↩
- IET. BS 7671 FAQs: Consumer Units and Protective Devices. Checked 11 November 2026. ↩
