What is a Dual
RCD Board?
A homeowner guide to dual RCD (split-load) consumer units. How the two RCDs split your circuits, when it became the UK standard plus how it stacks up against a modern RCBO board in 2026.
A dual RCD board (also called a split-load consumer unit) is a UK consumer unit fitted with two main RCDs instead of one. Each RCD covers a bank of MCBs which in turn cover groups of circuits. It became the UK domestic standard from around 2008 once 17th Edition BS 7671 required RCD protection on most circuits. It is still compliant in 2026 yet has been largely overtaken by full RCBO boards on new installs because of one practical weakness: a single fault can take half the house offline.
Four figures that
define the design
These figures are baked into the design of every UK dual RCD board fitted under BS 7671 from the 17th Edition onwards.
Main RCDs
The defining feature. Two 30mA RCDs sit on the busbar splitting the board into two protected banks of circuits.
Trip Threshold
The maximum residual current allowed before either RCD must disconnect the bank to protect against shock.
Typical Main Switch
The standard incoming rating on a UK domestic service. The main switch sits before either RCD on the busbar.
Edition Origin
Dual RCD boards became the practical UK domestic answer to the 17th Edition RCD protection requirements from 2008.
How a dual RCD board
is laid out
Open the front cover plus you find a familiar four-part layout. The main switch sits at one end, then two RCDs split the rest of the board into protected banks.
Main Switch
A double-pole 100A isolator at one end of the board. The first device the meter tails reach. Operates the whole installation in one move.
RCD 1 (Bank A)
The first 30mA RCD. Protects one half of the board. Typically covers downstairs sockets, downstairs lights plus the kitchen ring.
RCD 2 (Bank B)
The second 30mA RCD. Protects the other half of the board. Typically covers upstairs sockets, upstairs lights, the cooker plus the shower.
MCBs Per Circuit
Standard miniature circuit breakers for each circuit. They sit under whichever RCD bank the circuit belongs to. They handle overload plus short circuit only.
Dual RCD boards solved an old problem yet introduced a new one
A dual RCD board is a consumer unit with two main RCDs wired across the busbar. Each RCD covers a bank of MCBs. Each MCB protects one circuit. The phrase you will hear electricians use is split-load because the load (the circuits) is split across two RCDs rather than sitting under a single RCD or being individually protected.
The design solved a real problem. Before dual RCD boards became standard, a UK consumer unit had MCBs only. MCBs trip on overload plus short circuit. They do not detect earth faults. That meant a faulty appliance, a damaged cable or a person touching a live conductor could complete a circuit to earth without anything tripping. The 17th Edition of BS 7671, published in 2008, required RCD protection on most domestic circuits. Dual RCD boards were the practical industry response.
How the wiring is split
Inside a typical dual RCD consumer unit, the busbar is broken into two sections. The main switch feeds both RCDs in parallel. Each RCD then feeds its own busbar section onto which a bank of MCBs is fitted. There is also a separate neutral bar dedicated to each RCD bank because the RCDs are sensing the imbalance between the line plus neutral conductors that pass through them.
The way circuits are allocated between the two banks is governed by a simple rule from BS 7671: do not group circuits in a way that a single RCD trip leaves the property dangerous. In practice that means the lights plus sockets for the same area must not both sit on the same RCD. A common layout looks like this:
- RCD 1 (left bank): upstairs lights, downstairs sockets, garage circuit, outdoor sockets.
- RCD 2 (right bank): downstairs lights, upstairs sockets, cooker, shower.
That cross-distribution means if one RCD trips, the other RCD still keeps lighting on in the affected zones plus power available somewhere in the property.
The practical weakness
Here is the catch that has driven the move to RCBO boards. A single earth fault on a single circuit trips the whole RCD bank for that side of the board. If a kettle starts leaking to earth, the RCD covering the kitchen ring trips. That same RCD also covers downstairs sockets, garage plus outdoor sockets. All of them go off at once. Lights flicker. The freezer trips. The router resets. You troubleshoot a kitchen problem with half the house dark.
Nuisance trips are the other side of the same coin. Some cumulative earth leakage is normal across an entire RCD bank. A few appliances each leaking a tiny amount can sum to enough current to trip the RCD even without a true fault. Dual RCD boards are more prone to this than RCBO boards because more circuits sit under each device.
Is it still compliant?
Yes. A correctly designed dual RCD board with circuits properly grouped between the two RCDs is fully BS 7671 compliant in 2026. It will pass an EICR. It is legal to install. The shift toward RCBO boards is industry preference plus regulation guidance rather than a hard rule that bans split-load. That said, full RCBO is now the recommended standard on full rewires plus new builds because it eliminates the bank trip behaviour entirely.
Dual RCD board
install cost ranges
Dual RCD boards are still cheaper than full RCBO boards on parts. Labour is similar. Most homeowners weigh the part savings against the bank-trip behaviour.
Dual RCD Board Cost Bands
Prices include parts, labour, EIC certification plus Building Control notification. Remedial work uncovered during testing is quoted separately.
The four-stage path
to today's standard
UK consumer unit design has moved through four broad eras. Dual RCD boards represent stage three. The current standard is RCBO at stage four.
Rewireable Fuses
Plastic boards with rewireable fuse wire. No RCD protection. Slow plus dangerous to operate. Almost all gone now.
MCB Boards
Plastic boards with miniature circuit breakers. Faster operation than fuses but still no RCD shock protection on most circuits.
Dual RCD Boards
Plastic split-load boards with two main RCDs covering banks of MCBs. Major safety improvement. Still common in homes built or rewired in this window.
Metal RCBO Boards
Steel enclosure mandatory. Full RCBO protection (one device per circuit) is now the recommended standard on new installs.
Four facts about
dual RCD boards
Still compliant in 2026
A correctly designed dual RCD board passes an EICR plus is legal to fit new today. The shift to RCBO is preference not legal mandate.
One fault knocks out a bank
The defining quirk of a dual RCD board. A kettle fault drops the kitchen ring plus every other circuit on the same RCD bank.
Test both RCDs every 6 months
Press the test button on RCD 1 plus RCD 2 separately. Each must trip instantly. If either does not trip, call an electrician straight away.
Upgrade if you have nuisance trips
Persistent unexplained RCD trips on a dual RCD board often clear up with a move to full RCBO because cumulative earth leakage is split per circuit.
Consumer Unit Upgrades in Milton Keynes
C-Lec Electrical fits dual RCD boards plus full RCBO boards to BS 7671 across Milton Keynes, Bedford plus the surrounding Bedfordshire area. Free quote, certified install, EIC plus Building Control notification handled in full.
Dual RCD board vs
full RCBO board
Both are compliant in 2026. Both pass an EICR. Each suits a slightly different homeowner priority. Neither is wrong.
Lower cost route
- ●Two 30mA RCDs cover banks of MCBs across the board.
- ●Lower parts cost than an equivalent RCBO board.
- ●One earth fault drops a whole RCD bank, taking out half the circuits at once.
- ●More prone to nuisance trips because cumulative earth leakage adds up across the bank.
- ●Compliant choice on a budget-driven replacement where existing circuit groupings are sound.
Modern preferred route
- ●Each circuit has its own combined RCD plus MCB on a single device.
- ●Higher parts cost because every circuit gets its own RCBO.
- ●One earth fault only takes out the affected circuit. Lights, freezer plus router stay live.
- ●Far less prone to nuisance trips because each device sees only its own circuit.
- ●Recommended on new builds, full rewires plus EV charger installs.
For the wider context on consumer unit types, RCBO design, AFDDs plus when an upgrade is genuinely needed, head back to our full guide to consumer units where every common homeowner question is answered in one place.
Back to the Consumer Units Guide
This article sits inside our complete Consumer Units knowledge base. The hub covers everything from board types plus RCBOs through to landlord requirements plus BS 7671 wiring regulations.
More on consumer
unit design
If you have not seen a consumer unit broken down to its parts before, our short explainer on what is a consumer unit covers every component in plain language. To understand the modern alternative to a dual RCD layout, the explainer on what is an RCBO board sets out how every circuit gets its own dedicated protection. For a side-by-side breakdown of the two designs, our deep dive on RCBO vs dual RCD board covers cost, behaviour plus when each one is the right choice. If you need a board fitted in Milton Keynes or Bedford, our consumer unit upgrades service page is the fastest route to a quote.