RCDs in Domestic Consumer Units
How should RCDs be arranged in a domestic consumer unit? A domestic consumer unit protects socket outlets, lighting, and fixed appliances with 30 mA residual current devices, either as two RCDs feeding a split-load board or as one RCBO per circuit, to meet the additional-protection requirement in the current UK wiring regulations. Get the split wrong and a single earth fault on an outdoor socket takes down half the house, including the fridge and the alarm system. This article covers split-load vs RCBO-per-way boards, sensitivity selection for socket and lighting circuits, selectivity against the main switch, TT earthing in rural properties, and the nuisance-tripping patterns specific to domestic wiring.
Split-Load Boards: Two RCDs, One Failure Point Each
A split-load consumer unit puts two 30 mA RCCBs ahead of the MCBs, each one feeding half the ways. One side typically carries the socket outlets and the outdoor supply; the other carries lighting and fixed appliances. The logic is simple: if one RCCB trips on an earth fault, the other half of the house stays live. It rarely works out that clean in practice. Sockets, kitchen appliances, and an outdoor supply on the same 30 mA device means their leakage currents add up on one sense winding, and a marginal fault on any one of them trips the lot.
What we see in the field: a single leaky halogen transformer or an aging washing machine element trips the RCCB, and everything else on that half — including the fridge-freezer — goes with it. That is the single biggest argument for RCBO-per-way boards in any property with unattended appliances.
RCBO-Per-Way Boards: Individual Discrimination, Higher Parts Cost
An RCBO-per-way board fits a combined MCB and 30 mA RCD on every circuit instead of sharing two RCCBs across the whole board. A fault on the cooker circuit trips only the cooker way; the lighting, sockets, and freezer stay energized. This is close to standard practice on new UK domestic installations now, driven by the amendment that extended additional RCD protection to lighting circuits alongside sockets and outdoor mobile equipment. The trade-off is cost: an RCBO runs roughly two to three times the price of an MCB plus its share of a shared RCCB, and a twelve-way board full of RCBOs needs more enclosure depth than the same board with two RCCBs.
| Criteria | Split-Load (2x RCCB) | RCBO-Per-Way | Single Main RCD |
|---|---|---|---|
| Fault isolation | Half the board drops | Only the faulted way drops | Whole board drops |
| Parts cost | Lower | Higher per way | Lowest |
| Board depth needed | Standard | Larger enclosure | Standard |
| Typical use today | Retrofits, budget rewires | New builds, full rewires | Rarely used alone in domestic |
Retrofits and budget rewires still specify split-load boards because the existing enclosure and tails often can't take a deeper RCBO board without a full swap. New-build and full rewire jobs default to RCBO-per-way now, for the same reason panel builders moved industrial boards toward individual protection: nuisance trips get isolated instead of cascading. See our differences between RCCB, RCBO and MCB for the device-level breakdown behind this choice.
Sensitivity: 30 mA Is the Default, Not the Only Option
Every domestic socket outlet, and now every lighting circuit under current UK wiring rules, needs additional protection from a 30 mA RCD or RCBO. That figure is not arbitrary: 30 mA is the threshold below which most adults can release their grip and survive contact for the trip time defined in IEC 61008, which is why it is called personal protection rather than equipment protection. Higher sensitivities — 100 mA, 300 mA — belong upstream of large loads or on TT main switches, where the job is limiting fire risk from a sustained earth fault current, not stopping a shock. A domestic board almost never needs those higher values on a final circuit; they show up as a time-delayed main switch instead.
Use our RCD sensitivity breakdown when specifying a replacement device, and the RCD selection checklist for poles and type alongside sensitivity.
Selectivity Against the Main Switch: When a Domestic Board Needs It
Most domestic consumer units don't run a time-delayed S-type RCD upstream of the final-circuit devices, because the incomer is usually a plain main switch, not an RCD. TT properties are the exception. Where the supply has no reliable metallic earth path back to the transformer, some installations fit a 100 mA time-delayed RCD as, or ahead of, the main switch, sized so it operates only on an earth fault the downstream 30 mA devices failed to clear. Selectivity between the two needs the upstream device rated at roughly double the downstream IΔn with a short time delay, the same rule used in industrial boards, just at domestic scale.
TT Earthing in Rural and Older Properties: Why the RCD Isn't Optional
A property with no PME connection or a poor incoming earth relies on a local earth electrode, usually a rod, and that gives a loop impedance far too high for an MCB to clear a line-to-earth fault within a safe disconnection time. The RCD is what actually protects the installation in that scenario, not the MCB. That's also why TT earthing carries a maximum touch-voltage rule tying the electrode resistance directly to the RCD's sensitivity.
Formula: Maximum earth electrode resistance for TT touch-voltage limit — Source: IEC 60364-4-41, Clause 411.5.3
RA x IΔn ≤ 50 V
| Symbol | Description | Unit |
|---|---|---|
| RA | Resistance of the earth electrode plus protective conductor | Ω |
| IΔn | Rated residual operating current of the RCD | A |
| 50 V | Maximum permitted touch voltage under fault conditions | V |
Run the numbers and a 30 mA RCD (0.03 A) permits an earth electrode resistance up to roughly 1,666 Ω before the touch-voltage limit is exceeded, a figure most rod electrodes clear without difficulty. That's the practical reason 30 mA RCBOs, not just a good earth rod, are what makes a TT domestic installation safe rather than the rod alone. See RCDs in TT, TN and IT earthing systems for how this compares to TN properties, where the supply network's earth does most of the work instead.
Nuisance-Tripping Patterns Specific to Domestic Wiring
A freezer left unattended for two weeks over an aging split-load RCCB is the complaint every panel installer has heard. The fix isn't a bigger RCD, it's putting the freezer on its own RCBO way, so a fault or cumulative leakage anywhere else on the board can't take it down. Outdoor sockets are the second recurring pattern: a garden extension lead left out in the rain adds insulation leakage that pushes a marginal RCCB over the edge, even with no fault on the item actually plugged in. LED driver and halogen transformer leakage is smaller per unit but additive; a kitchen with a dozen LED downlights on one way can accumulate enough leakage current to sit close to the 30 mA threshold before any single fault occurs.
For step-by-step fault isolation, see why an RCD keeps tripping.
Frequently Asked Questions
Do all circuits in a domestic consumer unit need an RCD?
Current UK wiring rules require 30 mA additional protection on socket outlets, circuits supplying mobile equipment outdoors, and lighting circuits. A handful of circuits, a fire alarm panel for example, are sometimes exempted where continuity of supply outweighs shock protection, but the exemption has to be justified, not assumed.
What's the real difference between a split-load board and an RCBO board?
A split-load board uses two RCCBs, each covering half the final circuits, so a fault on any one circuit trips everything sharing that RCCB. An RCBO board fits an individual RCD into every way, so only the faulted circuit drops.
Can a consumer unit mix RCBOs and a shared RCCB?
Yes. A common layout puts high-leakage or critical circuits, a shower or a freezer, on their own RCBO, and leaves lower-risk circuits like a single lighting way on a shared RCCB. It's a reasonable middle ground between board cost and fault isolation.
Does a freezer need its own RCBO?
Not by regulation, but it's the single most common upgrade requested after a nuisance trip has spoiled food. Isolating it on its own 30 mA RCBO removes it from the leakage total of every other circuit on that RCCB side.
Why does my consumer unit have a 100 mA RCD as the main switch?
That's typical of a TT installation. The 100 mA time-delayed device backs up the 30 mA downstream RCBOs, clearing an earth fault they failed to clear rather than tripping on every downstream event, and it protects against the higher-impedance earth path a rod electrode gives compared with a PME connection.
Conclusion
A domestic consumer unit's RCD strategy comes down to how much a nuisance trip costs the household. Split-load boards are cheaper and still meet the regulations, but a marginal fault takes out everything sharing that RCCB, freezer included. RCBO-per-way boards cost more per way and need a deeper enclosure, but isolate faults to a single circuit, which is why they're now the default on new builds and full rewires. TT properties add a second requirement on top of sensitivity: the earth electrode and the RCD's IΔn have to satisfy the touch-voltage rule together, not separately. Get the sensitivity, the board architecture, and the earthing system right together, and the RCD stops being the household's most complained-about component. Start from our RCD protection guide for the full technical picture, or browse residual current devices and RCBOs to spec the right device for the board.
