Consumer Unit Regulations (Amd 3)
Amendment 3 consumer unit requirements — metal enclosures, AFDD, SPD, and circuit arrangement
Metal Consumer Unit Requirement
Amendment 3 to BS 7671 mandates non-combustible enclosures for consumer units in domestic premises, primarily to reduce the risk of fire.
Regulation 421.1.201 requires that in domestic (household) premises, a consumer unit or similar switchgear assembly must be enclosed in a non-combustible enclosure — in practice, this means a steel consumer unit. Plastic consumer units can no longer be installed in domestic premises. This applies to new installations and when an existing consumer unit is replaced.
Replacement Trigger
Consumer Unit Configurations
CU configuration comparison
BS 7671 and manufacturer guidance| Configuration | Description | Pros | Cons |
|---|---|---|---|
| Split-load (dual RCD) | Two 63A RCDs each protecting half the circuits | Lowest cost | Half circuits lost on RCD trip |
| High integrity | Two RCDs + main switch, with some circuits on non-RCD ways | Flexibility | Some circuits may lack RCD protection |
| Full RCBO | Individual RCBO per circuit | Best discrimination | Higher cost per way |
| Main switch only | Main switch + RCBOs on every way | Maximum flexibility | Highest cost |
Full RCBO boards are now the preferred configuration for new domestic installations.
RCD and RCBO Requirements
BS 7671 requires 30mA RCD protection for socket outlets up to 32A, mobile equipment up to 32A outdoors, cables installed in walls at less than 50mm depth without mechanical protection, and all circuits in domestic installations (with limited exceptions).
RCBO type selection guidance
BS 7671 Regulation 531.3| Circuit Type | RCBO Type | Reason |
|---|---|---|
| General sockets, lighting | Type A | Detects AC and pulsating DC faults |
| EV charger | Type A (+ Type B RDC-DD) | DC fault current from charger electronics |
| Cooker (induction hob) | Type A or Type F | High-frequency leakage from induction |
| VFD / inverter circuits | Type B | Detects smooth DC fault currents |
| Fire alarm | Type AC (if required) | Keep simple — avoid nuisance trips |
SPD and AFDD Provisions
SPD Installation
AFDD Considerations
A typical modern 3-bed house requires a minimum 12-way consumer unit: 2 x ring final or radial sockets, 2 x lighting, cooker, shower, immersion, smoke alarm, outdoor circuit, and spare ways for future additions. Always include at least two spare ways.