United Kingdom · Wiring Regulations examWhat does an 18th Edition consumer unit need?
- In domestic premises, consumer units must be made of non-combustible material or enclosed in a non-combustible cabinet (Regulation 421.1.201). The rule dates from the 17th Edition, BS 7671:2008+A3:2015.
- Luminaires in domestic premises need additional protection by a 30 mA RCD when they're on an AC final circuit (Regulation 411.3.4). This came in with the 18th Edition and applies to new circuits and existing ones that are significantly altered.
- Single dwellings need protection against transient overvoltage, a surge protective device, unless the value of the installation and equipment doesn't justify it (Regulation 443.4).
- Type AC RCDs are only suitable for sinusoidal residual current (Regulation 531.3.3). Residual DC from electronic equipment can blind one, so choose the type to suit the equipment.
- Overload protection can't be based only on diversity factors of the circuits downstream (Regulation 536.4.202).
Metal consumer units and TT supplies
Nothing stops you fitting a metal consumer unit on a TT system. The risk is a fault between the meter tails and the enclosure before the RCD: the high loop impedance may stop the distributor's device operating within the 1 second limit of Regulation 411.2.4, or at all. The IET suggests short meter tails, proprietary clamps and glands, and protecting the tails from damage.
RCDs, RCBOs and testing
An RCCB doesn't give overcurrent protection; an RCBO does. An RCCB has two ratings, its tripping current in mA and its current-carrying capacity, and it isn't designed to trip at the second one, so coordinate it with the upstream fuse.
Regulation 514.12.2 now says to press the RCD test button every six months, where older versions said quarterly. The IET suggests doing it when the clocks change.
Fitting and meter tails
Every connection must give durable electrical continuity and adequate mechanical strength (Regulation 526.1), and Regulation 134.1.1 means following the manufacturer's instructions, including the torque settings on the unit. BS7671 doesn't require fire-rated glands or intumescent seals; follow the manufacturer.
If the consumer unit is away from the cut-out, Regulations 433.2.2 and 434.2.1 say the unprotected conductors should be no longer than three metres unless installed to reduce the risk of a fault to a minimum. The distributor owns the cut-out fuse, so talk to the DNO, and ENA Engineering Recommendation G87 has guidance on meter tails.
Sources: IET · IET Wiring Matters. Checked October 2026.
- IET: Consumer units and protective devices FAQs ↗
- IET Wiring Matters: BS 7671:2018 frequently asked questions (September 2019) ↗
Rules and fees can change, so check the official site before your test.
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