Section 704 of BS 7671:2018+A4:2026 covers construction and demolition site installations: the temporary supply, the site distribution and every socket a plasterer's mixer plugs into. Its central rule is Regulation 704.410.3.10, which says any circuit feeding a socket-outlet up to 32 A, or hand-held equipment up to 32 A, must be protected by reduced low voltage (110 V centre-tapped, 55 V to earth), a 30 mA RCD, electrical separation or SELV/PELV. Add BS EN 60309-2 plugs, BS EN 61439-4 ACS boards, H07RN-F flex, a lockable main isolator and a three-monthly EICR, and you have the section in one paragraph. The rest of this guide shows how to design, test and certify it without getting caught out.
- Section 704 applies to the working site. Site offices, canteens, drying rooms and toilets are outside its scope and follow the ordinary rules of BS 7671.
- Circuits for sockets and hand tools up to 32 A need RLV, a 30 mA RCD, electrical separation or SELV/PELV (Reg 704.410.3.10). The UK default is 110 V centre-tapped-to-earth.
- Distribution units should be Assemblies for Construction Sites (ACS) to BS EN 61439-4, each with a lockable isolator on the incoming supply.
- Plugs and sockets are BS EN 60309-2 industrial type: yellow for 110 V, blue for 230 V, red for 400 V.
- Do not assume a PME earth. If the DNO refuses one, the site is TT: fit and test an electrode and put every circuit behind an RCD.
- RLV circuits are tested against Table 41.6, not the 230 V tables, and a 5 s disconnection time applies.
- IET Guidance Note 3 recommends periodic inspection every three months. Issue an EIC at first energisation and record each check as an EICR.
What does Section 704 actually cover?
Regulation 704.1 sets the scope: installations used during the construction of new buildings, repair, alteration, extension or demolition of existing buildings, engineering works, earthworks, and similar work. It also covers the temporary installation itself, meaning the incoming supply, the site distribution and the final circuits, until the permanent installation takes over.
What it does not cover matters just as much. Installations in the administrative parts of a site, such as offices, cloakrooms, meeting rooms, canteens, restaurants, dormitories and toilets, are excluded. A prefabricated site cabin with a fixed board, lights and sockets is treated like any other small commercial installation, so you would not, for example, insist on 110 V sockets in the site manager's office. The boundary is the door of the cabin: the supply cable feeding that cabin from the site distribution is Section 704 territory.
Two other documents sit alongside the wiring regulations: BS 7375:2010, the code of practice for distribution of electricity on construction and demolition sites, and the HSE's HSG141, Electrical safety on construction sites.
Why is 110 V the UK default for site tools?
Regulation 704.410.3.10 gives four acceptable ways to protect circuits supplying socket-outlets rated up to and including 32 A, and other circuits supplying hand-held equipment up to 32 A: reduced low voltage to Regulation 411.8; automatic disconnection of supply with additional protection by an RCD rated at no more than 30 mA; electrical separation with each socket-outlet supplied by its own separate source; or SELV/PELV.
The regulation does not say "110 V". But the HSE's HSG141 recommends reduced low voltage for portable tools and hand lamps, and every major contractor's site rules make it a condition of entry. A 110 V centre-tapped transformer to BS EN 61558-2-23 earths the middle of the winding, so the highest voltage anyone can touch to earth is 55 V (63.5 V on a three-phase 110 V system). A damaged drill lead on a wet slab at 55 V to earth is unpleasant; at 230 V it can be fatal.
Reduced low voltage has a separate set of rules in Regulation 411.8. The source must be a safety isolating transformer or equivalent, the secondary centre point must be earthed, exposed-conductive-parts are connected to the protective conductor, and the maximum disconnection time is 5 s. Table 41.6 gives the maximum earth fault loop impedance for RLV circuits, and the figures are far smaller than their 230 V cousins because there is only 55 V or 63.5 V available to push fault current. A 16 A 110 V lead run 50 m across a site can fail on Zs when its 230 V twin would sail through; the Maximum Zs Values BS 7671: Tables, 80% Rule and Ze guide explains why the design voltage drives the whole table.
Cabins and static plant that need 230 V or 400 V rely on the usual automatic disconnection, with a 30 mA RCD wherever they feed sockets up to 32 A.
What does a site distribution look like?
| Item | Requirement | Where it comes from |
|---|---|---|
| Incoming supply unit | Main switch or isolator that can be secured in the OFF position; usually a metered DNO supply or a generator | Regs 704.537.2.2, 704.313 |
| Distribution units | Assemblies for construction sites (ACS) to BS EN 61439-4, each with a lockable isolator on the incoming supply | Reg 704.511.1 |
| Plugs and sockets | BS EN 60309-2 industrial type; yellow 110 V, blue 230 V, red 400 V | Reg 704.511.1, BS EN 60309-2 |
| Transformers | 110 V centre-tapped to BS EN 61558-2-23, earthed at the centre tap | Reg 411.8 |
| Flexible cables | H07RN-F (BS EN 50525-2-21) or equivalent for RLV circuits; nothing across walkways or vehicle routes unless protected | Regs 704.522.8.10, 704.522.8.11 |
| Additional protection | RCD not exceeding 30 mA on socket circuits up to 32 A unless RLV, separation or SELV/PELV is used | Reg 704.410.3.10 |
| Periodic inspection | Recommended every 3 months | IET GN3, Table 3.2 |
Check the ACS standard, BS EN 61439-4, on the rating plate. A domestic consumer unit in a plywood box is not an ACS, however robustly it has been mounted; proper site units are IP44 or better with sockets angled so that plugs hang downward.
The BS EN 60309-2 colour code runs violet (20 to 25 V), white (40 to 50 V), yellow (100 to 130 V), blue (200 to 250 V), red (380 to 480 V) and black (500 to 690 V). The earth-pin position differs with the colour, so a 230 V plug physically cannot enter a 110 V socket, which is why cutting a site plug off to fit a 13 A one is such a dangerous shortcut.
Regulation 704.522.8.10 addresses the biggest cable risk: a lead across a site road gets flattened by a telehandler within the day, so crossings need a ramp or an overhead catenary. The flex itself must be H07RN-F or equivalent, because ordinary PVC flex cracks in cold weather and does not survive being dragged over blockwork.
TN-C-S or TT: which earthing arrangement will the site have?
A temporary builders' supply may arrive as TN-C-S, TN-S or with no earth terminal at all. Distributors have historically been reluctant to provide a PME earth terminal for construction sites, because PME relies on main bonding to extraneous-conductive-parts, and on a changing site with scaffolding, reinforcement and temporary pipework that bonding cannot be guaranteed. Some DNOs will provide one subject to conditions; others will not. Ask before you design.
If no earth terminal is provided, the installation is TT. That means an earth electrode, a measured electrode resistance that keeps RA × IΔn within the 50 V touch-voltage limit of Regulation 411.5.3, and RCD protection on every circuit, not only the socket circuits. The Earth Electrode Testing for TT Systems: Methods and Limits guide covers how to measure the rod properly, and Earthing Arrangements Explained: TN-S, TN-C-S and TT sets out the three systems side by side.
Generator supplies are a third case: a set with a neutral-earth link and its own electrode is effectively a TN-S island. Get the supplier's earthing diagram before you connect anything and record it on the certificate.
The distributor's decision also affects disconnection times. The Table 41.1 figures apply to final circuits: 0.4 s for a 230 V TN circuit and 0.2 s for TT. Section 704 also modifies the disconnection-time rules for larger circuits in Regulation 704.411.3.2.1, so read it before you rely on the usual 5 s allowance for a distribution circuit feeding a remote ACS.
How do you test a construction site installation?
The sequence is the ordinary one from IET Guidance Note 3. The site adds four wrinkles.
First, 110 V circuits are tested at their own voltage. Measure Zs at the socket on the secondary side of the transformer and compare it with Table 41.6 for a 5 s disconnection time. Check that your MFT will run a loop test at 110 V before you rely on it; some older instruments refuse below a minimum supply voltage.
Second, the distribution is almost always three-phase. Balance the transformers and single-phase loads across the phases and test phase rotation at every three-phase outlet; the Three-Phase Testing Basics for Domestic Electricians guide covers the sequence.
Third, RCD testing is not a one-off. A site RCD lives in a wet, dusty enclosure and is tripped a dozen times a week, so test at IΔn and 5 IΔn on every periodic visit and record the trip time (/guides/rcd-testing-explained).
Fourth, insulation resistance is where a site ages fastest. A 110 V lead that read 200 MΩ in March can read 0.3 MΩ in June once the sheath has been nicked and rain has found its way in, so test leads and transformers as part of the periodic, not only the fixed wiring.
What certification and inspection intervals apply?
Section 704 does not invent a new form. The temporary installation gets an Electrical Installation Certificate with schedules of inspections and test results when it is first energised, laid out to the current BS 7671 model forms. Every extension of the distribution or additional ACS is further work and needs its own EIC or, where genuinely minor, a Minor Works certificate (/guides/electrical-installation-certificate-eic-explained).
IET Guidance Note 3 recommends a maximum of three months between periodic inspections of construction site installations, one of the shortest intervals in its table. Each is recorded as an EICR with observations coded C1, C2, C3 or FI as normal, and the main contractor will usually want the report on the day for the CDM 2015 file.
The Electricity at Work Regulations 1989 sit over all of this: Regulation 4(2) requires systems to be maintained so as to prevent danger, which is the duty behind the three-monthly interval. HSG141 adds daily user checks of tools and leads and a weekly visual inspection of the distribution by a competent person.
Common site observations are predictable: 13 A extension leads at 230 V on the working site (C2 where a 30 mA RCD cannot be confirmed), damaged flex with exposed cores (C1 if live, C2 otherwise), missing padlocks on ACS isolators (C3), transformers with the earth pin broken off the plug (C2), and cables across vehicle routes without ramps (C2, and a word with the site manager the same day).
How Certio helps
Certio produces the EICR, EIC, Minor Works certificate and PAT records on the BS 7671 model forms and turns them into branded PDFs. Photograph the consumer unit and the circuit schedule drafts itself; point the phone camera at a Megger MFT-X1 and the reading goes straight into the schedule of test results, or dictate it while your hands stay on the probes. Suggested observation wording with the regulation reference is there for you to check and confirm. The judgement stays yours; the typing goes. Starter is free for 7 days, then £4.99 a month.