Construction site temporary installations use 110V CTE for portable tools, BS EN 61439-4 assemblies, one of four permitted protection methods on every socket-outlet circuit up to 32 A (Reg 704.410.3.10), and quarterly inspection. This guide covers BS 7375 and BS 7671:2018+A4:2026 Section 704 — with the regulation numbers and the Zs figures.
Free for 7 days · No charge until day 8 · Cancel anytime · Used by 1,000+ UK electricians
Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
The short answer
Portable hand tools and handlamps on UK construction sites are supplied at 110 V reduced low voltage — a 55-0-55 V centre-tapped-earth arrangement, so no live conductor sits more than 55 V above earth (BS 7671 Reg 411.8.1.2). Reg 704.410.3.10 requires every socket-outlet circuit rated up to and including 32 A to be protected by reduced low voltage, by automatic disconnection with a 30 mA RCD to Reg 415.1.1, by electrical separation, or by SELV/PELV. Site installations also follow BS 7375:2010 and are commonly inspected every 3 months.
1,000+
UK electricians
“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”
01 · Construction Guide
Everything below comes from BS 7671:2018+A4:2026 Section 704 (Construction and Demolition Site Installations) and Chapter 41. If you only read one part of this page, read this table.
| What | Requirement | Reg |
|---|---|---|
| Socket-outlet ≤ 32 A, or any circuit supplying hand-held equipment ≤ 32 A | Protect by one of: (a) reduced low voltage (Reg 411.8); (b) automatic disconnection of supply with additional protection by an RCD to Reg 415.1.1; (c) electrical separation, each outlet and item of hand-held equipment on its own transformer or separate winding; (d) SELV or PELV. | 704.410.3.10 |
| Socket-outlet circuits rated above 32 A | An RCD with a rated residual operating current not exceeding 500 mA, disconnecting within the time required by Reg 411.3.2.3 or 411.3.2.4. Reg 411.3.2.5 does not apply. | 704.411.3.2.1 |
| Reduced low voltage — nominal voltage | Not exceeding 110 V AC RMS between lines: 55 V to the earthed midpoint single-phase, 63.5 V to the earthed neutral three-phase. | 411.8.1.2 |
| Reduced low voltage — fault protection | Overcurrent device in each line conductor or an RCD; all exposed-conductive-parts earthed; disconnection within 5 s at every point of utilisation including socket-outlets. Maximum Zs in Table 41.6. If an RCD provides fault protection, IΔn × Zs must not exceed 50 V. | 411.8.3 |
| Means of earthing | A PME earthing facility shall not be used unless all extraneous-conductive-parts are reliably connected to the main earthing terminal per Reg 411.3.1.2. | 704.411.3.1 |
| Distribution assemblies | All assemblies distributing electricity on site shall comply with BS EN 61439-4 (Assembly for Construction Sites, ACS). | 704.511.1 |
| Plugs and socket-outlets | 16 A up to 125 A: BS EN IEC 60309-2. Above 125 A up to 800 A where interchangeability is not required: BS EN IEC 60309-1. | 704.511.1 |
| Isolation | Each ACS shall incorporate devices for switching and isolating the incoming supply, suitable for securing in the off position (padlocking facility or lockable enclosure). | 704.537.2 |
| Protective measures not permitted | Obstacles and placing out of reach; non-conducting location; earth-free local equipotential bonding; electrical separation supplying more than one item of current-using equipment. | 704.410.3.5, 704.410.3.6 |
Regulation 704.1.1 excludes the administrative locations of a construction site — offices, cloakrooms, meeting rooms, canteens, restaurants, dormitories and toilets. In those areas the general requirements of Parts 1 to 6 and Part 8 apply instead, so the ordinary 30 mA additional-protection rules of Regulation 411.3.3 govern the socket-outlets rather than the four methods of Regulation 704.410.3.10.
Free download
Every key change in the 2026 amendment on one page. AFDDs, TN-C-S protection, new schedule columns, model forms. Pinned on your van dash.
02 · Construction Guide
BS 7375:2010 (Code of practice for distribution of electricity on construction and building sites) is the practical guidance document for temporary electrical installations on UK construction sites. The enforceable requirements sit in BS 7671:2018+A4:2026 Section 704, which applies to temporary installations for construction and demolition work — new build, repair, alteration, extension, demolition, engineering works and earthworks — and covers both fixed and movable installations (Reg 704.1.1).
Construction sites are among the most electrically hazardous environments in the UK. Wet conditions, metallic structures, heavy machinery, moving plant and a workforce that is largely not electrically trained combine badly. The temporary installation has to deliver safe, reliable power in those conditions, often for years, while the site around it keeps changing shape.
The current edition is BS 7671:2018+A4:2026 — not A2:2022. The published amendment summary lists the following changes to Section 704: new requirements for external influences (Reg 704.512.2); a modification to Regulation 704.410.3.6 to add the non-use of the protective measure of electrical separation; changes to plug and socket-outlet requirements in Regulation 704.511.1; changes to the wiring system requirements (704.52); and a full redraft of the requirements for devices for isolation (704.537). Check that EICRs, specifications and design records quote the right edition.
Regulation 421.1.7 was redrafted at A4:2026. It is now a requirement to protect final circuits supplying socket-outlets rated not exceeding 32 A with arc fault detection devices in Higher Risk Residential Buildings, Houses in Multiple Occupation, purpose-built student accommodation and care homes. For all other premises — which includes construction site offices and welfare accommodation — the regulation recommends AFDDs for single-phase AC final circuits supplying socket-outlets not exceeding 32 A. Where AFDDs are specified, Regulation 532.6 places them at the origin of the final circuits to be protected and in AC single-phase circuits not exceeding 230 V.
03 · Construction Guide
A 110V centre-tapped-earth supply is a reduced low voltage (RLV) system under BS 7671 Regulation 411.8. It is the UK construction industry's primary defence against a fatal shock from a portable tool, and Regulation 704.410.3.10 NOTE 1 says the reduced low voltage system is strongly preferred for portable handlamps for general use, portable hand tools and local lighting up to 2 kW.
Regulation 411.8.4.1 permits only three sources: a double-wound isolating transformer complying with BS EN IEC 61558-1 and BS EN 61558-2-23; a motor-generator set with windings providing isolation equivalent to an isolating transformer; or a source independent of other supplies, such as an engine-driven generator. On site this is normally a 3.3 kVA, 5 kVA or 10 kVA yellow transformer, with larger units used for multiple-outlet distribution.
Regulation 411.8.4.2 requires the midpoint of the secondary winding of a single-phase transformer or generator — or the neutral star point on three-phase — to be connected to Earth. That splits the 110V output into two halves: 55 V from each live conductor to earth. The tool still sees 110 V between conductors, but nothing on the system sits more than 55 V above earth. On three-phase RLV the equivalent figure is 63.5 V to the earthed neutral (Reg 411.8.1.2).
If a worker contacts a live conductor while standing on wet concrete or touching scaffolding, the voltage driving the shock current is 55 V rather than 230 V. The shock is not harmless — it can still cause pain, burns and involuntary muscle contraction — but the risk of a fatal cardiac event is dramatically reduced.
Regulation 411.8.5 requires every plug, socket-outlet, luminaire supporting coupler, device for connecting a luminaire and cable coupler on an RLV system to have a protective conductor contact, and to be dimensionally incompatible with those used for any other system in the same premises. The yellow colour is the site convention on top of that; the physical keying is what actually stops a 110V tool being plugged into 230V.
Reduced low voltage still needs fault protection. Regulation 411.8.3 requires an overcurrent protective device in each line conductor or an RCD, all exposed-conductive-parts connected to Earth, and an earth fault loop impedance low enough that disconnection happens within 5 s at every point of utilisation, socket-outlets included. Table 41.6 gives the maximum Zs values.
| Rating (A) | Type B | Type C and D | gG fuse, BS 88-2 | |||
|---|---|---|---|---|---|---|
| 55 V | 63.5 V | 55 V | 63.5 V | 55 V | 63.5 V | |
| 3 | 3.48 | 4.02 | 1.74 | 2.01 | — | — |
| 6 | 1.74 | 2.01 | 0.87 | 1.01 | 2.90 | 3.35 |
| 10 | 1.05 | 1.21 | 0.52 | 0.60 | 1.63 | 1.89 |
| 16 | 0.65 | 0.75 | 0.33 | 0.38 | 0.95 | 1.10 |
| 20 | 0.52 | 0.60 | 0.26 | 0.30 | 0.67 | 0.77 |
| 25 | 0.42 | 0.48 | 0.21 | 0.24 | 0.52 | 0.60 |
| 32 | 0.33 | 0.38 | 0.16 | 0.19 | 0.42 | 0.48 |
| 40 | 0.26 | 0.30 | 0.13 | 0.15 | 0.31 | 0.35 |
| 50 | 0.21 | 0.24 | 0.10 | 0.12 | 0.24 | 0.27 |
| 63 | 0.17 | 0.19 | 0.08 | 0.10 | 0.19 | 0.22 |
| 80 | 0.13 | 0.15 | 0.07 | 0.08 | 0.13 | 0.15 |
| 100 | 0.10 | 0.12 | 0.05 | 0.06 | 0.12 | 0.14 |
| 125 | 0.08 | 0.10 | 0.04 | 0.05 | 0.08 | 0.09 |
Values in ohms, from BS 7671:2018+A4:2026 Table 41.6. The circuit-breaker columns cover devices to BS EN 60898 and the overcurrent characteristics of RCBOs to BS EN 61009-1; the fuse columns are general purpose (gG) fuses to BS 88-2, fuse systems E and G. Table 41.6 NOTE 1 states the values were determined using a value of 0.95 for Cmin; NOTE 2 states they should not be exceeded with line conductors at their maximum permitted operating temperature and cpcs at their assumed initial temperature, and that a reading taken at a different temperature should be adjusted accordingly (see Appendix 3). Where fault protection is by RCD instead, Regulation 411.8.3 requires the product of the rated residual operating current and the earth fault loop impedance not to exceed 50 V.
04 · Construction Guide
Site distribution boards are the backbone of the temporary installation. Regulation 704.511.1 requires all assemblies on construction and demolition sites for the distribution of electricity to comply with BS EN 61439-4 — an Assembly for Construction Sites, or ACS. That is the term designers, specifiers and inspectors use, and it is what the marking on the enclosure should say.
The primary distribution point, connected to the incoming builder's supply (400V three-phase or 230V single-phase from the DNO). Houses the main isolator, metering and outgoing ways for sub-boards, site transformers and fixed plant. Regulation 704.537.2 requires the incoming isolating device to be suitable for securing in the off position — a padlocking facility or a lockable enclosure.
Smaller assemblies positioned at work locations around the site, supplied from the main board via SWA. Each provides 110V CTE socket-outlets from a built-in or adjacent transformer plus 230V outlets for fixed equipment. IP44 is the usual minimum specification for outdoor positions; wetter or more exposed locations are commonly specified at IP55 or above.
Trolley-mounted or carry-handle units combining a step-down transformer with several 110V socket-outlets. Ideal for individual work areas and easily moved as the work progresses through the building — provided they are stable and secure wherever they are stood.
Regulation 704.512.2 requires consideration of the risk of damage from corrosive substances, movement of structures and vehicles, wear and tear, tension, flexing, impact, abrasion, severing, and the ingress of liquids or solids. In practice that means a robust enclosure (IK08 or better is the usual site specification), IP-rated glands at every cable entry, and no open knockouts.
Regulation 704.511.1 requires plugs and socket-outlets rated 16 A up to 125 A to comply with BS EN IEC 60309-2. Above 125 A up to 800 A, where interchangeability is not required, BS EN IEC 60309-1 applies. Colour follows the BS EN IEC 60309-2 voltage-band convention:
| Colour | Supply | Typical use on site |
|---|---|---|
| Yellow | 110 V CTE (reduced low voltage) | Portable hand tools, task lighting, transformers |
| Blue | 230 V single-phase | Fixed plant, welfare and cabin supplies |
| Red | 400 V three-phase | Hoists, tower cranes, large fixed plant |
Interlocked socket-outlets that cannot be connected or disconnected under load are common good practice rather than a BS 7671 requirement. Boards must be mounted on stable, level surfaces and kept clear of vehicle traffic. On multi-storey sites, floor-level boards are supplied via vertical risers from the main board, and every board position must stay accessible for operation, testing and emergency isolation.
In the app
BS 7671:2018+A4:2026 compliant temporary electrical distribution for site. Scan boards, auto-populate EICR schedules in minutes. Reduce paperwork by 80%.
05 · Construction Guide
Regulation 704.410.3.10 sets the required protection for every socket-outlet circuit rated up to and including 32 A and every circuit supplying hand-held equipment up to 32 A. It permits four alternative methods, and at least one must be applied. The 30 mA RCD belongs to method (b), where automatic disconnection of supply is the chosen protection — it is not a blanket requirement on every circuit. Regulation 411.3.3 makes the point explicitly: its 30 mA additional-protection requirements do not apply to reduced low voltage systems under Regulation 411.8.
Regulation 704.411.3.2.1 covers any circuit supplying one or more socket-outlets rated above 32 A. Regulation 411.3.2.5 is not applicable to those circuits; instead an RCD with a rated residual operating current not exceeding 500 mA must interrupt the supply within the disconnection time required by Regulation 411.3.2.3 or 411.3.2.4, as appropriate.
Regulation 531.3.3 restricts Type AC to fixed equipment where it is known that the load current contains no DC components — which rules it out for site socket-outlets. Type A is the practical minimum. Consider Type F where inverter-driven equipment such as variable speed drives and battery chargers is connected, and Type B where smooth DC residual current is possible.
The integral test button should be used daily by the user. Electrical verification is carried out at the rated residual operating current as part of the periodic inspection — quarterly on most sites. See the testing section below for what the standard requires of that test.
Unwanted tripping on site is usually moisture ingress, damaged cables or high standing earth leakage from construction plant. The answer is never to bypass the device — it is to find and fix the leakage. Splitting loads across several RCDs, rather than one device covering many outlets, limits the disruption when one does trip.
The RCD is one layer, not the whole defence. A properly designed site stacks reduced voltage, earthing and bonding, insulation, mechanical protection of cables and residual current protection. Each addresses a different failure mode, and together they cut the risk of a fatal shock dramatically.
In the app
16 certificate types, 70+ calculators, RAMS, quoting, invoicing, AI agents, and 46+ training courses — from £6.99/mo.
06 · Construction Guide
Cables on site face mechanical damage from construction activity, vehicle traffic, sharp edges and weather, and badly run cable is a serious tripping hazard. Regulation 704.522.8.11 fixes the cable types, and Regulation 704.522.8.101 governs where they may run.
| Application | Required cable |
|---|---|
| Reduced low voltage systems (110V CTE) | Low temperature 3182/3/4/5A thermoplastic cable to BS 6004, or equivalent flexible cable (Reg 704.522.8.11). |
| Applications exceeding reduced low voltage | H07RN-F to BS EN 50525-2-21, or equivalent heavy-duty flexible cable (Reg 704.522.8.11). |
| Flexible cable subject to movement | H07RN-F to BS EN 50525-2-21 or equivalent, resistant to abrasion and to water (Reg 704.522.8.101). |
| Semi-permanent distribution runs | Steel wire armoured (SWA) is the usual site choice — mechanical protection, weather resistance, and suitable for surface, tray or buried routes. |
Regulation 704.522.8.101 states that cables should not be run across site roads or walkways. Where that is unavoidable, adequate protection against mechanical damage and contact with construction plant machinery must be provided, and particular attention must be given to protecting surface-run and overhead cables against mechanical damage. In practice that means overhead runs, buried routes with cable covers, SWA on the surface, or proper cable ramps and bridges at crossings. Standard PVC flex laid loose on the ground meets none of those requirements.
Extension leads for 110V tools should be the shortest practical length, made from cable of the type required by Regulation 704.522.8.11, fitted with BS EN IEC 60309-2 plugs and sockets, and inspected before use. Fully unwind a lead in use so it does not overheat. Damaged cables come out of service immediately — a repair with insulation tape is not a repair. Route leads away from walkways and work areas, and use ramps or bridges wherever a crossing is unavoidable.
Cable routes need reviewing as the job moves on. A run that was safe during groundworks can become a hazard once crane operations, scaffolding and heavy plant arrive. The site electrician should re-walk the routes regularly and reposition cables before they get damaged, not after.
07 · Construction Guide
Testing a site installation follows the standard BS 7671 sequence, with extra attention to Section 704 and to the practical business of testing in a live construction environment.
Regulation 652.1 requires the frequency of periodic inspection and testing to be determined having regard to the type of installation. For construction sites the recommended maximum is 3 months; many principal contractors add monthly visual inspections on top of the quarterly EICR.
Measure Zs at the most distant socket-outlet on each circuit. For 110V CTE circuits compare against Table 41.6, which is based on a 5 s disconnection time at Uo of 55 V single-phase or 63.5 V three-phase — the values are much lower than the 230V figures. Table 41.6 NOTE 2 assumes conductors at operating temperature; where the test is made at a different temperature, adjust the reading accordingly (see Appendix 3).
Regulation 643.3.2 requires the test with current-using equipment disconnected, using the test voltages in Table 64 — 500 V DC and a minimum of 1.0 MΩ for circuits up to and including 500 V, or 250 V DC and 0.5 MΩ for SELV and PELV. Regulation 643.3.3 adds a second stage: where connected equipment would influence the result or be damaged, test before connecting it, then after connection apply a 250 V DC test between live conductors and the protective conductor, which must read at least 1 MΩ. Site circuits often read lower than permanent ones because of moisture and cable condition — investigate before you record.
Regulation 643.8 requires the effectiveness of automatic disconnection by RCDs to be verified using suitable test equipment to BS EN 61557-6. Its note states that, regardless of RCD type, effectiveness is deemed verified where the device disconnects with an alternating current test at the rated residual operating current (IΔn) within 300 ms maximum for a general non-delay type. The former time/current table (Table 3A) was deleted at A4:2026 — there is no ½× or 5× requirement in BS 7671. Record the trip time at IΔn for each device.
Check every assembly, transformer, socket-outlet, cable route and earth connection for damage from construction activity, water ingress, corrosion and unauthorised modification. Site installations change constantly — new circuits, repositioned boards, re-routed cables — and every change needs verifying.
The EICR for a site temporary installation goes to the principal contractor, who keeps it in the project health and safety file. Say plainly on the certificate that the installation is temporary, and set out the scope of what was inspected.
08 · Construction Guide
Site installations take constant abuse, modification and weather. These are the defects that turn up on quarterly inspections again and again.
| Defect | What you find |
|---|---|
| Damaged cables | Crushed by vehicles, cut by excavation machinery, trapped under scaffold boards or chafed on sharp edges. Insulation-tape "repairs" are common and unacceptable — damaged cable gets replaced. |
| Water ingress in assemblies | Open knockouts, cracked enclosures, missing covers and boards stood in flood-risk positions. Water inside a live assembly normally warrants C1, danger present. |
| RCDs bypassed or not operating | Devices taped in the on position to stop tripping, devices that fail to disconnect within 300 ms at IΔn, or devices removed from the board altogether. Normally C1. |
| 230V portable tools in use | Drills, grinders and saws at 230V instead of the 110V equivalent, and domestic white 13 A extension leads brought onto site. |
| Unauthorised modifications | Circuits added without testing or certification, joints made without proper connectors, and assemblies loaded beyond their rating. |
Document each defect with a photograph, a clear description and the correct observation code, plus — for anything coded C1 — a record of the immediate action taken to make the installation safe. The EICR goes to the principal contractor, who is responsible for the remedial work.
09 · Construction Guide
Site work rewards efficiency and documentation. Quarterly EICRs, monthly visual checks and CDM records have to be produced consistently and handed over promptly. The electrician who finishes the inspection, produces the certificate and gives it to the site manager on the day of testing keeps the contract.
Elec-Mate is built for exactly that workflow:
Probes in hand, hard hat on, stood at a board on the third floor of an unfinished building? Speak the results: "Socket circuit 3, Zs 0.28, insulation resistance 200 megohms, RCD trip time 22 milliseconds." Elec-Mate fills the schedule in while you work. No clipboard in the rain.
Found a C1? Document it, classify it, and generate a remedial works quote — materials, labour, margin — from the same app. Hand over the EICR and the quote in one visit.
Use the RAMS generator to produce risk assessments and method statements for site electrical work, and keep the CDM documentation trail complete without the paperwork evening.
Construction work is steady, well paid, and often leads to the permanent installation contract once the building is finished. Professional documentation, on time, every quarter, is what wins that.
In the app
Join 1,000+ UK electricians creating professional certificates with voice entry, AI defect coding, and instant PDF delivery.
Verified reviews from the UK App Store.
Elec-Mate is my go to app for business and electrical work. It's feature rich without feeling cluttered. A true all in one app for quotes, certs, calculations, RAMS, EICRs, and more. I use it every day without fail, and it makes my workflow much smoother since I'm not jumping between apps anymore. The price-to-feature ratio is excellent. Any issues I've had, the developer responds within the hour and usually fixes them the same day. 100% recommend.
I've used the app and the web based version for a while now and it's well worth the investment. If you're an apprentice or experienced Spark give it a go, you won't be disappointed.
I've been using Elec-Mate for a while now, and honestly, it's one of the best apps I've ever downloaded. Every aspect of it feels thoughtfully designed, from the clean and intuitive interface to the powerful features that make everything so easy to manage. It's clear that a lot of care and attention went into building this app, and it shows in every detail.
Real feedback from real sparks
“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”
Daniel Palmer
Sole Trader · DP Electrical
“I've won two contracts this month because I could turn quotes around same-day with the AI cost engineer.”
Nathan Perry
Electrician · NP Electrical Services
“The study centre got me through my AM2. Mock exams and flashcards are brilliant.”
Jake Pizey
3rd Year Apprentice · Apprentice
7-day free trial
Join 1,000+ UK electricians creating professional certificates with voice test entry, AI defect coding, and instant PDF delivery. Built for construction site conditions. 7-day free trial, cancel anytime.
Start your free trialFrom £6.99/mo after the trial — no charge until day 8.
“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”
Or download the app
We use cookies to improve the app and measure what works. Cookie Policy