Temporary Installations: BS 7909, Events, and Construction Sites

What governs which job

BS 7671 Regulation 110.1.1 lists construction and demolition sites, exhibitions, shows and stands, and temporary fairground installations among the installations the Regulations apply to — so the Wiring Regulations are the framework in every row above.

Where BS 7909 is the working code

• Events and entertainment. Structures, sets and mobile units used for public or private events, touring shows, and theatrical, radio, TV or film productions — the exact wording BS 7671 uses when it excludes these systems from Sections 706 and 711.
• Temporary structures. Marquees, temporary stages, grandstands and hospitality builds erected for a limited period and needing a supply.
• Generator-supplied event systems. Where the source is a generating set rather than a DNO supply, BS 7671 Section 551 still governs the earthing and the protective arrangements — see Regulations 551.4.3.2.1 and 551.4.4.2.

Construction and demolition sites are the exception: they are covered directly by BS 7671 Section 704 and by BS 7375:2010, the Code of practice for distribution of electricity on construction and building sites cited in the note to Regulation 704.411.3.1. BS 7909 is not the governing code for that work.

Design decisions, in order

• Supply source. A temporary builder’s supply from the DNO, a generating set, or both with changeover. A single construction site may be served by several sources — public supply and generating set together — and equipment must be identified with, and compatible with, the particular supply energising it (Reg 704.313.3). Safety and standby supplies must be connected by devices arranged to prevent interconnection (Reg 704.537.2).
• Load assessment. Establish maximum demand with diversity applied, covering lighting, tools or stage equipment, heating, welfare and any specialist plant, then size the supply, main cables and protective devices to it with headroom for the site growing.
• Distribution layout. On a construction site, current-using equipment must be supplied via an Assembly for Construction Sites (ACS) incorporating overcurrent protective devices, devices affording fault protection and socket-outlets where required. Each ACS needs its own means of switching and isolating the incoming supply, suitable for securing in the off position — a padlockable device or a lockable enclosure (Reg 704.537.2). At exhibitions, every separate temporary structure and each distribution circuit supplying outdoor installations needs its own readily accessible, identifiable means of isolation (Reg 711.537.2.1.1).
• Cable selection. On construction sites, reduced low voltage systems use low temperature 3182/3/4/5A thermoplastic cable to BS 6004 or equivalent flexible cable; anything above reduced low voltage uses H07RN-F to BS EN 50525-2-21 or equivalent heavy duty flexible cable, and flexible cables subject to movement must be H07RN-F or equivalent, resistant to abrasion and water (Reg 704.522.8.11). Rate every cable for the current it will actually carry, allowing for ambient temperature and grouping.
• Cable routing. Cables should not be run across site roads or walkways; where that is unavoidable, provide adequate protection against mechanical damage and contact with construction plant. Surface-run and overhead cables need particular attention (Reg 704.522.8.101). At exhibitions, flexible cables must not be laid in areas accessible to the public unless protected against mechanical damage, and wiring cables must be copper of at least 1.5 mm² (Reg 711.52).

Equipment standards on construction sites

Regulation 704.511.1 names the standards outright — there is no room for interpretation here.

Minimum IP ratings by environment

BS 7671 does not publish one IP figure for temporary work — Regulation 704.512.2 requires external influences to be assessed instead. The figures below are the practical working minima. Select for the worst conditions the equipment will actually see.

Where BS 7671 does give a figure it is location-specific: at exhibitions, enclosures providing basic protection for SELV or PELV must be at least IP4X or IPXXD (Reg 711.414.4.5), and a floor-mounted socket-outlet must be adequately protected against accidental ingress of water and strong enough for the expected traffic load (Reg 711.55.7).

TT fault protection: the numbers

Regulation 411.5.3 sets two conditions. Disconnection must be within the time required by Regulation 411.3.2.2 or 411.3.2.4, and R_A × I_Δn must not exceed 50 V. The requirement is met if the earth fault loop impedance meets Table 41.5.

The 1667 Ω and 500 Ω figures carry a note in Table 41.5: the resistance of the installation earth electrode should be as low as practicable, and a value exceeding 200 Ω may not be stable (see Reg 542.2.4). That 200 Ω is a stability caution, not the compliance limit — the compliance limits are the ones tabulated above. On poor ground, drive more electrodes.

Maximum disconnection times

Table 41.1 applies to final circuits rated up to 63 A with one or more socket-outlets, and up to 32 A supplying only fixed connected equipment (Reg 411.3.2.2). Figures below are for AC at U₀ of 230 V.

Where disconnection on a TT system is achieved by an overcurrent protective device and the protective equipotential bonding is connected to all extraneous-conductive-parts in accordance with Regulation 411.3.1.2, the TN times may be used instead.

Earthing systems for temporary supplies

Where a generating set provides a switched alternative to the public supply, fault protection shall not rely on the connection to the earthed point of the public distribution system — a suitable means of earthing must be provided (Reg 551.4.3.2.1).

Bonding conductor sizes

• Main protective bonding, no PME (Reg 544.1.1). Not less than half the cross-sectional area required for the earthing conductor of the installation, at least 6 mm², and need not exceed 25 mm² in copper or equivalent conductance in another metal.
• Main protective bonding where PME conditions apply (Table 54.8). Selected against the PEN conductor of the supply: 10 mm² for a PEN of 35 mm² or less, 16 mm² over 35 up to 50 mm², 25 mm² over 50 up to 95 mm², 35 mm² over 95 up to 150 mm², and 50 mm² above 150 mm². It is not a flat 10 mm². Where there is more than one source to which PME conditions apply, size against the largest PEN conductor.
• Supplementary bonding (Reg 544.2). Between two exposed-conductive-parts, conductance not less than the smaller protective conductor (Reg 544.2.1). Between an exposed-conductive-part and an extraneous-conductive-part, not less than half the protective conductor connected to the exposed-conductive-part (Reg 544.2.2). Between two extraneous-conductive-parts, not less than 2.5 mm² (Reg 544.2.3). In every case, where mechanical protection is not provided the minimum is 4 mm².
• What to bond. Extraneous-conductive-parts — metallic structures, scaffolding, staging and trusses, metallic services — connected to the main earthing terminal. At exhibitions, structural metallic parts accessible from within the stand, vehicle, wagon, caravan or container shall be connected through the main protective bonding conductors to the main earthing terminal within the unit (Reg 711.411.3.1.2).

RCD requirements by situation

On construction sites the reduced low voltage option means a nominal voltage not exceeding 110 V AC RMS between lines — 63.5 V to earthed neutral three-phase, 55 V to earthed midpoint single-phase (Reg 411.8.1.2). Regulation 704.410.3.10 notes that reduced low voltage is strongly preferred for portable handlamps for general use, portable hand tools and local lighting up to 2 kW, and that SELV is strongly preferred for portable handlamps in confined or damp locations.

RCD type selection (Reg 531.3.3)

Choose the lowest type that fully covers the residual-current waveform the load can produce. Type AC shall only be used to serve fixed equipment where it is known that the load current contains no DC components.

All RCDs connected in series should be appropriate for the residual currents expected from the loads. Entertainment lighting and stage electronics routinely produce pulsating DC or composite residual currents that a Type AC device will not see — start at Type A and move up.

Overcurrent devices and selectivity

• Breaking capacity. Every circuit needs an appropriately rated MCB or RCBO providing both overload and fault current protection, with a breaking capacity at least equal to the prospective fault current at the point of installation — which is measured, calculated or otherwise determined at the origin and at other relevant points (Reg 643.7.3.201).
• Selectivity between RCDs (Reg 536.4.1.4). BS 7671 calls this selectivity, not discrimination, and it is not achieved by rating alone. Two conditions must both be met: the upstream RCD is of selective type — type S, or time-delayed with an appropriate delay setting — and the ratio of upstream to downstream rated residual operating current is at least 3:1. A 100 mA type S device upstream of 30 mA devices satisfies the ratio. At exhibitions, selectivity between RCDs installed in series shall be provided (Reg 711.536.4.1.4).
• Switchgear access. At exhibitions, shows and stands, switchgear and controlgear must be placed in closed cabinets that can only be opened with a key or a tool, except for parts designed and intended to be operated by ordinary persons (Reg 711.51).

The verification sequence

1. Visual inspection. Cables checked for damage; connections tight and correctly terminated; enclosure IP ratings appropriate to the assessed external influences; routing free of trip hazards, water exposure and plant damage; every circuit and board labelled.
2. Dead tests. Continuity of protective conductors including main and supplementary bonding; ring final circuit continuity where present; insulation resistance (Reg 643.3) — for circuits up to and including 500 V, 500 V DC test with a minimum of 1.0 MΩ per Table 64; polarity (Reg 643.6).
3. Insulation resistance where equipment is connected (Reg 643.3.3). Where connected equipment is likely to influence the result or be damaged, the Table 64 test is applied before that equipment is connected. After connection, a 250 V DC test is applied between live conductors and the protective conductor connected to the earthing arrangement, with a value of at least 1 MΩ. On temporary installations full of switch-mode drivers and control gear, this two-stage route is usually the practical one.
4. Live tests. Earth fault loop impedance to verify disconnection times; prospective short-circuit and earth fault current at the origin and other relevant points (Reg 643.7.3.201); phase sequence on polyphase circuits (Reg 643.9); RCD verification under Regulation 643.8 using equipment to BS EN 61557-6 — a single alternating current test at the rated residual operating current, I_Δn, whatever the device type, with 300 ms maximum for a general non-delay device. Amendment 4 deleted Table 3A of Appendix 3, so the ½× and 5× tests are no longer part of the required sequence.
5. Functional testing (Reg 643.10). Switchgear and controlgear assemblies, drives, controls and interlocks proved to operate correctly; main switches and RCD test buttons operated; generator changeover proved where fitted.

What has to exist before hand-over

• Single-line diagram. Supply source, main and sub-distribution, cable types and sizes, protective devices, earthing arrangement and loads. This is the reference for anyone who works on the installation after you.
• Electrical Installation Certificate. Issued by the persons responsible for design, construction and verification, to the person ordering the work (Reg 644.4), before the installation goes into service.
• Schedules (Reg 644.3). The Certificate must state the extent of the work and include the Schedule(s) of Inspection and the Schedule(s) of Circuit Details and Test Results, based on the models in Appendix 6.
• Recommended re-inspection interval. The interval between initial verification and the first periodic inspection must be recorded on the Certificate (Reg 644.4). For temporary work this is the field that carries your inspection regime — set it deliberately.
• Risk assessment and method statement. Covering installation, commissioning, operation and decommissioning, completed before work begins. Where the documented risk assessment exception to Regulation 411.3.3 is used, it must be provided with the certificate.
• Periodic records. An Electrical Installation Condition Report on each periodic inspection (Reg 653.1), retained with the project file.

Certificates and Minor Works Certificates may be produced in written or electronic form, provided their authenticity and integrity — and the fidelity of any copy — can be verified by a reliable process (Reg 644.4.202).