Requirements for the use of reduced low voltage (RLV) systems on construction sites | NICEIC

Requirements for the use of reduced low voltage (RLV) systems on construction sites | NICEIC

BS 7671 permits the use of a range of supplies to equipment on construction sites. This article from the experts at NICEIC summarises the requirements in Section 704 for the use of reduced low voltage (RLV) supplies in such locations.

When considering the use of RLV, it is important to meet the requirements of both Section 411.8 of BS 7671 and also those of BS 4363: 1998 (2013) – Specification for distribution assemblies for reduced low voltage electricity supplies for construction and building sites.

Where the use of extra-low voltage is impracticable for functional reasons and/or there is no requirement for the use of SELV or PELV, an RLV system may be used (411.8.1.1).

Such types of system have been used on construction sites for many years as a means of reducing the risk and severity of injury from electricity whilst providing an adequate supply voltage for maintaining correct functionality of site equipment.

For this reason, Note 1 of Regulation 704.410.3.10 provides a strong preference for the use of RLV for the supply to portable hand lamps for general use, portable hand tools and local lighting up to 2 kW.

RLV is a supply in which the nominal circuit voltage does not exceed 110 V AC between lines. For three-phase circuits, the voltage should not exceed 63.5 V between a line and earthed neutral and, for single-phase circuits, 55 V between a line and earthed midpoint (411.8.1.2). This reduced voltage level provides the primary means of protection against electric shock in the RLV system. As such, historically, a disconnection time was not specified.

Permitted sources of supply

The supply to an RLV circuit should be derived from one of the following sources:

– A double-wound isolating transformer meeting the requirements of BS EN 61558-1 and BS EN 61558-2-23

– A motor-generator in which the windings provide a degree of isolation equivalent to that provided by the windings of an isolating transformer

– A source independent of other supplies, such as an engine-driven generator (411.8.4.1).

The secondary windings of a transformer or generator providing the supply to an RLV system must be connected to Earth. As shown in Fig 1, for three-phase supplies, the connection is made at the neutral (star) point and for single-phase supplies the connection is made at the midpoint (411.8.4.2).

Fig 1. Three-phase and Single-phase RLV supplies

Basic protection

Basic protection must be provided by either basic insulation suitable for the maximum nominal voltage of the RLV system or by the use of barriers or enclosures (411.8.2).

Fault protection

All exposed-conductive-parts of the RLV system must be connected to Earth. For automatic disconnection of supply during a fault to earth, each line conductor should be protected by either an overcurrent protective device or an RCD.

Additionally, Clause 5.5 of BS 4363 requires any circuit-breakers used on output circuits shall disconnect all circuit conductors simultaneously. Regulation 411.8.3 of BS 7671 calls for a maximum disconnection time of 5 seconds for an RLV system.

The maximum value of earth fault loop impedance to achieve this disconnection time can be found by the application of one of the following formulae derived from the expression given in Regulation 411.4.4:

For fuses or circuit-breakers:


U0 is the nominal AC rms voltage to Earth, 55 V for 1-ph RLV or 63.5 V for 3-ph RLV (411.8.3)

Ia is the current in amperes (A) causing the automatic operation of the disconnecting device within 5s

Cmin is the minimum voltage factor, taking account of voltage variations depending on time and place, changing of transformer taps and other considerations (typically taken as 0.95)

Alternatively, Table 41.6 of BS 7671 provides maximum earth fault loop impedance values for circuit-breakers to BS EN 60898, RCBOs to BS EN 61009-1 and for general purpose fuses to BS 88-2 fuse systems E and G.

It should be noted at this point that Clause 7.1 of BS 4363 recommends for a single-phase transformer assembly (TA/1) that:

– any single-phase output circuits having a rating up to 32 A supplying socket-outlets, and

– any circuits in excess of 32 A, whether supplying circuits directly or via socket-outlets, must be protected by double-pole circuit-breakers.

Similarly, in a three-phase transformer assembly (TA/3), any three-phase circuit having a rating up to 32 A supplying socket-outlets should be protected by triple-pole circuit-breakers (Clause 7.3). This precludes the use of fuses at the origin of circuits in such site assemblies.

Clause 7 also states that any socket-outlets in excess of 32 A rating must be mechanically or electrically interlocked to prevent the on-load removal of the plug.

Where fault protection is provided by an RCD, the following condition must be met:

IΔn × Zs ≤ 50 V


(IΔn) is the rated residual operating current of the RCD providing fault protection.

However, where an RCD is relied upon for fault protection for example, where the earth fault loop impedance value is high, an appropriate overcurrent protective device must still be provided in each line conductor in accordance with Chapter 43 (411.4.5; 411.5.2).

Particular requirements where the installation forms part of a TT system

BS 7671 permits the use of a Class I enclosure to house an RCD at the origin of an installation forming part of a TT system subject to the use of live conductors having double or reinforced insulation on the supply side of the incoming device (531.

Similarly, Clause of BS 73751 states that a metallic enclosure may be used if insulating glands, gland plates or other measures, providing the equivalent Class II insulation between the incoming supply conductors and equipment metalwork are taken to reduce the possibility of earth faults.

However, this clause also states that, wherever practicable, an insulating enclosure should be used to avoid the risk of earth faults between incoming cables and metalwork on the supply side of the principal protective device(s).

It should also be recognised that any specific requirements imposed by a Distribution Network Operator (DNO) as conditional to the provision of a supply on a construction site must also be met. For example, Clause of Engineering Recommendation G12, issue 42 does not permit any exposed-conductive-parts before, or enclosing, an RCD. As a result, an RCD at the origin would have to be housed in an enclosure of Class II or equivalent all-insulated construction.


Every plug, socket-outlet, luminaire supporting coupler, device for connecting a luminaire and cable coupler used in RLV systems should have a protective conductor contact that is not dimensionally compatible with those used for any other system in use on the particular construction or demolition site (411.8.5).

All plugs and socket-outlets including, cable couplers and cable connectors – of current rating 16 A up to 125 A should meet the requirements of BS EN 60309-23 (704.511.1). Unless otherwise specified in that standard, the requirements of BS EN 60309-14 are also applicable. Accessories to BS EN 60309-2 are classified as non-interchangeable with respect to their current and voltage ratings.

Clause 7.7 of BS EN 60309-1 states that the operating voltage of accessories may be indicated by colour. Accessories suitable for operation at 100 – 130 V, should be identified by the colour yellow. While there is no requirement to do so, this colour coding system is often also applied to the outer sheath of flexible cables.


The reduced low voltage (RLV) system provides both a safe system for use in the harsh environment typically associated with construction and demolition sites whilst providing a source of supply that is sufficient for the majority of site electrical equipment to function properly.

When an RLV system is employed it is important that all relevant requirements relating to the installation of such systems are met. This will require reference to a number of British Standards including in particular BS 7671 and BS 4363.

Key references:

1 BS 7375: 2010 Distribution of electricity on construction and demolition sites – Code of practice

2 Engineering Recommendation G12 Issue 4 – Requirements for the Application of Protective Multiple Earthing to Low Voltage Networks

3 BS EN 60309-2:2022 Plugs, fixed or portable socket-outlets and appliance inlets for industrial purposes. Dimensional interchangeability requirements for pin and contact-tube accessories

4 BS EN 60309-1:2022 Plugs, socket-outlets and couplers for industrial purposes. General requirements

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