to it may not. For example, if the insulation resistance of an older appliance, such as a cooker, has over time significantly decreased, then it may cause unwanted tripping of an RCD/ RCBO when reconnected. Consequently, whilst the final circuit may be satisfactory to reconnect to the new unit the item of equipment is not. Where no electrical records are available for the installation, the client should be advised to have a periodic inspection carried out before proceeding with the consumer unit replacement.
Fig 2 Isolation and protection of the supply to a consumer unit
Remote consumer unit
SWA cable (distribution circuit)
Isolation and protection of the meter tails
MAIN SWITCH
Typically, the Distribution Network Operator’s (DNO’s) sealed cut-out fuse is used as the means of isolation for the meter tails in a domestic installation (Regulation 537.1.3 refers). Therefore, in advance of the work starting, arrangements will need to be made for the DNO to withdraw their fuse so work can commence, and to reinstate and reseal the fuse once the work is completed. In no circumstances should the contractor withdraw the DNO’s fuse unless prior authorisation has been obtained. Additionally, the client should also be made aware of the benefits of installing a double-pole isolator in the meter tails so that the new unit can, in the future, be conveniently and safely isolated without affecting the DNO’s fuse. Depending on the length of the meter tails, fault protection may also be required. Where the DNO’s cut-out fuse provides overcurrent protection for the existing meter tails, this is subject to the conditions imposed by the distributor. In particular, the length of meter tails is generally restricted to a maximum length of not more than 3 metres; where necessary confirmation should be obtained from the local DNO. Where the existing meter tails need to be extended or replaced with longer cables, due to the new unit being located in a different position, the DNO’s fuse should not be relied upon to provide overcurrent protection (Regulations 433.3.1 and 434.2.1 refer). In such circumstances, a distribution circuit should be installed as shown in Fig 2, and protection provided by an appropriately rated fuse or circuit-breaker or a device that combines the functions of isolation and protection, such as a fused isolator. However, the installation of an RCD to provide additional protection against shock should be avoided, as its operation is likely to cause disruption and inconvenience. Note: In some circumstances temporary supplies may be required to ensure the safety of persons within the premises or to minimise any power disruption for those who work from home. If the meter tails are concealed in a wall or
Electricity distributor’s cut-out fuse
Switch/fuse
Meter tails
partition then the requirements for protection against impact must be satisfied (Regulation 522.6.202 refers). However, an RCD used to provide additional protection against shock is required to have a residual operating current of not more than 30 mA, which means that discrimination cannot be achieved between this RCD and any downstream RCD housed within the consumer unit. Because of this, the cables used to supply the unit should be installed in such a manner that avoids the need for RCD protection. As shown in Fig 2, a cable that provides suitable protection, such as a steel-wire armoured cable, should be used. Note: Where the position of the unit is altered, the existing circuits may also require extending. Where this is the case, the relevant requirements of BS 7671 must be satisfied, in particular those of Section 526 relating to Electrical connections and Section 514 for Identification and notices.
Consumer unit The replacement unit must satisfy the requirements of Regulation 421.1.201. In addition to complying with BS EN 61439-3, as amended, the new unit and any associated switchgear should be either made from a non-combustible material, such as steel, or be enclosed within a non-combustible cabinet or enclosure. Where a metal-clad consumer unit is installed in an installation forming part of a TT system, an earth fault occurring between the incoming line conductor of the meter tails and the metallic enclosure, as shown in Fig 3, is unlikely to disconnect the fuse protecting the tails, 43 AU T U MN 2 017
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