The 17th edition wiring regulations BS7671 must be applied to all new installations designed after 30th June 2008, and there are various options that must be considered in order to satisfy these requirements.Starting with the wiring scheme and the building design, consideration must be given to the purpose of the building, and to the persons who will be using the installation, as different regulations will be applicable, e.g. is the installation under the control of skilled or instructed persons, or ordinary persons, or perhaps the installed location contains a bath or shower. Answers to these points will start to focus in on the regulations that need to be followed.
Consideration must be given to the way cables will be fitted within the installation, either on the surface in trunking or conduit, or else concealed beneath the surface of the wall either buried within the plaster or inside the cavity of a hollow partition wall. The chosen installation methods and wiring accessories or loads being fed, will also dictate whether additional protection by means of 30m A RCD's is required on those circuits or not.
The wiring regulations are open to interpretation in some areas, where reference is made to "minimise inconvenience in the event of a fault", acceptable levels of inconvenience can be somewhat subjective.
In most circumstances the regulations now dictate that a consumer unit will be required to have two or more 30m ARCD's, enabling the load circuits to be spread over a number of RCD protected zones or outgoing ways.
The examples shown on the next page illustrate just some of the many options available, to a contractor, to achieve a 17th Edition compliant installation. Some options provide a much higher level of circuit integrity than others.The final choice of which may well be made based on this and the cost /ease of the installation.
The advent of the New 17th Edition of the Wiring regulations brings with it new challenges for installers and manufacturers alike, one such challenge involves the much greater use of RCD's within the electrical installation. The new regulations BS7671:2008 published in January 2008 came into force at the end of June 2008, when the previous 2001 version was withdrawn.
States that additional protection by means of a 30mA RCD is to be provided for all socket outlets with a rated current not exceeding 20A for use by ordinary persons. The only exceptions allowed are for socket outlets for use under the supervision of "skilled" or "instructed persons" e.g.some commercial / industrial locations, or a specific labelled socket provided for connection of a particular item of equipment, e.g. a freezer circuit.
In specific locations such as those containing a bath or shower there is a requirement now to provide RCD protection on all circuits, including the lighting and shower circuits.
Requires that every installation shall be divided into circuits as necessary to avoid danger and minimise inconvenience, in the event of a fault. Also reducing the possibility of unwanted RCD tripping, due to excessive protective conductor currents but not due to an Earth fault. Separate circuits may be required for parts of the installation, which need to be separately controlled in such a way that they are not affected by the failure of other circuits. The appropriate subdivision should take account of any danger arising from the failure of a single circuit eg.an RCD trip on a socket outlet causing the unwanted failure of alighting circuit and its associated hazards.
Now requires a much greater use of RCD's to protect the wiring concealed in walls or partitions even where installed in previously defined "Safe Zones". These regulations effectively mean that all concealed wiring at a depth of less than 50mm from the surface now requires protection by a30mA RCD unless provided with earthed mechanical protection.
The 17th Edition of the IEE wiring regulations (BS7671), detail a number of regulations relating to protection against electric shock, including the need for additional protection.
The use of RCD's (Residual Current Devices) with a residual operating current not exceeding 30mA is the recognised means of providing this additional protection in the event of failure of the provision for basic protection and or the provision for fault protection or carelessness by users.
Such RCD's should not be used to provide the sole means of protection and do not obviate the need to apply one or more of the recognised protective measure as detailed in the regulations.
Under the new regulations an installation is required to incorporate one or more RCD's, depending upon the circumstances.
In addition to the protection requirements of the outgoing circuits / loads, the requirements of the installed cabling also must be taken into account.
Where a cable is concealed in a wall or partition at a depth of less than 50mm from the surface, even if installed in the "safe zone", if not provided with earthed mechanical protection e.g. Metal trunking or conduit, it must be provided with additional protection by means of a 30mA RCD.
Whilst it may be desirable to have one or two circuits fed via an unprotected circuit e.g. an identified / dedicated freezer circuit, the installation of the wiring may still dictate that the circuit must be RCD protected. The protection of a circuit by means of a 30mA RCD is also required where cables are concealed in walls constructed with metal stud partitions which are common in modern buildings, irrespective of the depth from the surface, unless provided with protection in the form of earthed metallic covering, trunking, conduit or other mechanical protection so as to avoid damage to the cable during installation or construction of the wall.