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The new 17th edition Wiring Regulations BS 7671 where published on the 1st January 2008 and came in to force at the end of June 2008, and call for much more extensive provision for RCD (residual circuit protection) of both domestic and non-domestic dwellings
The practicalities of meeting those requirements for protection of cables buried in plaster or enclosed within partition walls means that most circuits will now need 30mA RCD protection. It is also a requirement that all 13amp sockets for use by ‘ordinary persons’ should normally be protected by a 30mA RCD. In bathrooms for example RCD protection is now required for all circuits including lighting circuits.
There are a number of approaches to fuse board design to meet the new regulations, some of which utilise traditional RCD main switch consumer units and split load consumer units, while others rely on the new type of dual RCD protected consumer units. In general the use of RCDs and RCBOs is now greater and there is a trend towards larger consumer units with more ways available.
Outgoing Circuits
An RCD with a residual operating current not exceeding 30mA is the recognised device for providing additional protection in the event of:
1. Failure of the basic protection
2. Electrical fault
3. Carelessness by the user or operator
Such RCD’s should not be used as the sole means of protection and do not avoid the need to apply one or more of the protective measures detailed in the Regulations.
Under the new Regulations, an installation is required to incorporate one or more RCDs depending on the circumstances. These include:
All socket outlets not exceeding 20A but with certain exceptions. One such exception would be a circuit labelled or otherwise suitably identified, socket outlet for connection of a particular piece of equipment (e.g. Freezer).
Mobile equipment with a current rating not exceeding 32a for use outdoors.
Electrical circuits installed in special installations and locations as defined in Part 7 of the Regulations (e.g. Swimming pools and saunas).
All circuits, including shower and lighting circuits, in rooms with a fixed bath or shower (e.g. bathrooms and bedrooms with en-suite).
Protection of Cabling
In addition to the needs of the outgoing circuits described above, the requirements of the installed cabling must also be taken in to account.
Where a cable is concealed in a wall or partition at a depth of less than 50mm form the surface, if earthed mechanical protection is not included (e.g. metal trunking or conduit), it must be provided with additional protection by means of a 30mA RCD. This applies even when it is installed in a Safe Zone.
Whilst it may be desirable to have one or two circuits which do not have RCD protection, (e.g. Clearly labelled circuit ‘freezer’) the installation of the wiring may still dictate that the circuit must be protected by an RCD.
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, irrespective of the depth from the surface, unless provided with earthed mechanical protection so as to avoid damage to the cable during installation or subsequent alteration.
Solutions
The most comprehensive solution is to provide individual RCBO protection on each outgoing circuit. A typical solution would be a CED 12 way consumer unit, and individual rcbos for each circuit,each rcbo giving overload and short circuit protection to that circuit.
Another way is to use a split load type board with two rcds and a main switch and to split circuits across the rcds,this however will mean that in the event of one of the rcds tripping all circiuts on that rcd will become non operative until the rcd is reset. |
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