Designing for the 17th Edition

Posted in Regulations

Bill Allan of N.A.P.I.T. looks, in this very useful and informative article, at designing electrical installations so that they meet the requirements of the 17th Edition:

After all the speculation and debate and rumour, it's here. BS 7671:2008 - the 17th Edition - has now been published and takes exclusive effect on July 1st (Amendment coming in July 2011). Whether we like them or not, whether we agree with them or not, these are the new rules and we have to comply with them. In this article, we'll jump right in with both feet and consider how to design a typical domestic premises to the 17th Edition.

Definitions:

Before we get into specifics, we'd better clarify a few definitions which are important in the design of electrical installations, including those in domestic premises. Part 2 of the Wiring Regulations contains definitions and the following, although they were all in the previous edition of BS 7671, are of particular importance in the 17th Edition and we'll be referring to them in this article:

Skilled person - 'A person with technical knowledge or sufficient experience to enable him/her to avoid dangers which electricity may create'.

Instructed person - 'A person adequately advised or supervised by skilled persons to enable him/her to avoid dangers which electricity may create'.

Ordinary person - 'A person who is neither a skilled person nor an instructed person'.

Socket-outlets circuits:

Now we'll consider the requirements for socket-outlet circuits. Regulation 411.3.3 concerns additional protection, and requires that all socket-outlets in domestic premises be protected by a 30mA RCD - not only those which may supply portable equipment for use outdoors. The wording is that socket-outlets with a rated current not exceeding 20A that are for use by 'ordinary persons' and are intended for general use are to be so protected. The definition of an 'ordinary person' is given above but, however you describe yourself, it includes you. It's intended to cover all who dwell in houses, flats, etc.

The note to Regulation 411.3.3 permits two exceptions:

  • Socket-outlets which are under the supervision of skilled or instructed persons, eg. in some commercial or industrial locations, and,
  • 'A specific labelled or otherwise suitably identified socket-outlet provided for connection to a particular item of equipment'.

The second exception above could be applied to a socket-outlet which supplies a freezer in a domestic kitchen. It implies that, as long as we have a sign indicating that the particular socket-outlet is for the freezer only, we don't need to have RCD protection for that socket-outlet. Obviously, we don't want the freezer going off because of a temperamental RCD while we're away on holiday. However, the idea that freezers are a frequent cause of RCD tripping is probably somewhat overstated.

Impact protection:

The other matter we must carefully consider at the outset are the requirements of Regulation 522.6.6 and Regulation 522.6.7 regarding impact protection of cables because these Regulations affect the design of the whole installation. Regulation 522.6.6 concerns the protection against impact of cables concealed in a wall or partition at a depth of less than 50mm from the surface of the wall or partition, which is likely to be the case in domestic premises. Regulation 522.6.6 lists measures that can be used to provide mechanical protection for such cables. These are as follows:

1) Use a cable which complies with the requirements of Regulation 522.6.6 (i) for impact protection.

2) Run the cable in earthed steel conduit or trunking or ducting.

3) Provide mechanical protection sufficient to prevent penetration of the cable by nails, screws and the like.

None of these wiring systems looks particularly appealing for domestic premises. Of course, we could always run the cable on the surface in PVC minitrunking. Come to think of it, that's not particularly appealing either. Will any electrician seriously consider installing cables other than 'PVC twin and earth' for domestic installation work? At the present time, I doubt it. While I wouldn't discount the possibility of some type of cable complying with item (1) of the above Regulation being used at some future date, possibly for switch drops, for the purpose of this article, I'll stick with the familiar PVC twin and CPC sheathed cable.

Regulation 522.6.7 requires that, where the installation is not intended to be under the supervision of a skilled or instructed person, and where measures to provide mechanical protection, such as those listed above, have not been applied, then cables must be both:

  • Run in 'safe zones' (ie. run horizontally or vertically to the outlet point), and,
  • Protected by a 30mA RCD.

The definitions of a 'skilled person' and an 'instructed person' are given above but Regulation 411.3.3 associates electrical installations that are under the supervision of skilled or instructed persons with commercial and industrial locations, not domestic premises.

(Regulation 522.6.8 requires that where a cable is run without adequate mechanical protection in a partition which is internally constructed of metallic parts, then it must be protected by a 30mA RCD, regardless of the depth from the surface.)

The consumer unit:

Having noted the above requirements, we can now discuss how to comply with the 17th Edition. We'll do this by considering three possible consumer unit arrangements which will comply with the new Regulations, although there are other arrangements that could also comply. The reason for demonstrating three arrangements is to show two methods of supplying the fire detection and alarm circuit in new or materially altered dwellings (Figs. 1 and 2), and to show an arrangement which may be suitable when carrying out a rewire in an existing dwelling (Fig.3). In cases of doubt, consult Table 1 in Part 6 of BS 5839:2004 - 'Code of practice for the design and installation of fire detection and alarm systems'.

Fig. 1 shows a consumer unit with two 30mA RCDs and a 30mA RCBO supplying 10 circuits. The fire alarm circuit is protected by the 30mA RCBO. The other four circuits on the left side are protected against overcurrent by four MCBs with additional protection being provided by means of a 30mA RCD. The remaining five circuits are protected against overcurrent by five MCBs and additional protection is provided by the other 30mA RCD. This type of arrangement will 'minimise inconvenience in the event of a fault' as required by Regulation 314.1(i). I've split the circuits as seems reasonable. For example, the downstairs lights are on the same RCD as the upstairs sockets - but not the downstairs sockets. The reason for this is that, if the RCD trips and the downstairs lights go off, the downstairs sockets are still live and you can plug a tablelamp into one of them.

We've mentioned the fire alarm circuit and we'll comment further on it later. For now, we'll discuss the other circuits as appropriate.

Lighting circuits - In the 16th Edition, lighting circuits would not use 30mA RCDs for additional protection because of the possibility of nuisance tripping, leading to the potentially dangerous scenario of people being left in the dark. In the 17th Edition, however, the requirement of Regulation 522.6.7 means that such additional RCD protection applies also to lighting circuits. To eliminate the risk of both lighting circuits going off simultaneously because of nuisance tripping, I've put each lighting circuit on a separate RCD.

The bathroom lighting circuit - Supplementary equipotential bonding may be omitted within a bathroom or shower room where the conditions of Regulation 701.415.2 are complied with. These conditions include the requirement that all final circuits within the location have additional protection by means of a 30mA RCD. The bathroom lighting circuit is supplied by means of a separate circuit, as shown in Fig. 1.

Socket-outlet circuits - This particular freezer is located in the kitchen. I don't fancy the methods of impact protection for my PVC/PVC twin and CPC cable listed in Regulation 522.6.6, nor do I want to run it on the surface, so I've supplied it from the kitchen socket- outlet circuit.

Fig. 2 is similar to Fig.1 but differs in that the fire alarm circuit is protected against overcurrent by an MCB and has additional protection via the adjacent 30mA RCD, which is shared with four other circuits. This may surprise some readers, so I would refer you to the accompanying box article entitled, 'Supplying the fire detection and alarm circuit' for the technical justification.

Fig. 3 again depicts a dual 30 mA RCD consumer unit, but this time, the fire detection and alarm system is not shown because it is not mains-operated. A Grade F system comprising battery-powered smoke alarms has been used. Where existing dwellings are rewired, it is acceptable in the majority of all single -family dwellings to have a minimum system of Grade F (again, Table 1 of BS 5836-6 must be consulted for the full details).

Of course, there are other ways of complying with the requirements of BS 7671:2008 than the methods I've shown in this article. Some will suggest dispensing with MCBs altogether and using only RCBOs. This is a much more expensive option.

 

Conclusion:

That's it for now. Means of avoiding the need for additional protection by means of a 30mA RCD are likely to emerge. Some will probably be quite ingenious. I expect we'll be discussing the implications of the 17th Edition for some time to come!


Supplying the fire detection and alarm circuit

In Fig. 2, the fire detection and alarm system, which is Grade D, is wired in PVC/PVC sheathed cable like all the other circuits. It is shown protected by a 6A MCB with the common 30mA RCD providing additional protection. Some may feel that it should be on an RCBO, so I will explain why I have opted for the design shown.

The legal requirement for fire alarm and detection systems in England and Wales is Part B of the Building Regulations 2000. For domestic premises, Part B essentially requires compliance with BS 5839-Part 6:2004 Code of practice for the design and installation of fire detection and alarm systems in dwellings.

Table 1 of BS 5839-6 requires that, for new or materially altered dwellings, the majority of all single-family dwellings (bungalows, flats, maisonettes, two and three-storey houses) must have a minimum system of Grade D (this Table should be consulted for the full requirements). A Grade D system is a system of one or more mains powered smoke alarms, where each alarm has an integral standby supply. Clause 15.5 of BS 5839-6 recommends that the mains supply to smoke alarms and heat alarms take the form of either:

  • An independent circuit at the dwelling's main distribution board, in which case no other electrical equipment should be connected to this circuit (other than a dedicated monitoring device installed to indicate failure of the mains supply to the smoke alarms and any heat alarms); or
  • A separately electrically protected, regularly used local lighting circuit.

When we compare the recommendations for power supplies for Grade D - as given above - with those of Grade E (see Clause 15.6), we can discern the probable intent of item i. A Grade E system is a system of one or more mains powered smoke alarms with no standby supply. This Clause specifically requires that the RCD protection of smoke alarm circuits in Grade E systems should operate independently of any RCD protection for circuits supplying socket-outlets or portable equipment. No such restriction is made for Grade D systems. We refer again to Fig. 2. Were the common 30mA RCD - which supplies the Grade D fire alarm system - to trip, the internal batteries would operate to ensure the proper functioning of the system. Furthermore, because other circuits would also lose their supply, warning is given that the RCD needs to be reset. This would be especially needful if you went away on holiday for a week or two. If an RCBO were to be used instead, no such warning would be given.

Consequently, the fire alarm circuit, as shown in Fig. 2, does not seem to infringe the likely intent of item i in Clause 15.5 of BS 5839-6, although it is emphasised that the code should be studied to ensure that all relevant recommendations are complied with.

Further information on the requirements of BS 5839-6:2004 for fire alarm and detection systems can be found in articles by Don Holmes in the Competent Person, Number 3, 2007 and Number 5, 2007.

Tags: Electrical Electrical safety

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