Consumer units and the 17th Edition of the Wiring Regulations

Published:  18 May, 2009

CM
Consumer units that comply with the 17th Edition — Colin McAhren.

Are you looking for a 17th Edition consumer unit? If so, you probably need to know that there’s no such thing, says Colin McAhren of Moeller Electric. However, he explains how to match the configuration of the consumer unit to the wiring scheme so that the complete installation conforms to the 17th Edition.

Wouldn’t it be nice to buy a consumer unit off the shelf that was guaranteed to meet the requirements of the 17th Edition of the IEE Wiring Regulations? Unfortunately, despite what some suppliers are saying, you can’t. The reason is very simple; the consumer unit has to be arranged to suit the installation’s wiring. There is no single arrangement that can guarantee compliance.

Let us take a closer look. The 17th Edition identifies five important conditions that must be satisfied in standard applications.
1. Socket outlets for general use must be protected by residual-current devices (RCDs) (Reg. 411.3.3).
2. All circuits in locations containing a bath or shower must be RCD protected (Reg. 701.411.3.3).
3. Unprotected cables buried less than 50 mm deep in a wall must be RCD protected (Regs. 522.6.6-8).
4. To prevent nuisance tripping, unnecessary hazards and to minimise inconvenience, circuits should not be connected to a single upstream RCD (Reg. 314.1).
5. Separate circuits must not be affected by the failure of other circuits (Reg. 314.2).

Although these conditions may at first seem very precise, the regulations are in practice open to a degree of interpretation, particularly regarding the division of installations, where acceptable levels of inconvenience resulting from a fault can be somewhat subjective.

Because of this, the remainder of this article discusses consumer-unit configurations that, depending on the installation design, will offer a fully compliant assembly (FCA), or a partially compliant assembly (PCA). It also discusses possible pitfalls that make an installation potentially dangerous and, therefore, result in a non-compliant assembly (NCA).

Let us consider some practical examples.

A dual RCD board (Fig. 1) is often promoted as a fully compliant 17th Edition solution. In reality, however, it can only be a PCA at best. This arrangement satisfies conditions 1, 2 and 3 in the list above, but it is most unlikely to satisfy conditions 4 and 5. This is because, in the event of a fault on either set of MCBs, the associated RCD may also trip. This creates an unwanted disconnection of the MCBs where no fault exists, which is contrary to conditions 4 and 5.

In addition, the smoke alarms are protected by the same RCD as lighting circuits, and an incandescent bulb failing almost always trips the RCD. Under these circumstances, with the arrangement shown, the smoke-alarm circuit will be disabled, potentially putting the occupants of the house at risk. This arrangement must, therefore, be considered as an NCA; it is dangerous and unsatisfactory.

Now consider the consumer unit arrangement in Fig. 2, a split-load board with independent RCBOs (residual current circuit breaker with overcurrent protection). Once again, this satisfies conditions 1 to 3 above, but it does not fully satisfy conditions 4 and 5 as, for example, a fault on the shower may result in the disconnection of all socket outlets. Nevertheless, none of the fault conditions with this arrangement result in specific hazards, so it can be considered as a PCA.

There is one other point that requires careful attention. As shown, all of the socket outlets are fed from one RCD. Because of their in-built mains filters, many electronic devices such as televisions and computers have significant earth leakage even when turned off. If too many devices of this type are plugged into the socket circuits, nuisance tripping of the RCD is very likely. It would probably be a good idea, therefore, to separate the upstairs and downstairs socket circuits in a house.

Fig. 3, with separate RCBOs for all circuits, is an example of an FCA, which unconditionally meets all the conditions listed earlier. Where possible, the use of this arrangement is strongly recommended as it not only satisfies the regulations but also provides the best possible protection for life and equipment.

composite
FROM TOP:

Fig. 1: Although often presented as a fully compliant 17th Edition solution, this assembly can only be partially compliant at best.

Fig. 2: Although this split-load board has independent RCBOs, it can still only be considered as a partially compliant assembly to the 17th Edition.

Fig. 3: The use of separate RCBOs for all circuits makes this arrangement fully compliant with the 17th Edition.

While Fig. 3 provides the best possible consumer-unit arrangement for meeting the requirements of the 17th Edition, pressure to keep installation costs to a minimum may mean that it is not possible to use an RCBO on every circuit. In these cases, looking at the wiring scheme carefully may allow economies to be made.

If, for example, cable runs for burglar and smoke alarms are run in metal trunking rather than buried in the walls, they do not require RCD protection, and can be fed from ordinary MCBs. Likewise it may be possible to arrange for the feeds to the immersion heater and electric cooker to be surface mounted in conduit or buried deeper than 50 mm within the wall, once again eliminating the need for RCD protection.

In these cases, the consumer unit can be considered as an FCA, provided that RCBO protection is provided for each of the remaining circuits. This gives a good balance between safety, convenience and cost.

It is also possible to configure a consumer unit with even fewer RCBOs if it is accepted that faults on certain circuits will affect others. For example, if one RCD feeds the shower and sockets throughout the house, it may be considered acceptable for a fault on the shower to interrupt the supply to the sockets. Installers should, however, be clear that an arrangement of this type does not satisfy the regulations in their current form and, for that reason, it cannot be recommended as best practice.

The 17th Edition of the Wiring Regulations is still quite new, and, no doubt, the details of its implementation will become clearer over time.

Hopefully this article will have provided a few useful pointers and, for those who require more information, Moeller Electric has produced a new free publication ‘Moeller consumer units and the 17th Edition Wiring Regulations, which discusses in more detail the examples used in the article.

Colin McAhren is with Moeller Electric.

 

 



comments powered by Disqus

Search

Welcome

Welcome to Modern Building Services Online, the web edition of Modern Building Services (MBS) journal and the UK's most popular Building Services engineering site. Modern Building Services covers the entire Building Services Engineering industry. This site contains archived content from the journal, plus web-specific content.

When you go to our digital edition, you can also access the archive of digital editions.
August 2017: DIGITAL EDITION
ARCHIVE OF DIGITAL EDITIONS

Modern Building services has a group
on Linkedin - join us!

Construction Week

Show

Jobs

  • Field Sales Executive

    Successful professional technical salesperson required to sell to consultants and contractors. Unique opportunity to sell ultra-low energy, patented technology, ventilation products from a UK manufacturer. ...

  • Property Surveyor

    Property Surveyor/ Building Service Surveyor needed to work within the Technical Services team for the management of water hygiene/ legionella risk within DCC's property portfolio. We are looking for an experienced and dynamic team member to join our T......

more jobs »

Poll

"Is the Building Services industry lagging behind in the implementation of BIM?"



Calendar