Now we are 17
Published:  16 June, 2008

One of the biggest changes to the IEE Wiring Regulations is the requirement for residual-current protection. Colin McAhren explains the reasons behind it and how to meet it.

The 17th Edition of the IEE Wiring Regulations was published at the beginning of 2008 and has now come into full effect.

One of the biggest changes in the latest edition of the Wiring Regulations is a greatly increased requirement for circuits to be provided with residual-current protection which, in practice, means using either RCDs (residual-current devices) or RCBOs (residual-current circuit breaker with over-current protection).

RCDs are now required for all general-use socket outlets rated up to 20 A. Two exceptions are allowed. One is a socket outlet used under the supervision of skilled or instructed persons — which could exempt commercial premises such as offices. The other is a socket outlet identified for connection to a specific piece of equipment; one suggestion is a freezer to protect it against nuisance tripping.

All circuits — including lighting — in a room that contains a bath or shower must have residual-current protection, as must almost all circuits supplying socket outlets. Only a few socket-outlet circuits, such as those used exclusively for the supply of a domestic freezer, are potentially exempt from this requirement — and then only if they are pre-connected to an outlet and the cable is more than 50 mm below the surface of the wall. More than one condition needs to be satisfied to meet the 17th Edition.

Is this change a good thing? It would be very hard to argue otherwise. The prices of RCDs and RCBOs are now lower than they used to be. They are just as easy to fit and use as an MCB, so even if only a few lives are saved, fitting the extra protection required by the new regulations is definitely a worthwhile step.

A possible downside in that the wider use of RCDs may result in more nuisance tripping. To a certain extent, contractors can help to minimise this problem by, for example, taking steps to limit the risk of insects and other foreign matter getting into light fittings, socket outlets and the like, but this can never be a complete solution. The problem is particularly acute when an RCD on the incoming supply is used to protect a group of circuits, which is very common practice.

Potentially, this could mean that a transient earth fault on the lighting circuit — by far the most common cause of RCD tripping — could lead to, for example, a freezer being disconnected. If this happens while there is no one at home, the results can be expensive and inconvenient.

Such problems are covered in the 17th Edition, although little attention seems to have been given to the relevant section.

Nevertheless, the intention of Regulations 314.1 and 314.2 is quite clear, since they state, ‘Every installation shall be divided into circuits as necessary to avoid danger and inconvenience in the event of a fault, take account of danger that may arise from failure of a single circuit such as a lighting circuit, reduce the possibility of unwanted tripping of RCDs etc.’ and, ‘Separate circuits to be provided for parts of the installation that need to be separately controlled in such a way that those circuits are not affected by the failure of other circuits.’

What this means in practice is that it is up to individual specifiers and installers to interpret the regulations, It seems apparent that the best way to meet the requirements mentioned is not to use RCD incomers, but to use RCBOs on most, if not all, of the outgoing circuits.

There are other reasons to prefer this approach. It is often forgotten that electronic equipment such as computers, printers, televisions and hi-fi systems almost always incorporate a mains filter that produces an earth-leakage current as part of normal operation. This current can amount to as much as 3 to 4 mA per device.

If several such devices are fed from a consumer unit with only incoming RCD protection, the cumulative leakage currents can become very close to the tripping current of the RCD. In this situation, regular nuisance tripping is virtually inevitable. Feeding individual circuits with separate RCBOs is, once again, the answer.

To satisfy the new regulations, contractors will have to make changes to their current practices. One of the most significant is likely to be the move away from split-load consumer units, and the adoption, hopefully, of RCBOs rather than MCBs as the most common form of protection device.

This change in emphasis on protection products has generated concern that the introduction of the new regulations will lead to a shortage of RCDs and RCBOs, which will, in turn, provide manufacturers with an opportunity to increase their prices. These concerns are, however, unfounded — certainly in the case of Moeller Electric, which will continue to offer both RCDs and RCBOs at very affordable prices.

The introduction of the 17th Edition of the IEE Wiring Regulations should not have come as a surprise. They have been under discussion for years, and the content has been clear in outline for well over a year. Manufacturers of protective devices have, therefore, had plenty of time to prepare themselves for the changes and to ensure that they have adequate stock of, and manufacturing capacity, for RCDs and RCBOs.

With no product shortages expected, increased demand is in the long run likely to ensure that prices at least remain steady and quite possibly fall.

There is, however, a need to sound a note of caution. The rapid growth in demand for residual-current protection devices will be seen as an opportunity for counterfeiters and those who import cheap devices of uncertain performance and reliability. It is worth bearing in mind that a contractor who knowingly uses such a device may well be prosecuted if its use results in accident or injury.

The way to avoid this problem is, of course, to buy only devices with well-known and respected brands — and only to source them from an established supplier. The old maxim ‘if it looks too good to be true, it probably is’ most certainly applies in this case. Never forget that residual-current devices are not fitted just to satisfy the regulations but to protect lives. Penny pinching just isn’t acceptable!

The changes introduced by the 17th Edition of the Wiring Regulations are progressive and, without doubt, they are to be welcomed by electrical contractors and property owners alike. Once the inevitable transition period is over, they are unlikely to cause problems with contractors and may even create opportunities for updating existing installations.

It is important, however, for contractors to bear one important point in mind — safe, dependable installations will always depend on using reliable, properly tested products from an established supplier.

Colin McAhren is with Moeller Electric.



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.

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

Construction Week



  • 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 »


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