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  Simon Clarke, Training Manager at MK Electric, provides his take on the subject, and considers how the role - and usage - of the RCD will change under the new edition of the IEE Wiring Regulations:

MK Electric has produced a video explaining and illustrating the requirements contained in this article; along with information on its 17th Edition Product Solutions concerning consumer unit configurations. The high-quality, informative video is available for viewing, at www.mk.learninglounge.com/resources/sott17esv.html, for other iniatives and solutions please visit www.switchonmk.com

RCDs have been installed in electrical installations for many years, but have become more commonplace since the 14th Edition of the IEE Wiring Regulations, and in its subsequent updates through the 15th and 16th Editions, given the British Standard reference in 1991 of BS 7671 - Requirements for Electrical Installations.

But what is the role of the RCD? Although coming in many guises, put simply its job is to automatically disconnect the electrical supply in the event of fault conditions for protection against fire and harmful thermal effects within installations - such as agricultural premises - but more commonly against Indirect Contact, and for supplementary protection against Direct Contact.

These two definitions have changed with the publication of the 17th Edition in January. Direct Contact will be known as 'Basic Protection' - protection from electric shock under fault free conditions - while Indirect Contact will be referred to as 'Fault Protection' - protection against electric shock under single fault conditions. However it is not only the terminology that will change with the advent of the new edition, as the use of RCDs is set to increase dramatically.

As a necessary safety device, the installation of RCDs has long been required in instances where increased personal protection is required. In domestic situations, this has been to protect socket outlets and other electrical equipment that is intended to be used for outside the home, as well as 'Special Locations': those areas where, for various reasons, increased risk of electric shock is present. These locations include, for example, swimming pools, saunas, construction sites, horticultural premises and caravan parks. These are currently found in Part 6 of the 16th Edition, but will now be found in Part 7, with the new publication being brought into line with the numbering of IEC publications. Part 7 will also include new areas such as marinas, exhibition stands, fairgrounds and floor and ceiling heating systems, where the use of RCDs will be required.

A major change:

And as mentioned above, so as to increase safety, the usage of RCDs is set to increase hugely. Three areas of the Regulations are set to affect them in a major way.

First: Chapter 41 Protection Against Electric Shock. This has requirements for the additional protection of AC systems. Socket outlets with a rated current not exceeding 20A for use for general purposes by ordinary persons, and mobile equipment with a current rating not exceeding 32A for use outdoors are, to be protected by an RCD complying with Regulation 415.1.1, that is a device with a rated residual operating current not exceeding 30mA and an operating time not exceeding 40ms at five times the rated operating current. Exceptions are made for socket outlets under the supervision of skilled or instructed persons, or a specifically labelled or identified socket outlet intended for the connection of a particular item of equipment.

Second: Section 701 Locations Containing a Bath or Shower. This has a requirement for additional protection of all circuits within these areas by one or more RCDs with the previously mentioned characteristics.

But third, probably the largest single impact can be found within Chapter 52 Selection and Erection of Wiring Systems. This includes a number of new and some modified regulations, one of the changes that will have the largest effect can be found in:

522.6.6 - Cables Installed in a Wall or Partition. This says that: 'A cable installed in a wall or partition at a depth of less than 50mm from the surface of the wall or partition shall':

Incorporate an earthed metallic covering which complies with the requirements for a protective conductor of the circuit concerned, the cable complying with BS 5467, BS 6346, BS 6724, BS 7846, BS EN 60702-1 or BS 8436, or;

• be enclosed in earthed conduit, or;
• be enclosed in earthed trunking or ducting, or;
• be mechanically protected against damage, sufficient to prevent penetration of the cable by nails, screws and the like, or;
• be installed in the traditional 'safe zones' - 150mm from the top of the wall or partition or within 150mm of an angle formed by two adjoining walls/partitions, and horizontally or vertically to any point, accessory and switchgear.

A new regulation has been included - 522.6.7 - which covers all circuits. It says: 'Where 522.6.6 applies and the installation IS NOT INTENDED TO BE UNDER THE SUPERVISION OF A SKILLED OR INSTRUCTED PERSON a cable installed in accordance with 522.6.6 (v) (safe zones) and not complying with 522.6.6 (i), (ii), (iii), (iv) (mechanically protected) SHALL BE PROVIDED WITH ADDITIONAL PROTECTION BY MEANS OF AN RCD having the characteristics specified in Regulation 415.1.1' - again by means of a 30mA device.

Domestic installations, and some commercial, will not be under the control of a skilled or instructed person. Final circuit cables would generally be run in the 'safe zones' at a depth less than 50mm; but are unlikely to be installed with one of cable types above or the mechanically protected options given, leaving all final circuits to be protected by a 30mA rated RCD.

However, we must remember the time honoured provision of dividing an installation into circuits to avoid hazards and minimize inconvenience still applies as found in 314.1 (i). Other requirements within this section that will affect the configuration of the circuit protective measures are:
(iii) 'take account of danger that may arise from the failure of a single circuit such as a lighting circuit', and;
(iv) 'reduce the possibility of unwanted tripping of an RCD by circuit protective conductor currents produced by equipment in normal use'.

In essence, consumer unit configuration will change from what is currently the norm. This may be by increased use of RCBOs to provide both residual and over-current protection to individual circuits - a single fault only affecting one final circuit, or by multiple RCDs connected in parallel protecting a smaller number of outgoing final circuits, balancing circuits across these devices such as an upstairs lighting circuit being protected by a different RCD to the downstairs circuit.