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Divide & Conquer the Requirements of the 17th Edition
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Chris Thomas – CEO of Electrium and former BEAMA President explains why RCBOs offer the best option for compliance with the 17th Edition IEE Wiring Regulations.
The much anticipated 17th Edition of the IEE Wiring Regulations was published in January. At first glance the implications seem most onerous upon domestic installations where, in the majority of cases, the entire installation will require additional protection by 30 milliamp RCDs.
Presently the majority of domestic properties would have a part of the installation protected by a 30mA RCD which would be contained within a split load consumer unit. The RCD would most likely be protecting circuits such as socket outlets, electric shower and power in the garage or outside. With this type of arrangement when a fault causes the RCD to trip all of the lighting remains on. The benefits of this good practice are already obvious. Even so under the 17th Edition the effects of a single fault – particularly upon a lighting circuit – are now prescribed as a standard design consideration in Chapter 31 (Division of the installation).
However, if under the new regulations all circuits require 30mA RCD protection there could be a situation whereby power to some lighting circuits could be lost even though the lighting circuits are without fault. These circuits should be arranged in such a way as to avoid such risks.
Electrical designers will have to carry out a risk analysis to assess just how many circuits need to be independent and how many can utilise shared RCD devices before specifying a consumer unit. For example, it may be an inconvenience to lose all socket outlets at once but this is probably unlikely to cause another hazard, as would be the case if lighting circuits were affected.
There are other considerations such as smoke alarm circuits. BS5839 part 6 (Code of Practice for design installation and maintenance of fire detection and alarm systems in dwellings) recommends independent circuits for certain grades of mains powered smoke alarms systems, and further recommends that if RCD protection is required then the RCD should serve only that circuit.
The 17th Edition requires every installation to be divided up into as many circuits as necessary to avoid hazards and minimise inconvenience in the event of a fault, and take account of the effect of a single fault, particularly on a lighting circuit. Designers should also reduce the possibilities of unwanted tripping of RCDs and facilitate safe inspection and maintenance as well as providing separate controls for circuits requiring independent controls.
Independently protected circuits throughout is the safest option, alternatively four or five RCBOs in a Split Load Unit would provide independence to essential circuits such as lighting and smoke alarms. Using RCBOs, is by far the best way to divide and conquer the RCD requirements.
