17th EDITION REGULATIONS
The new national standard to which all domestic and industrial wiring must conform, BS: 7671:2008 Requirements for Electrical Installation (IEE Wiring Regulations 17th Edition) came into force on 1st July and all designs now have to be to the new standard, and installed and certified as such. An important new regulation that particularly affects domestic and simillar installations, requires that all cables buried beneath the plaster surface of a wall or partition (at less than 50mm and without earthed mechanical protection) and all cables concealed in metal stud partitions (common in new builds) should be protected by 30mA RCDs. Key changes also include: * The maximum permitted volt drop where supplied directly from a public distribution system is now 3% for lighting and 5% for all other applications. * All socket outlets up to 20A rating and intended for general use by ordinary persons should be protected by 30mA RCDs - whether on the ground floor of a house or the top floor of a high-rise apartment block. * External sockets rated not exceeding 32A must also have 30mA RCD protection. *A socket outlet is permitted in bathrooms (if 3meters from the edge of a bath or shower and rcd protected) All circuits in a room with a fixed bath or shower should be protected by one or more 30mA RCDs. * The maximum permitted earth loop impedance - Zs values have been adjusted, based on 230V nominal and this has slightly reduced these, for example for 32amp type ‘B’ MCB protection the maximum permitted is 1.44 ohm and was previously 1.5 ohm. Chapter 41 of the 17th edition - Protection against electric shock - contains many regulations that have been reworded but the new requiements are not significantly changed. The terms ‘Direct Contact’ and ‘Indirect Contact’ are no longer used. They are replaced with Basic Protection and Fault Protection. ‘Basic Protection’ is protection against touching live parts. ‘Fault Protection’ is protection against receiving a shock from conductive parts that have become live due to a breakdown of insulation or damage to equipment.
To enable the industry to meet the new requirements of the 17th Edition, the Electrical Contractors’ Association has published a comprehensive book for electrical contractors, apprentices, designers and installers. Simply laid out using a series of clear tables and diagrams, ‘Guide to the wiring Regulations BS7671: 2008′ addresses all areas of the new requirements in straightforward terms. To read in conjunction with the Regulations the ECA is offering the guide and a complete copy of of BS7671: 2008 to all existing and new members free of charge, see the website www.eca.co.uk
CONSUMER UNITS Because the RCD is sensitive to current imbalances, it is not practical to fit just one device to protect an entire house. If a fault develops within one circuit, all circuits would be switched off immediately. By using individual and grouped RCD protected ways in combination with one another, the required protection can be provided whilst maintaining continuity of supply to other non-affected zones within the installation.
There are four types of Consumer Units that may be used. 1: MAIN SWITCH CONTROLLED UNIT. Utilising RCBOs throughout would provide a secure dedicated 30mA protected supply to every circuit. 2: SPLIT LOAD UNIT. With one RCD protected section using MCBs, and the main switch side using RCBOs. 3: DUAL RCD CONSUMER UNIT. A main switch feeding two RCD protected sections using MCBs throughout. 4: HIGH INTEGRITY DUAL RCD CONSUMER UNIT. A main switch feeding two RCD protected sections for MCBs, and also feeding a number of dedicated ways (High Integrity Circuits) for the use of 30mA RCBOs.
Some circuits may require independence from a shared RCD but their installation method may still dictate 30mA RCD protection. Examples may be smoke alarms, burgular alarms, door entry systems.
CONSIDERATION. Consideration for appropriate circuit division has never been greater. Specific reference is made within the new standard to taking account of and reducing possibilities of danger and inconvenience arising from the failure of a single circuit such as a lighting circuit. For example; the result of an RCD operating due to a fault at the shower should not create a low light hazard in the bathroom.
