http://www.electricalreview.co.uk/features/117916/17th_edition_-_Ringing_the_changes.html
The 17th Edition of IEE Wiring Regulations was published in January 2008. Installations designed after 30th June 2008 are required to comply with BS 7671: 2008. As with the introduction of earlier Editions, the 17th Edition takes account of the evolutionary progress of standards in the international and, more particularly, in the European arena where it is necessary to incorporate the changes agreed in CENELEC with our European contemporaries.
This is a summary of some of the changes which are likely to have the most impact.
Protection against electric shock
The terms direct contact and indirect contact, which contractors have become accustomed to over many years, have now been replaced by the concepts of basic protection and fault protection, respectively.
Chapter 41 Protection against electric shock has perhaps seen the most radical changes in terms of layout and now incorporates requirements which were previously included in Section 471.
The protective measures of automatic disconnection of supply, double or reinforced insulation, electrical separation and extra-low voltage provided by SELV or PELV are recognised in BS 7671: 2008 (Regulation 410.3.3 refers) with the measure of Automatic Disconnection of Supply being the measure that is employed in almost every electrical installation. The protective measure of Automatic Disconnection of Supply was previously known as Earthed Equipotential Bonding and Automatic Disconnection of Supply (EEBADS).
For a 230/400V a.c. system, maximum disconnection times for final circuits not exceeding 32A are now 0.4s for a TN system and 0.2s for a TT system. 5s disconnection times (TN systems) and 1s disconnection times (TT systems) are only permitted for final circuits exceeding 32A, distribution circuits and street lighting circuits.
Where the protective measure of Automatic Disconnection of Supply is employed, socket-outlets with a rated current not exceeding 20 A for general use (by ordinary persons) are now required to be provided with additional protection by means of an RCD with a rated residual operating current I-delta-n of not more than 30mA. Similar protection is required for mobile equipment with a current rating not exceeding 32A for use outdoors. An exception is permitted for socket-outlets for use under the supervision of skilled or instructed persons, eg. in some commercial premises or a specific labelled or otherwise suitably identified socket-outlet provided for connection of a particular item of equipment.
The data given for limiting earth fault loop impedance values (now contained in Tables 41.2, 41.3 and 41.4) has been modified to take account of the nominal voltage of 230V as opposed to 240V, reducing the limits to about 95% of their previous values.
Protection against overcurrent
Chapter 43 Protection against overcurrent now includes the requirements previously given in Section 473 in the 16th Edition. Guidance on the overcurrent protection of conductors in parallel is given in Appendix 10.
Protection against voltage disturbances
Chapter 44 Protection against voltage disturbances includes a new Section 442, dealing with protection of low voltage installations against temporary overvoltages due to earth faults in the high voltage system and due to faults in the low voltage system. Protection against overvoltages of atmospheric origin or due to switching in Section 443 retains the existing requirements but enables designers to use a risk assessment approach when designing installations which may be susceptible to overvoltages of atmospheric origin.
Selection and erection of wiring systems
Chapter 52 - Selection and erection of wiring systems, now requires cables concealed in a wall or partition (at a depth of less than 50mm) to be protected by a 30 mA RCD where the installation is not intended to be under the supervision of a skilled or instructed person, if the normal methods of protection including use of cables with an earthed metallic covering, mechanical protection (including use of cables with an earthed metallic covering, or mechanical protection) cannot be employed. The requirements also apply to a cable in a partition where the construction includes metallic parts other than fixings, irrespective of the depth of the cable.
Table 52.2, giving data relating to cables surrounded by thermal insulation, provides slightly more onerous de-rating factors, to take account of the ready availability of material with improved thermal insulation.
Chapter 52 now includes busbar trunking systems and powertrack systems.
Chapter 52, in Section 525 and Appendix 12 refers to voltage drop. The maximum permitted voltage drop for lighting circuits is now 3% and for other uses 5%.
Protection, isolation, switching, control and monitoring
Simplification brings together the requirements previously in Chapter 46, Sections 476 and 537 of the 16th Edition in a single Chapter 53, which also includes a new Section 532 Devices for protection against the risk of fire, and another new Section 538 - Monitoring devices.
Earthing arrangements and protective conductors
Chapter 54 -;Earthing arrangements and protective conductors, retains the requirement that a metallic pipe of a water utility supply shall not be used as an earth electrode but also states that other metallic water supply pipework (such as a metallic water supply pipework of a privately owned water supply network) shall not be used as an earth electrode unless precautions are taken against its removal and it has been considered for such a use.
The requirements relating to the installation of equipment having high protective conductor currents previously in Section 607 of the 16th Edition are now no longer considered to be a special location or installation and are moved to the General Rules. The requirements are to be found in Regulation Group 543.7.
Low voltage generating sets
Requirements for low voltage generating sets including small-scale embedded generators (SSEGs) such as wind turbines and photovoltaic (pv) generators are now to be found in Chapter 55.
Luminaires and lighting installations
A new series of requirements for fixed lighting installations, outdoor lighting installations, extra-low voltage lighting installations, lighting for display stands and highway power supplies and street furniture (previously in Section 611) are given in Section 559 - Luminaires and lighting installations.
Safety services
Requirements for safety services such as for emergency escape lighting, fire alarm systems, installations for fire pumps, fire rescue service lifts, smoke and heat extraction equipment, are dealt with under Chapter 35 (safety services) and Chapter 36 (continuity of service), the latter requiring an assessment to be made for each circuit of any need for continuity of service considered necessary during the intended life of the installation. In line with IEC standardisation, Chapter 56 - Safety services, has been expanded.
Inspection, testing, verification and certification
A fundamental requirement has been introduced under Regulation 132.13 requiring documentation (including that required by Chapter 51, Part 6 and Part 7) to be provided for every electrical installation.
The requirements for inspection, testing and certification previously in Part 7 of the 16th Edition are now to be found in Part 6. Very little has changed but perhaps worthy of note is that the minimum values for insulation resistance is raised to 0.5Mohm for SELV and PELV (when measured at 250V) and 1.0Mohm for low voltage circuits up to a nominal voltage of 500V (when measured at 500V), and for nominal voltage above 500V (when measured at 1,000V).
Measuring instruments and monitoring equipment should be chosen to meet the requirements of the relevant parts of BS EN 61557 (see Appendix 2 of the 17th Edition). Where such equipment does not meet the requirements of this Standard, it must provide for no less a degree of performance and safety.
A caution is provided to warn that when testing in potentially explosive atmospheres appropriate safety precautions are necessary.
Special installations or locations
A number of changes have taken place in relation to special installations or locations. For a start, the requirements are now embodied in Part 7 (rather than in Part 6 as in the 16th Edition). The main changes are in the following Sections:
Section 701: Locations containing a bath or shower
Section 702: Swimming pools and other basins
Section 703: Rooms and cabins containing sauna heaters
Section 704: Construction and demolition site installations
Section 705: Agricultural and horticultural premises
Section 706: Conducting locations with restricted movement
Section 708: Electrical installations in caravan/camping parks and similar locations
New special installations and locations
Section 709: Marinas and similar locations
Section 711: Exhibitions, shows and stands
Section 712: Solar photovoltaic (pv) power supply systems
Section 717: Mobile or transportable units
Section 721: Electrical installations in caravans and motor caravans (previously in Section 608 of the 16th Edition)
Section 740: Temporary electrical installations for structures, amusement devices and booths at fairgrounds, amusement parks and circuses
Section 753: Floor and ceiling heating systems.
Section 611 of the 16th Edition relating to highway power supplies is now incorporated into Section 559.
Appendices 1 to 7
Necessary changes to Appendices 1 to 7 have been made, including in particular the Installation and Reference Methods and tables in Appendix 4.
New appendices
Appendix 8: Current-carrying capacity and voltage drop for busbar trunking and powertrack systems
Appendix 9: Definitions - multiple source, d.c. and other systems
Appendix 10: Protection of conductors in parallel against overcurrent
Appendix 11: Effect of harmonic currents on balanced three-phase systems
Appendix 12: Voltage drop in consumers' installations
Appendix 13: Methods for measuring the insulation resistance/impedance of floors and walls to Earth or to the protective conductor system
Appendix 14: Measurement of earth fault loop impedance: consideration of the increase of the resistance of conductors with increase of temperature
Appendix 15: Ring and radial final circuit arrangements, Regulation 433.1
