Ask the experts/Technical
Fully charged Designing for external influences – impact Objective To show that cables buried in the ground or installed in walls, floors and ceilings pose an increased risk of impact as they are typically not instantly visible to, for example, persons carrying out building works, and consequently BS 7671 requires robust measures.
A
n electrical installation should be designed, amongst other things, to be suitable for the environment to which it will be exposed. The designer should make an assessment of the external influences which may affect the design and safe operation for a planned installation (Regulation 132.5.1 and Regulation 301.1 refer). For such reasons, Section 522 (Selection and erection of wiring systems in relation to external influences) of BS 7671 contains a number of external influences that the designer should consider when selecting wiring systems and associated equipment for a particular location. This list of external influences is not intended to be exhaustive and there may be instances where other factors need to be considered. One such external influence and the subject of this article is the risk of ‘Impact’ (Regulation Group 522.6 refers). This article will highlight potential sources of impact to electrical installations (cables, fixed equipment and accessories) and provide a brief overview of the requirements given in BS 7671 to minimise impact to cables, fixed equipment and accessories.
• vehicles, where wiring systems are mounted on walls in car parks • plant and machinery, where flexible cables are used on construction sites • footballs and other projectiles associated with sport, where light fittings are mounted in gymnasiums and arenas • floor cleaning or similar equipment, where socket-outlets are mounted in floors • pick axes, shovels and other excavating equipment, where cables are buried in the ground • stones, bricks and other projectiles associated with vandalism, where light fittings are mounted outdoors • misuse by persons of readily accessible accessories, such as emergency stop buttons. All parts of an electrical installation should be selected taking into account the identified sources of impact (and other external influences) in the local environment. However, it is permitted to install equipment that does not, by its own construction, provide relevant protection if appropriate additional protection is provided in the erection of the installation. The additional protection, where provided, is not adversely to affect the operation of the equipment (Regulation 512.2.2 refers). Severity of impact The external influence of impact is recognised in Appendix 5 of BS 7671. Three distinct codes are used to describe the severity of impact. Code
Level of Application Severity
AG1
Low
Household and similar premises
AG2
Medium
Industrial premises having standard conditions. For example, a lightengineering workshop
AG3
High
Industrial premises having severe conditions. For example, certain locations in a shipyard
Sources of impact Impact to electrical installations can come from a number of sources and have a varying degree of severity. The sources of impact to an electrical installation will depend on how the building and associated land are used, the characteristics of the users of the building and associated land and the location(s) of the electrical installation under consideration. The typical sources of impact to electrical installations and likely locations include: • nails, screws and the like, where cables are installed in ceilings, floors, walls or partitions 52
Wiring systems Regulation 522.6.1 requires wiring systems to be selected and erected to minimise damage that may arise from mechanical stress caused by impact during installation, use or maintenance.
Spring 2016 Connections
42-61_Technical.FINAL.indd 52
05/04/2016 11:55