Important terms/topics covered in this chapter:
▪ Assessment of general characteristics
▪ Basic protection
▪ Fault protection
▪ IP and IK codes
▪ The earth fault loop path
▪ Supplementary equipotential bonding
▪ Overcurrent
▪ Let-through-energy
▪ Selectivity
▪ Undervoltage
▪ Overvoltage
▪ Isolation and switching
▪ Design current
▪ Diversity
▪ Nominal rating of protection
▪ Rating factors
▪ Current carrying capacity of conductors
▪ Voltage drop
▪ Shock risk and thermal constraints.
At the end of this chapter the reader should,
▪ understand the need to assess all relevant characteristics of the installation,
▪ have reinforced their knowledge of Basic and Fault protection and how such protection is achieved,
▪ understand how ‘let-through-energy’ can cause damage to cables,
▪ be able to distinguish between ‘Isolation’ and ‘Switching’,
▪ be able to determine suitable wiring systems for particular applications,
▪ be able to carry out basic design calculations to determine cable sizes,
▪ recognize various types of installation diagram.
Any design to the 18th Edition of the IET Wiring Regulations BS 7671 must be primarily concerned with the safety of persons, property and livestock. All other considerations such as operation, maintenance, aesthetics, etc., while forming an essential part of the design, should never compromise the safety of the installation.
The selection of appropriate systems and associated equipment and accessories is an integral part of the design procedure, and as such cannot be addressed in isolation. For example, the choice of a particular type of protective device may have a considerable effect on the calculation of cable size or shock risk, or the integrity of conductor insulation under fault conditions.
Perhaps the most difficult installations to design are those involving additions and/or alterations to existing systems, especially where no original details are available, and those where there is a change of usage or a refurbishment of a premises, together with a requirement to utilize as much of the existing wiring system as possible.
So, let us investigate those parts of the Wiring Regulations that need to be considered in the early stages of the design procedure.
ASSESSMENT OF GENERAL CHARACTERISTICS
Regardless of whether the installation is a whole one, an addition, or an alteration, there will always be certain design criteria to be considered before calculations are carried out. Part 3 of the 18th Edition, Assessment of General Characteristics’, indicates six main headings under which these considerations should be addressed. These are:
1. Purpose, supplies and structure
2. External influences
3. Compatibility
4. Maintainability
5. Recognized safety services
6. Assessment of continuity of service.
Let us look at these headings in a little more detail.
Purpose, supplies and structure
▪ For a new design, will the installation be suitable for its intended purpose?
▪ For a change of usage, is the installation being used for its intended purpose?
▪ If not, can it be used safely and effectively for any other purpose?
▪ Has the maximum demand been evaluated?
▪ Can diversity be taken into account?
▪ Are the supply and earthing characteristics suitable?
▪ Are the methods for protection for safety appropriate?
▪ If standby or safety supplies are used, are they reliable?
▪ Are the installation circuits arranged to avoid danger and facilitate safe operation?
External influences
Appendix 5 of the IET Regulations classifies external influences which may affect an installation. This classification is divided into three sections, the environment (A), how that environment is utilized (B) and construction of buildings (C). The nature of any influence within each section is also represented by a number. Table 1.1 gives examples of the classification.
With external influences included on drawings and in ...
