2.4.3 - External influences

This subject is a very clear example of the way that the lEE Wiring Regulations follow an international pattern. As no final agreement on the subject of external influences had been reached when the Sixteenth Edition was published, [Chapter 32] was effectively missing from the edition. It will be provided by an amendment when an agreement is reached internationally which also meets the requirements of the UK. A great deal of extra data on external influences has now (January 1997) been published in Appendix. C of the second edition of Guidance Note 1.

[Appendix 5] gives a list of how external influences are to be indicated in three categories, A, B and C. the second letter refers to the nature of the external influence. These letters are different for each of the categories, and are shown in {Table 2.3}. Next follows a number, rising in some cases to 8, which indicates the class within each external influence. For example:-

BA5 indicates:
B = utilisation
A = capability
5 = skilled persons

The ability of an enclosure to withstand the ingress of solid objects and of water is indicated by the index of protection (IP) system of classification. The system is detailed in BS EN 60529, and consists of the letters IP followed by two numbers. The first number indicates the degree of protection against solid objects, and the second against water. If, as is sometimes the case, either form of protection is not classified, the number is replaced with X. Thus, IPX5 indicates an enclosure whose protection against solid objects is not classified, but which will protect against water jets.

Table 2.3 - External influences
Environment (A)	AA	Ambient temperature
	AB	Humidity
	AC	Altitude
	AD	Water
	AE	Foreign bodies
	AF	Corrosion
	AG	Impact
	AH	Vibration
	AJ	Other mechanical stresses
	AK	Flora (plants)
	AL	Fauna (animals)
	AM	Radiation
	AN	Solar (sunlight)
	AP	Seismic (earthquakes)
	AQ	Lightning
	AR	Wind
Utilisation (B)	BA	Capability(such as physical handicap)
	BB	Resistance
	BC	Contact with earth
	BD	Evacuation (such as difficult)
	BE	Materials (fire risk)
Building (C)	CA	Materials (combustible or non-flammable)
	CB	Structure (spread of fire etc.)

Other letters are also used as follows:-

W  - placed after IP indicates a specified degree of weather protection
S  - after the numbers indicates that the enclosure has been tested against water penetration when not in use
M - after the numbers indicates that the enclosure has been tested against water penetration when in use.

A great deal of extra data on external influences has now been published in Appendix C of the 2nd edition of Guidance Note 1. A more complicated system involving the use of additional and of supplementary letters has been adopted internationally, as has an impact protection code (the 1K Code). These details are beyond the scope of this Guide but full details are found in Appendix B of the 2nd Ed. of Guidance Note I

An abbreviated form of the information concerning the meanings of the two numbers of the IP system is shown in {Table 2.4}.

The EMC (Electromagnetic Compatibility) Regulations are now in force and require that electrical installations are designed and constructed so that they do not cause electromagnetic interference with other equipment's or systems and are themselves immune to electromagnetic interference from other systems. The full implications of these Regulations for electrical installations are not yet fully understood.

Table 2.4 - Numbers in the I P system
First Number	Mechanical protection against	Second Number	Water protection against
0	Not protected	0	Not protected
1	Solid objects exceeding 50mm	1	Dripping water
2	Solid objects exceeding 12mm	2	Dripping water when tilted up to 15°
3	Solid objects exceeding 2.5mm	3	Spraying water
4	Solid objects exceeding 1.0mm	4	Splashing Water
5	Dust protected	5	Water jets
6	Dust tight	6	Heavy seas
-	-	7	Effects of immersion
-	-	8	Submersion

One extremely important external influence which is often overlooked by the electrician (as well as the designer) is the effect of testing. The application of a potential of 500 V d.c. during an insulation test can be fatal to many parts of an electrical installation, such as movement indicators (PIRs), electronic starter switches, computers, etc. A sensible policy is to provide a clear and durable notice at the mains position listing the details and location of all pieces of equipment which must be disconnected before an insulation test is carried out.