Continuing our look at the new changes introduced in ET101 the 4th edition … Chapter 52 contains a number of major clarifications rather than changes in the rules. Although for some it will be seen as changes! In particular I refer to the requirements for wiring concealed in walls etc. These “changes” appeared first in Amendment Number 2 to ET101 2000 published in September 2005 and have been incorporated in the new rules.
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522.6.1 states “Wiring systems shall be selected and erected so as to avoid damage arising from mechanical stress due to impact, penetration or compression during installation, use and maintenance”. More importantly 522.6.2 states “In fixed installations where impacts of medium severity (AG2) or high severity (AG3) can occur, protection shall be afforded by one or more of the following: |
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- the mechanical characteristics of the wiring system,
- the location selected,
- the provision of additional local or general mechanical protection.”
The main area of emphasis in the new rule is “the location selected” for example 522.6.4 states “that wiring embedded in solid walls or in un-battened dry-lined walls shall be protected by an earthed metal screen, armouring, metal conduit or trunking, against damage by impact or penetration by drills or nails except where the following two conditions apply:
a) The distance measured horizontally between the wiring and the reverse side of the wall is not less than 50mm.
b) The wiring is installed:
- in a straight vertical or horizontal run going directly to a point, accessory or switchgear mounted on the wall, or
- within a vertical distance of 150 mm from a ceiling, or
- within a horizontal distance of 150mm from a corner formed by two adjoining walls.”
The above requirement is structured so that any wiring that is not installed to comply with conditions (a) and (b) must have an earthed protective screen / shield encircling the hidden conductor. In the event of a nail /drill etc. piercing the cable insulation the offending metallic intrusion would cause an earth fault which would isolate the circuit in question and “protect” the individual concerned.
The intention is that the requirement is meant to be a preventative measure and as time progresses people involved in the construction industry will become familiar with the logic behind the rule. Consequently they should become competent to identify an area on a wall where drilling or using nails or screws would be less likely to cause damage to “live cables” installed beneath the surface.
The new annex 52F gives a summary of the requirement.
523. CURRENT-CARRYING CAPACITIES OF CONDUCTORS
In this section recognition is given to a situation in a three phase circuit where harmonic distortion is significant that the cross sectional area of the neutral conductor is not permitted to be smaller than the corresponding phase conductors. Gone are the days when the traditional tendency was to size down the neutral conductor in a three phase “balanced” load situation. Modern solid state devices can cause difficulties for sizing neutral conductors even when the load appears to be balanced. This problem is caused by the fact that the normal 120º phase displacement may support a harmonic content e.g. third harmonic with a frequency of 150HZ. Instead of cancelling out when added these phase currents add up and are returned in the neutral. This fact is recognised and the problem addressed as we read through
524. CROSS-SECTIONAL AREAS OF CONDUCTORS.
We will continue our look at changes in the 4th edition of Irish National Wiring Rules in next month’s issue of Voltimum.