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17th Edition Amendment 3 - What You Need To Know

Posted by Andy Hay-Ellis on 1st July 2015

The six months transitional period from the Green book to the Yellow book is over! From 1st July 2015 BS7671 17th Edition Amendment 3 must be used for the design of electrical installation. So apart from a colour change to the cover and a change in the numbering system, what things should we have implemented by now?

Fire Safety.

The headline grabbing changes were those introduced for fire safety and after consultation with the Fire Brigade.

421.1.201 requires that within domestic dwellings switchgear assemblies including consumer units have their enclosure manufactured from non-combustible material or be enclosed in a cabinet or enclosure constructed of non-combustible material. Steel is an example of non-combustible material and most manufacturers of consumer units have adopted this method of compliance. However, this does not get implemented until January 2016 so more on this in a later blog.

521.11.201 requires that wiring systems within escape routes shall be supported so that they are not liable to premature collapse in the event of fire. This precludes the use of non-metallic cable clips, cable ties or cable trunking as the sole means of support, so yes we still can use plastic trunking conduit but not as the sole support. Manufacturers therefore have started to produce metal cable retaining clips for use inside plastic trunking and stainless steel cable ties are available for use with cable tray.

Reduction in maximum Earth Fault Loop Impedances.

Amendment 3 introduced Cmin which is the minimum voltage factor to take account of voltage variations depending on time and place, changing of transformer taps and other considerations. For a public supply in the UK, Cmin will be 0.95 of the declared voltage. This has therefore resulted in a reduction in all maximum earth fault loop values. Time to throw away those old tables!

RCD Protection of Socket Outlets

We have all got used to the requirements for RCD protection of socket outlets in domestic dwellings but what about commercial / industrial? Reference to the use of socket outlets by ordinary persons in 411.3.3 has been removed and there is now a requirement that a 30 mA RCD be used to protect socket outlets up to 20A for ALL installations, including commercial / industrial The exception to this for a specific labelled socket outlet remains and a further exception, which is not applicable to domestic dwellings, is made where a documented risk assessment determines that RCD protection is not necessary. The exception for an installation ‘under the control of a skilled person’ has been removed. This is probably one area that is likely to show up time and time again on periodic inspections.

Revised Inspection and testing Documentation

Appendix 6 introduced a new Schedule of Inspections for initial verification, complete with relevant regulation numbers which is similar in layout to the one used for periodic inspection and the inclusion of the regulation numbers should make it easier to understand and complete.

The Electrical Installation Condition Report includes the requirement for inspection to be carried out in accessible roof spaces where electrical equipment is present. What sort of equipment is likely to be in roof spaces? Lights, Solar PV inverters, ventilation equipment, TV amplifiers and boosters, the list could be endless.

So apart from using revised documentation (download your version here) we should also be now be carrying out periodic inspections in line with BS 7671 17th Edition Amendment 3.

What’s next?

Well January 2016 sees the implementation of non-combustible consumer units within domestic dwellings, the IET have a number of Codes of Practice in the pipeline included one on solar PV installations and of course the 18th Edition will not be far away. Whatever happens TS4U will always keep you up to date.

If you want more detail on 17th Edition Amendment 3 then you can watch our free online seminar here:

<<< Click Here for a free online video seminar on 17th Edition 3rd Amendment >>>

 

 

 

 

 

 

 

 

 

 

Does a metallic cable tray require earthing or bonding?

Posted by Andy Hay-Ellis on 11th May 2015

metallic tray

A. In short the answer is 'depends' but the answer could be ‘earthing’, ‘bonding’ or ‘neither’ depending on the circumstances. Looking at each of these in turn:

1. If the tray is an exposed conductive part it requires earthing.

2. If the tray is and extraneous conductive part it requires bonding.

3. If the tray is neither of the above it requires neither earthing nor bonding.

So once again the answer comes down to the definition of ‘exposed conductive part’ and ‘extraneous conductive part’

Exposed-Conductive-Part – Conductive part of equipment which can be touched and which is not normally live, but which may become live under fault conditions.

Extraneous-Conductive-Part – A conductive part liable to introduce a potential, generally Earth potential, and not forming part of the electrical installation.

So two further questions now arise:

Q1.  Is a metallic tray an exposed-conductive-part?

Note: It should be remembered that the purpose of earthing is to provide a path for fault current to flow and operate the overcurrent protective device.

A(i).  If the tray is used as a protective conductor as allowed by 543.2.1, then yes it is and it should be earthed.

A(ii). If the tray is used to carry cables and is not used as a protective conductor two scenarios exist:

1. The cable tray carries metallic sheathed cables, such as bare micc. - In the event of a fault on the circuit the fault path will be the metallic sheath of the cable and therefore the tray is not an exposed-conductive-part and does not require earthing. If the tray was to be connected to the MET, under fault conditions the tray would only serve to distribute further any touch voltage.

2. The cable tray carries cables with a non-metallic sheath. -  In this case the cables are deemed to provide the same basic and fault protection as class II equipment (see Regulation 412.2.4.1) and as such, in the event of an insulation fault in the cable a fault current cannot flow in a conductive part and hence the tray does not require earthing.

In either of the scenarios above the tray is not required to be earthed and in some cases earthing could increase the shock risk under fault conditions.

Q2.  Is a metallic tray an extraneous-conductive-part?

Note: It is worth noting that in the definition of an extraneous-conductive-part, the word ‘Earth’ is capitalised as it is a proper-noun and therefore refers to the planet or the ground we stand on.

A. To answer this question one must ask another question; can the tray introduce a potential that does not already exist in the installation? Largely the answer to this question is no and therefore normally there is no need to connect bonding or supplementary bonding to the tray.

If the tray were to carry services into a building from outside of that building and that tray was in contact with Earth potential outside, then this potential could be introduced to the location and yes there would be a need to provide bonding.

The above would also apply to cable basket but not to metallic conduit or trunking which houses insulated cables without a outer sheath.

Further details on the above subject can be found in Section 10.11 of Guidance Note 8 published by the IET.

Published by our Director of Education - Andy Hay-Ellis

andy hay ellis