3.6: Confirmation of main protective bonding conductor sizes

Provision Of Earthing/Bonding Labels At Appropriate Locations

3.6: Confirmation Of Main Protective Bonding Conductor Sizes

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When undertaking 18th edition electrical inspections, whether part of an EIC (Electrical Installation Certificate) or an EICR (Electjrical Installation Condition Report), the third section, 3.xx is full of visual inspections that must be completed on the earthing arrangements and protective bonding conductors.

*** It’s worth noting, this particular set of inspections are separate to those for Supplementary Bonding which is completed as part of section 6 (special locations)

The 3.xx ‘series’ of inspections can all be completed at roughly the same time. For example, as you complete inspection number 3.3 (Provision of Earthing/Bonding Labels at all Appropriate Locations) then it is really easy to complete confirmation of main protective bonding conductor sizes at the same time. Whilst you are doing this, you are also effectively checking inspection number 3.7 (Condition & Accessibility to Main Protective Bonding Conductor Connections) at the same time

Inspection 3.6 itself is one which is to give confirmation of main protective bonding conductor sizes.

This inspection is visual only and involves checking the sizes of the main bonding conductors and ensuring that they are of the required size specified by BS7671

Confirmation of main protective bonding conductor sizes
Confirmation of main protective bonding conductor sizes

There are 2 different ways to select main protective bonding conductor sizes. One is via a table and on most supplies (especially in domestic & light commercial) will require that the bonding conductors are at least half the size of the main earthing conductor.

The alternative method applies to TN-C-S supply earthing arrangements. This, in a nut shell, requires that the bonding conductor sizes are a minimum of 10mm2.

We will examine these further below. Firstly…….:

Ensure Safety First At All Times When Completing Electrical Inspections!

The risks involved in checking the main protective bonding bonding conductor sizes are quite low to be fair. These conductors carry no voltage (unless under fault conditions) and there is no need to disconnect them in order to check the specific size of the conductor in question. However, the specific risks encountered could be:

  • If the power to any part of the property is to be left on whilst you complete these inspections, be aware to not remove the bonding clamps and leave them disconnected for any length of time during testing. This can introduce a risk to anyone still using the installation at the time. This sort of situation is rare in domestic EICRs as the power can be isolated for the duration of the test. However, large commercial jobs may need certain circuits leaving live for business continuity purposes.
  • This website is intended as a study/work aid to assist qualified/working electricians in order to keep current with changes in fields such as inspection & testing. It is also something of a repository for those studying in the field to gain general knowledge. It is in no way intended to be taken as DIY Electrical Advice and should never replace the services of a qualified & insured electrician. If you do need to source an electrician for work, please check The Competent Persons Register: Electrical. This website lists all electricians who are registered with a CPS (Competent Person Scheme)
  • electrical-assistance.co.uk assume no liability for any accident or injury, howsoever caused, whilst attempting electrical works. If you are working with electricity please ensure that you follow all local regulations in force in your particular area.

For further information on this please visit our regulations pages to learn more.

What Am I Checking When I Inspect This?

The first thing to do when getting confirmation of main protective bonding conductor sizes is to ascertain the supply earthing type.

This will likely be one of 3 main types (there are others but they are rare and used in very particular circumstances):

  • TNS – (Terra Neutral Separate) This is where the supply to the property has a distinct separate “earth” conductor back from the transformer. The conductor could take the form of an actual separate conductor in the supply cable or even the sheath of the supply cable itself. It should be noted that the “earth” and neutral are separate (as the name suggest!!)
Main Protective Bonding Conductor Sizes - How to Select to BS7671
Example of TNS cable using outer sheath as incoming ‘earth conductor’
  • TNC-S – (Terra Neutral Combined Separate) This is where the supply to the premises is just Line & Neutral. There is a combined block within the service head to which the incoming neutral conductor connects. This then splits to the Neutral & “earth” conductors to the actual installation. As the name would suggest, the “earth” & neutral conductors are combined (up to the service head) and are then separate (within the installation). This type of earthing arrangement is sometimes called Protective Multiple Earthing (PME), which relates to the fact that there are multiple earth electrodes included along the path of the Neutral conductor
  • TT – (Terra Terra) This is where the incoming supply to the property has only Line & Neutral. However it differs from the example above in that there is no distributors earthing arrangement. In this type of supply, there must be an earth electrode included as part of the installation itself. This could take the form of an “earth spike”, matting, or even the metal framework of commercial buildings. It must not, however, use any incoming services (ie: water or gas) as the installation earth electrode.
  • From this you then need to calculate the required bonding conductor size as per regulation 544.1
  • Any undersized main protective bonding conductors would be recorded as a C2 – POTENTIALLY DANGEROUS fault, particularly on PME TNC-S installations

How Do I Record These Inspections?

If you are completing 18th edition inspection & testing, to be honest you are going to need something a little more robust than just a pen and paper!

With the cost of inspection schedule pads, test result pads & main EIC or EICR pads, it can soon add up to more costly than a modern software based approach anyway!

In the instance of EICRs, multiple ‘small’ problems such as lack of earthing/bonding labels often get overlooked, and as such it is vital to leave some kind of evidence what the problem is.

Adding photos to an EICR also allows you to have a visual record, easily stored within the PDF file of that EICR. As such, you can easily revisit the exact issues and check the photos for yourself or to instruct a 3rd party. This can be useful when preparing quotes for remedial works and the like.

See more information about how we recommend you record these types of inspections here: Best 18th Edition Certification Software for 2020

Further Resources & Information on ‘Confirmation Of Main Protective Bonding Conductor Sizes’

Here are some further resources relating to checking the “Confirmation Of Main Protective Bonding Conductor Sizes”:

  • Northern Powergrid – Distribution Network Owner for much of the North of England. This includes Yorkshire, Humberside, and upto Newcastle, Sunderland, etc…
  • 105 Phone Number Scheme – A government scheme to enable people to be able to get in touch with their local DNO during power cuts, etc.. without having to remember different numbers. Useful to save to your phone in case you need to report any faults to the DNO

Applicable Regulations:

The particular regulations in BS7671:2018 (The 18th Edition of IET Wiring Regulations) which relate to this inspection is 544.1.

This states (a brief summary) the following regarding the main protective bonding conductor sizes:

Except where PME conditions apply, a main protective bonding conductor shall have a cross-sectional area not less than half the cross-sectional area required for the earthing conductor of the installation, and not less than 6mm2. The cross sectional area need not exceed 25mm2 if the bonding conductor is made of copper or a cross-section area affording similar conductance if made from other metals. (IE: steel, aluminium)

There are differing regulations for both highway power supplies & street furniture supplies. These are covered in the appropriate section on special locations.

If the main supply type is a TNC-S (PME), then selection of main protective bonding conductor sizes is completed by reference to table 54.5 of BS7671.

To summarise this table, sizing is completed by reference to the size of the PEN (Protective Earth Neutral) conductor of the supply. PEN conductors of anything less than 35mm2 require a minimum of 10mm2 bonding conductors. PEN conductors between 35mm2 & 50mm2 require a 16mm2 bonding conductor to be used. The sizes of the required bonding conductors increase significantly as the size of the PEN conductor rises.

PME main bonding conductors are not subject the 25mm2 maximum requirement of any other supply type. The largest required bonding conductors on any PME supply is with an incoming supply PEN conductor of more than 150mm2 which requires a considerable 50mm2 bonding condutor to be installed!

NEXT INSPECTION: 3.7 (Condition & Accessibility to Main Protective Bonding Conductor Connections)

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