Extraneous conductive parts

The very idea of Supplementary Bonding and it’s meaning has troubled and confused me often. Having committed much of Lock-down to reading the On-Site Guide and dipping into the Requirements for Electrical Installations BS 7671:2018 (...), I believe that I do now have a grasp of what is being referred to as Supplementary Bonding, although (in my extremely limited experience), I have never seen it installed fully and correctly in a domestic dwelling.


As I understand matters, Supplementary Bonding is intended to create an ‘equipotential zone’ within a location containing a bath and/or a shower (mostly). The bonding cable is supplementary to the ‘circuit protective conductors’ of the electrical cables supplying the location with electrical energy (or, any cables running through zones 1 and 2...? I prefer to remember the regulation as, any cables going anywhere near the bathroom...!). Supplementary meaning, ‘completing or enhancing something’ (refer to dictionary) i.e. completing or enhancing the electrical earthing of the bathroom (making it safer).


Without RCD protection of the electrical circuits, Supplementary Bonding is still required. Fortunately, I imagine, both for professionals and consumers alike, modern RCDs (RCBOs, amongst other devices), make life simpler and safer. Evidence of misunderstanding in relation to Supplementary Bonding is apparent whenever you come across bonding cables attached to radiator brackets and bathtubs, amongst other unlikely places. For Supplementary Bonding to be effective, you need to run a bonding cable from each Earth point of every ‘exposed conductive part’ within that location (i.e. between each light fitting and switch to every piece of electrical equipment), to every ‘extraneous conductive part’ entering that location (i.e. generally, copper pipes but possibly old metal  waste pipes, or even steel girders if you happen to live in an ‘urbanised’ dwelling, like a New York loft apartment... or more likely, an old school conversion if you live in London). To install the supplementary bonding cable correctly, you must attach it to the pipes using metal straps to the correct British Standard with a label stating ‘SAFETY ELECTRICAL CONNECTION DO NOT REMOVE’. Also, the connection must be accessible for testing purposes. By doing this, any electrical fault which introduces an electrical potential within the location is quickly discharged to earth via a variety of routes (or the quickest one), reducing the risk of doing harm to a person in that bathroom. You do not need to connect the bonding cable to each and every copper pipe making an appearance within that bathroom, only to those pipes which are considered ‘extraneous conductive parts’ i.e. of external origin.


According to the Wiring Regulations, you can nowadays install RCDs to protect the electrical circuits supplying the bathroom and do not need Supplementary Bonding so long as the ‘extraneous conductive parts’ are Main Bonded (I don’t believe that I am fudging this, but my terminology may be a little skewed without looking at the OS Guide). Hence, my question about using the copper pipes as electrical conductors... Knowing that the copper pipes are all connected to the Main Electrical Earthing Bond of the mains water pipe via the gas boiler, why would it be necessary to bond the pipes one to another just outside the bathroom (which would require access for testing). If you were concerned about the plumbing installation being replaced with plastic pipes and the connection to main bonding being broken (or, temporarily disconnected, when a new boiler was being installed...), you would be obliged to run a main bonding cable across the pipes (in that accessible location just outside the bathroom), and string it down to the earth bar on the distribution board/consumer unit. Straight forward if you look at it that way, but it still begs the question, is it entirely necessary when the regulations permit you to use a copper pipe in the way which I have described?


It turns out that I do like to understand, rather than simply being told what to do (...).

The very idea of Supplementary Bonding and it’s meaning has troubled and confused me often. Having committed much of Lock-down to reading the On-Site Guide and dipping into the Requirements for Electrical Installations BS 7671:2018 (...), I believe that I do now have a grasp of what is being referred to as Supplementary Bonding, although (in my extremely limited experience), I have never seen it installed fully and correctly in a domestic dwelling.


As I understand matters, Supplementary Bonding is intended to create an ‘equipotential zone’ within a location containing a bath and/or a shower (mostly). The bonding cable is supplementary to the ‘circuit protective conductors’ of the electrical cables supplying the location with electrical energy (or, any cables running through zones 1 and 2...? I prefer to remember the regulation as, any cables going anywhere near the bathroom...!). Supplementary meaning, ‘completing or enhancing something’ (refer to dictionary) i.e. completing or enhancing the electrical earthing of the bathroom (making it safer).


Without RCD protection of the electrical circuits, Supplementary Bonding is still required. Fortunately, I imagine, both for professionals and consumers alike, modern RCDs (RCBOs, amongst other devices), make life simpler and safer. Evidence of misunderstanding in relation to Supplementary Bonding is apparent whenever you come across bonding cables attached to radiator brackets and bathtubs, amongst other unlikely places. For Supplementary Bonding to be effective, you need to run a bonding cable from each Earth point of every ‘exposed conductive part’ within that location (i.e. between each light fitting and switch to every piece of electrical equipment), to every ‘extraneous conductive part’ entering that location (i.e. generally, copper pipes but possibly old metal  waste pipes, or even steel girders if you happen to live in an ‘urbanised’ dwelling, like a New York loft apartment... or more likely, an old school conversion if you live in London). To install the supplementary bonding cable correctly, you must attach it to the pipes using metal straps to the correct British Standard with a label stating ‘SAFETY ELECTRICAL CONNECTION DO NOT REMOVE’. Also, the connection must be accessible for testing purposes. By doing this, any electrical fault which introduces an electrical potential within the location is quickly discharged to earth via a variety of routes (or the quickest one), reducing the risk of doing harm to a person in that bathroom. You do not need to connect the bonding cable to each and every copper pipe making an appearance within that bathroom, only to those pipes which are considered ‘extraneous conductive parts’ i.e. of external origin.


According to the Wiring Regulations, you can nowadays install RCDs to protect the electrical circuits supplying the bathroom and do not need Supplementary Bonding so long as the ‘extraneous conductive parts’ are Main Bonded (I don’t believe that I am fudging this, but my terminology may be a little skewed without looking at the OS Guide). Hence, my question about using the copper pipes as electrical conductors... Knowing that the copper pipes are all connected to the Main Electrical Earthing Bond of the mains water pipe via the gas boiler, why would it be necessary to bond the pipes one to another just outside the bathroom (which would require access for testing). If you were concerned about the plumbing installation being replaced with plastic pipes and the connection to main bonding being broken (or, temporarily disconnected, when a new boiler was being installed...), you would be obliged to run a main bonding cable across the pipes (in that accessible location just outside the bathroom), and string it down to the earth bar on the distribution board/consumer unit. Straight forward if you look at it that way, but it still begs the question, is it entirely necessary when the regulations permit you to use a copper pipe in the way which I have described?


It turns out that I do like to understand, rather than simply being told what to do (...).