Extraneous conductive parts

Consider a class 1 appliance in the bathroom that is not connected to the plumbing., perhaps a towel rail or a fan heater.

It is perhaps possible that this develops a live to earth fault. Let us consider 2 cases, one where there is a direct live to case fault, and the second where  a fault occurs part way along the element to the case of the equipment.


In the first case, the voltage of the supply is divided between the live conductors and the return path - without bonding this is the live core of the twin and earth that supplies it and the CPC.

Until the fuse or breaker operates, the case may then reach some intermediate voltage -normally not enough to hurt a healthy person dry, making a moderately good contact, more of a painful belt and a surprise. But wet handed, and well earthed the shock current will be higher and the survival time shorter.

Altering the potential division between the return path and the fault by local bonding to the plumbing, means the person with a hand on the taps and leaning on the faulty appliance  receives a much weaker shock.


 Near instant tripping RCDs do mean this is not as important as it is in older installations without one. - the shock current is high, but the RCD is faster than the old hot wire fuse.

For the second sort of fault, the increase in current may not be enough to fire an MCB or blow a fuse, but the RCD will still catch it.

Arguably you do best when you have both methods in place.

The OP doesn't mention any electrical appliances in the bathroom, but there must be some form of illumination, and for those who feel the need of one, possibly a shaver socket. I can see that in olden days* that a class 1 luminaire might have had supplementary bonding to the hot and cold water pipes (no point in bonding just one of them) but was there ever a reason for bonding the hot and cold pipes at every pair of taps? (Yes, perfectly foreseeable that one might turn both taps simultaneously.)


*Before 1 July 2018

but wouldn’t anyone doing that plumbing be obliged to consider this risk at that time?

Unfortunately not - UK plumbers traditonally haven't been trained to know much about electricity - even the 'safety electrical connection - do not remove' label appears to confuse them on ocasions. In certain circumstances where there are strict procedures (like some particularly stringent work places) you might be able to get it written into site procedures that metallic plumbing shouldn't be replaced by plastic without consideration of the bonding requirements, but as a general rule - certainly for ordinary domestics - we can't rely on plumbing maintaining its continuity.

Why would the copper pipes need to be main bonded one to another close to the bathroom (in an accessible place for testing, like in an airing cupboard)?

Normally you wouldn't - main bonding would usually only be applied around the perimeter of an installation to deal with anything that might introduce a potential into the installation. Bonding for a bathroom would normally be supplementary bonding (again bonding everything that would introduce a potential into the bathroom). The only situation I can think of where you'd need to main bond internal pipework for a bathroom is where you're trying to omit supplementary bonding but the condition for any extraneous-conductive-parts entering the bathroom being main bonded hasn't been satisfied. That condition is there to stop faults from elsewhere in the installation entering the bathroom via the metallic pipework - say from an immersion heater in an adjacent bedroom cupboard (as was the fashion a few decades ago). Often (but perhaps not always) it would be easier and cheaper just to supplementary bond within the bathroom rather than a run main bonding conductors up to the bathroom from the MET.


   - Andy.

Thinking about a domestic dwelling with main bonding to incoming water and gas pipes (even though most are plastic nowadays); all the electrical circuits within the dwelling are protected by RCDs; the only extraneous conductive parts to the bathroom being copper water pipes and copper central heating pipes... Why would the copper pipes need to be main bonded one to another close to the bathroom (in an accessible place for testing, like in an airing cupboard)?

 

They would not - and it would be Supplementary Bonding.

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 OSG is not the best publication to "study".


It usually quotes a worst case scenario to install so that you don't have to think.


For example fit 25sq.mm. tails and all will be well - BUT you might not have to.



Also it contains some contradictions and mistakes.

Supplementary Bonding simply means Additional Bonding. It is not a "special thing".


Its purpose is to reduce the impedance between two simultaneously accessible items so that the touch voltage during a fault is reduced to a maximum of 50 Volts which is considered safe.


If there is, for example, due to a fault, 200A (the current which will operate a B40A MCB in the required time. 40 x 5 = 200)  flowing in the earthed and bonded parts then the maximum impedance between two simultaneously accessible parts is 0.25 Ohms. 200 x 0.25 = 50.


If it is more than 0.25 Ohms then supplementary bonding between the two parts will reduce it.


With RCDs fitted the 200A can be discounted as the current required to operate the RCD is only 30mA, so the impedance can be 1,666 Ohms (50 / 0.03). Were the impedance to be above this 1,666 Ohms then SB would still be required but this is very unlikely.

A diagram that is as simple as it gets.


A wash basin with separate taps can end up with mains voltage on the hot tap with zero volts on the cold, if you then hold both of them you are in trouble. 


A piece of wire as BONDING between the pipes keeps them at the same voltage.

Challenge met...


On-Site Guide BS 7671:2018


Page 93


Locations containing a bath or shower


8.1 Summary of requirements

...

Supplementary bonding of locations containing a bath or shower is required unless all the following requirements are met:

* all circuits... meet... disconnection times

* all circuits... have additional protection by 30mA RCDs

* all extraneous-conductive parts... are effectively connected by main bonding conductors to the main earthing terminal of the installation


Page 94


Note: An example of this is where a metallic water service pipe enters the building in the bathroom and would be connected to the main earthing terminal of the electrical installation by means of a main bonding conductor


And...


Requirements for Electrical Installations BS 7671:2018


Page 241


701.415.2 Supplementary protective equipotential bonding

...may be omitted where all of the following conditions are met:

...

(vi) All extraneous-conductive-parts of the location are effectively connected to the protective equipotential bonding according to Regulation 411.3.1.2

NOTE: The effectiveness of the connection of the extraneous-conductive-parts in the location to the main earthing terminal may be assessed, where necessary, by the application of Regulation 415.2.2


Page 74


415.2.2 The resistance R between simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts shall fulfil the following condition:

R < or = 50 V/Ia       in AC systems

...

where Ia is the operating curr3nt in amperes (A) of the protective device or:

(i) for RCDs, I^n

(ii) for overcurrent devices, the 5 s operating current.

       

Page 58


411.3.2.2 Maximum disconnection times stated in Table 41.1 shall be applied to final circuits...


Page 59


TABLE 41.1 - Maximum disconnection times

...

NOTE 1: Disconnection is not required for protection against electric shock but may be required for other reasons, such as protection against thermal effects.


Page 241


701.411.3.3 Additional protection by RCDs

Additional protection by the use of one or more RCDs...

(I) serving the location

(I) passing through zones 1 and/or 2 not serving the location


Page 58


411.3.1.2 Protective equipotential bonding

...main protective bonding conductors... shall connect to the main earthing terminal extraneous-conductive-parts including the following:

(i)Water installation pipes

(ii) Gas installation pipes

(iii) Other installation pipe work and ducting

(iv) Central heating and air conditioning systems

(v) Exposed metallic structural parts of the building




An acquired taste, this required reading.