wiki: Earthing and Bonding article

I've just Persaud-ed John Rumm's piece from the thread 'Earth Bonding' into a brief article on the wiki:     http://wiki.diyfaq.org.uk/index.php?title κrthing_and_Bonding
Would folks like to cast an eye over it and suggest (or better still, write :-)) any improvements.
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John Stumbles

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John Stumbles wrote:

Shall we add:
Conductor sizes and clamps required a list of things that do not require bonding but are often incorrectly assumed to be in need of it? (Kitchen sinks etc)
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John.

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In an earlier contribution to this discussion,

If appropriate, I would like to see each of the different incoming mains earthing schemes described, together with the implications of each for consumer wiring and earthing/bonding. This is something which I've never totally got my head round, and having it described in the Wiki would be great.
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Cheers,
Roger
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It's currently in the FAQ. http://www.diyfaq.org.uk/electrical/electrical.html#system
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Andrew Gabriel
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Andrew Gabriel wrote:

There is more in the wiki as well:
http://wiki.diyfaq.org.uk/index.php?title κrthing_Types http://wiki.diyfaq.org.uk/index.php?title=TT_Earthing http://wiki.diyfaq.org.uk/index.php?title=Ufer_Earthing
That earthing and bonding article could be a good place the hang links to the rest of the the other bits as well.
Not sure if the implications of different earthing schemes are perhaps better dealt with in the articles describing things that are affected - e.g. the taking electricity outside article talking about implications of exported earths with PME earthing. I can see pros and cons. Perhaps a little duplication would not be a bad thing in this case.
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John.

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A nice helpful article.
A few details to chew on - I'm putting them here as some I'm sure are best discussed first.

TT earthing is unable to limit V rise to 50v, and TT is no rarity. Also some older non-TTs fail to limit to 50v.
suggestion: (under 50v for some installation types) or (under 50v in many cases)


suggest: taps would expose one to a 230V potential difference, causing a risk of electrocution.
There are a few reasons why its a risk rather than a cert.


Its often said, and theres a valid point in there of course, but i'm not convinced its actually true. A wet person standing on a concrete ground floor is surely liable to experience a painful pd between taps and floor.


Conductive items or surfaces connected to earth are not a problem in such a scheme, as long as they're bonded. And this is normal. The fact that equi bonding is in practice normally earthed is perhaps one of the reasons for popular confusion.
If we take it a step further, equi bonding also (often but certainly not always) is effective where an earthed bathroom item is _not_ part of the equi bonding, for the reason that under fault conditions the rise of pd between the equi bonded and the earthed items isnt high enough to cause a nasty. Even with a TT install this is usually the case, since almost any fault causing a rise in V on the equi would also cause the same V rise on the earthed item - not in 100% of cases but normally yes.
It may also be worth mentioning the various arguments against equi bonding - and bear in mind the 17th is significantly relaxing the requirement for equi, so its pretty relevant if one wants to understand it properly.


... or more often via copper pipes
NT
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snipped-for-privacy@care2.com wrote:

Hence why additional protection (RCD) is required...

I would expect a solid connection between mains and earth across the hands would be fatal in the vast majority of cases.

voltage (230V) under fault conditions, but touching both exposes one to zero volts of potential difference.

Not often you have a bare concrete floor. Normally there will be tiles, or carpet, or vinyl, or cork etc.

as the earth wires of any circuits feeding power into the room as well as metal plumbing) are securely connected together and there are no conductive items or surfaces connected to earth.

Yup, true. Although its not a requirement that is is. It works just fine without.

I think the 17th recognises the complexity if the concept and funds ways to make it a non issue.

protective conductors of any circuits that are included in the equipotential bonding.

well both if done right...
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John.

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On Sun, 06 Jul 2008 01:01:30 +0100, John Rumm wrote:
--8<-- snippetty-snip --8<--
OK, taking on board what you folks (JR & NT) in particular and others have said I've rewritten the first few sections:
"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
Earthing and bonding are often confused (sometimes even referred to as "earth bonding"). This article attempts to clarify the subject.
Earthing ======= Earthing ensures that in the event of a fault a large fault current will flow causing rapid operation of a Circuit Protective Device (fuse or circuit breaker) promptly disconnecting the supply. For example suppose that a live wire in a washing machine has been positioned incorrectly so that it rubs on a sharp metal edge of the casing when the machine is running, and the insulation has now worn away enough that the wire makes contact with the metal of the case. Since the case is connected (via its 13A plug) to mains earth a high current will flow which will blow the fuse in the plug.
Earthing is also designed to limit the voltage rise of anything earthed that one can touch to a safe level (specified as being under 50V AC) during the fault. Thus if someone is touching the washing machine case when the fault occurs the voltage they are exposed to, and the time for which they are exposed, are small enough that they should not be at significant risk of shock injury.
The size of earthing conductors and characteristics of fuses and other protective devices are designed to ensure that a fault is cleared within 5 seconds for all faults, and within 0.4 seconds for circuits feeding socket outlets (to which our washing machines etc, which may be touched, are connected).
Bonding ======Two types of bonding are recognised: Main and Supplementary
Main Bonding ------------
Main bonding is the electrical connection together of incoming service pipes (usually water and gas or oil) to the main electrical earth (at the incoming electricity cable and meter). This ensures that (metallic) gas and water installation pipework running through a building are at the electrical earth potential.
Supplementary bonding ---------------------
Supplementary, equipotential or cross bonding is usually found in special locations containing a bath or shower. Unlike earthing it is not designed to clear a fault or limit the absolute touch voltage. What it does is electrically tie together all extraneous-conductive parts (pipe or other services etc) that could under fault conditions introduce a dangerous potential into the room. For example suppose an immersion heater or inline instantaneous electric heater develops a fault which makes the hot water pipework electrically live. (Of course this also supposes that the pipework is not earthed and the appliance is not properly earthed: which should never happen but the regulations adopt a belt and braces approach.) Without bonding such a fault would result in the hot taps being at 230V while the cold taps might offer a path to earth via the rising main: this would be a very dangerous situation since touching both hot and cold taps would expose one to a 230V potential difference across the arms and chest (including heart) probably causing severe injury or death.
However if the pipework feeding both hot and cold taps is bonded together then both taps might be at mains voltage (230V) under fault conditions, but touching both exposes one to a potential difference of zero volts. (Actually the bonding may fail to tie all elements together at exactly the same potential, but it is designed to limit any potential difference to 50V or less.)
For equipotential zones to work all extraneous-conductive parts (such as the earth wires of any circuits feeding power into the room as well as metal plumbing - central heating as well as hot and cold water) must be securely electrically connected together, and there must be no conductive items or surfaces connected to earth.
Note that it is the conductive pipework (etc) entering the special location (bathroom etc) which must be bonded: if there is a mixture of metal and plastic pipework, plastic plumbing connectors etc, then it is the pipework coming into the room, not any downstream of plastic sections, which must be bonded.
Note also that there is no requirement to explicitly connect supplementary bonding to the main earth terminal in or adjacent to the consumer unit, though in practice there will often be a connection by default via the circuit protective conductors of any circuits that are included in the equipotential bonding.
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YAPH http://yaph.co.uk

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YAPH wrote:

Yup, that seems to read quite well...

I wonder if it is worth including a note somewhere about the extra significance of bonding in PME installations (with regard to disconnection of a suppliers PEN conductor in particular - which is probably a more likely real world danger for users with overhead supplies and PME)
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John.

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On Sun, 06 Jul 2008 16:01:04 +0100, John Rumm wrote:

Maybe, but perhaps it should go in the Types of Earthing article (I also excised some lines of NT's to the discussion page with a suggestion they should go in that article too.)
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YAPH http://yaph.co.uk

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I have to disagree. Main bonding is there to ensure that the incoming services (or any extraneous conducive part such as structual metalwork) is at the same potential as the suppliers earth or on a TT system the owners earth.
A metal incoming water supply will be at earth potential. There is no need to bond it to the CU to make it an electrical earth potential. The reason for main bonding is in case of a problem with the incoming supply. For example if the earth from the supplier was to become live then all the metalwork in the sockets, switches, washing machines could become live. Main bonding will bring all the earthed points (the extraneous condutive parts) up to mains potential as well so that you cannot touch an earthed stoptap etc and a now live metal lightswitch.

There way well be an extraneous condutive parts in these special locations. These would have to be protected under the main bonding though. Stop taps in bathrooms are not uncommon.

Just my opinion. Not a dig at your work.
Adam
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Should say "Not a dig at your work on the article you wrote" :-)
Adam
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On Sun, 06 Jul 2008 15:10:26 +0000, ARWadworth wrote:

Well you could read "at the electrical earth potential" as meaning that, but it could do with clarifying if that's what it does mean. TBH I dodged that issue as I'm not sure that is the reason: I just know one's got to do it!

What you say makes sense and I'm inclined to agree with your analysis, but I'd like to see chapter and verse from the regs or suchlike, not just your or my interpretation/opinion, so I know what's going in the article is gospel[1]

True, but in that case you have main bonding *and* supplementary bonding in the bathroom, and your eqipotential bonding is also earthed. I don't see any contradiction.
[1] or true, as the case many be ;-)
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The regs tell you what needs to be done not why they need to be done. Your main bonding part is virtually reg 413-02-02 of the 16th edition. It tells you what needs/may need main bonding. My query was with your statement "This ensures that (metallic) gas and water installation pipework running through a building are at the electrical earth potential." Maybe a more constructive suggestion would be "This ensures that all extraneous conductive parts such as (metallic) gas and water services entering a house are at the same potential as the suppliers earth"
You supplementary part uses the term extraneous conduvtive part instead of exposed-condutive part. I will re read it later with suggestions. It looks good though.
Adam
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ARWadworth wrote:

"as the electrical system's earth" may be better (to account for TT systems)
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YAPH wrote:

The fault current doesn't necessarily have to be 'large'. In principle 30 mA will do the trick if the device doing the automatic disconnection is an RCD.

Or RCD in some circumstances. (OK "circuit breaker" in its most general sense includes RCCBs, but saying "fuse, MCB or RCD" might be clearer for most readers.)

That's wrong. Touch voltage can be much higher during an earth fault: half-mains or more, due to the voltage drop in the CPC. Shock protection is achieved by limiting the fault duration, not the fault voltage. Only in particular cases is it necessary to provide local supplementary bonding to limit the voltage too.

This is out of date information now. Permissible disconnection times have changed significantly in the 17th ed. and the old distinction between socket circuits and fixed equipment has gone. For 230 V AC supplies (incl. 230/400 V 3-ph) the following maxima now apply:
System TN TT ----- ----- Final circuits <= 32 A 0.4 s 0.2 s Final circuits > 32 A 5 s 1 s Distribution circuits 5 s 1 s
Remember the mantra: earthing protects by reducing the duration of touch voltages. Bonding protects by reducing their magnitude.
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xwell.myzen.co.uk> scribeth thus

So is there a "safe time" for excessive current through a body?..
Suppose not...
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Tony Sayer



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Safe as in guaranteed -- no.
Safe as in unlikely to do any harm -- certainly.
There are lots of graphs showing the current verses time for various bad effects on the body. Whilst they disagree quite markedly in magnitude, they all agree that the longer and the higher the current flow, the more harmful.
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Andrew Gabriel
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John Stumbles wrote:

I now understand the concept as explained by John, but one thing I'm not clear on.
For example, in a wash basin. The hot tap is supplied via copper pipe & fittings. The cold is supplied via copper pipe & a plastic tap connector. The bonding isn't now continuous?
If the plastic tap connector were replaced by one of the metal braided flexible tap connectors http://www.screwfix.com/prods/18417/Plumbing/Flexible-Hoses/Flex-Tap-Connector-Cold-Water-Valve-15mm-x
Will that conduct electricity & complete the bonding.? Sorry, probably not using the correct terms.
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The Medway Handyman wrote:

http://www.screwfix.com/prods/18417/Plumbing/Flexible-Hoses/Flex-Tap-Connector-Cold-Water-Valve-15mm-x
Forgot to add - would things be OK as long as both taps were bonded in the same way?
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