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Topic Title: Omitting bathroom supplementary bonding
Topic Summary: Is there a gap in the regulations' logic?
Created On: 08 May 2018 02:22 PM
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 08 May 2018 02:22 PM
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AJJewsbury

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Following on from a recent discussion about omitting supplementary bonding from bathrooms, I think we agreed that as well as bathroom circuits having to meet modern disconnection times for ADS, there were two basic requirements:

30mA RCD protection for all circuits feeding the bathroom - so that duration of high touch voltages from earth faults on those circuits would be very short (typically <40ms) (for all circuits even those >32A).

and that all extraneous-conductive-parts of the bathroom were main bonded so that they couldn't persist at a dangerous voltage above that of the MET.

It strikes me however that the above approach doesn't consider long duration hazardous voltages being imported into the bathroom from other circuits via bathroom circuit c.p.c.s.

For example, say during a bathroom refurb, the upstairs lighting was given 30mA RCD additional protection (and the downstairs lighting remained as it was, with no additional protection) - but the upstairs lighting c.p.c. is bridged to the downstairs lighting c.p.c. at the hall lightswitch (2G one for the landing one for the hall). Any earth faults on the downstairs lighting (which if installed to earlier regulations could well persist for up to 5s) are then imposed on the upstairs lighting c.p.c. and the upstairs lighting RCD will do nothing about it. OK there's perhaps some mitigation in that the fault current will have two c.p.c.s in parallel (although not equal length/impedance) which should reduce the touch voltage to some extent, but there doesn't seem to be any guarantee that any reduction in voltage (or consequent ADS clearance time) would be sufficient.

Similarly if all (or worse still, some) of the bathroom circuits were fed from a CU via a distribution circuit- earth faults on that distribution circuit would impose a significant voltage on the CU's earth bar and so to all its outgoing circuits - with (even by today's regulations) disconnection times of up to 5s. With extraneous-conductive-parts bonded to the MET rather than the remote CU's earth bar, there could be a significant touch voltage for a significant duration within the bathroom.

Should we really be omitting supplementary bonding quite as often as the regs currently permit? Would we be better keeping it, or perhaps limit its omission to situations where all circuits of the installation have 30mA RCD protection?

- Andy.
 08 May 2018 03:39 PM
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gkenyon

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I think that's up to the designer to consider the risks ... which on occasion, may be as your example.

Even without electrical "upgrades" to the 17th Edition, considering existing installations to earlier Editions of the Regs, the use of sections of plastic pipe could sever the connection with the supplementary bonding, and permit importing a hazardous touch voltage, for up to 5 s ... doesn't need an Electrician to mess it up

So, I think the new requirements could be seen as a better attempt at enabling an electrician to be able to "fix" this type of unsatosfactory installation (or at least "report" on it in EICRs - per advice in GN 3, inspections should be done to latest BS 7671).


But it all goes to show that BS 7671 is merely a set of requirements, that often leads to a safe installation, but it's not in itself a "safety rulebook" which guarantees safety (or legal compliance) ... there's still some thinking to do on the part of the Designer, and the HSE's introductory note to BS 7671 effectively says that (as well as saying that in some circumstances you may be able to comply with legislation without full conformity to the current BS 7671.)

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EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 08 May 2018 04:04 PM
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tattyinengland

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I certainly think that in an installation with all metal pipework, that supplementary bonding certainly adds to the bathrooms safety.

I'm fairly sure that supplementary equipotential bonding was poorly understood and wasn't getting done properly (if at all in some cases)before the exemption allowed us to leave it out if all circuits were 30mA RCD protected, now we've had a period of not doing it at all, there's no chance it'll get done in most cases.

4mm bits of earth everywhere in the bathroom..........I kind of hope it doesn't come back as a requirement with no get out clause (All items (iv), (v), 30mA RCDs and (vi) of 701.415.2)

Assuming item (vi) is adhered to - (all extraneous conductive parts are effectively connected to the protective equipotential bonding according to reg 411.3.1.2) then the supplementary equipotential bonding may well add to the safety

I assume that supplementary equipotential bonding was bought in because the pipework could possibly be at different levels of fault path ohms back to where the current wants to go.............

So back to supplementary equipotential bonding = a good thing in the circumstances you describe......

Edited: 08 May 2018 at 04:12 PM by tattyinengland
 08 May 2018 04:46 PM
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geoffsd

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Originally posted by: AJJewsbury

and that all extraneous-conductive-parts of the bathroom were main bonded so that they couldn't persist at a dangerous voltage above that of the MET.

I must keep on, sorry.
I thought we had decided that it wasn't main bonding ((10sq.mm.) conductor from point to MET) that was required; merely effective connection which can be achieved by supplementary bonding to something which is.

It strikes me however that the above approach doesn't consider long duration hazardous voltages being imported into the bathroom from other circuits via bathroom circuit c.p.c.s.

Something may be lacking.

Should we really be omitting supplementary bonding quite as often as the regs currently permit? Would we be better keeping it, or perhaps limit its omission to situations where all circuits of the installation have 30mA RCD protection?

Perhaps.
 08 May 2018 04:59 PM
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AJJewsbury

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I must keep on, sorry.
I thought we had decided that it wasn't main bonding ((10sq.mm.) conductor from point to MET) that was required; merely effective connection

Yes, sorry, effectively main bonded (which could be achieved in any number of ways).
- Andy.
 08 May 2018 05:28 PM
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Weirdbeard2

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Originally posted by: gkenyon




Even without electrical "upgrades" to the 17th Edition, considering existing installations to earlier Editions of the Regs, the use of sections of plastic pipe could sever the connection with the supplementary bonding, and permit importing a hazardous touch voltage, for up to 5 s ...


Hi GK, how could the addition of an insulating section in a pipe make it more likely to permit the importation of a hazardous touch voltage?



So, I think the new requirements could be seen as a better attempt at enabling an electrician to be able to "fix" this type of unsatosfactory installation (or at least "report" on it in EICRs - per advice in GN 3, inspections should be done to latest BS 7671).




Which new requirements are you referring to, is it the one due out soon or 2008?
 08 May 2018 06:09 PM
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chrispearson

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Originally posted by: AJJewsbury

... disconnection times of up to 5s. With extraneous-conductive-parts bonded to the MET rather than the remote CU's earth bar, there could be a significant touch voltage for a significant duration within the bathroom.

Swiss cheese!

Such a scenario needs a number of holes to line up. I think that it is a useful basis for discussion here, or an exam question, but how likely is it to occur?

Why a 5s disconnection time? A distribution circuit suggests a block of flats or perhaps HMO and 5s implies a type D CB protecting the distribution circuit. Even then, somebody or something has to be touching the exposed and extraneous parts simultaneously within 5s of the fault occurring.

More likely, IMHO, is an earth fault that is caused by the manipulation of a piece of equipment (with shiny metallic exterior, say a luminaire over a wash basin) whilst the individual is also touching something that is bonded to the MET (e.g. a tap). This scenario does not require coincidence and of course the 0.4s max disconnection time of a final circuit is long enough to injure. So perhaps supplementary bond the shiny luminaire.

The only risk in my bathroom appears to be the radiant heater above the wash basin, but I don't think that I am going to rush to supplementary bond the circuit or fit RCDs.
 08 May 2018 06:31 PM
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geoffsd

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Ah.

Could it be that the scenario with a left-over 5s. disconnection time is actually not compliant with of 701.415.2.1(iv) and therefore does not meet the conditions for omission of supplementary bonding?
 08 May 2018 06:39 PM
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Zoomup

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There appears to be a greater risk of electrocution by people using portable electrical appliances in bathrooms via extension leads. This is now being encouraged by the allowance of sockets in bathrooms. The 3 metre rule does little to improve safety. People are encouraged to use appliances in bathrooms perhaps even on the side of the bath. The old rule prohibiting the installation of socket outlets in bathrooms was a good one. That simple rule encouraged people NOT to use portable appliances in bathrooms. That made sense. People knew that keeping portable appliances out of bathrooms was for their safety. There have been more recent cases of electrocution in bathrooms reported by the use of portable appliances like phone chargers than deaths causes by non-compliant or non-existent bonding.

http://www.theguardian.com/uk-...-while-charging-phone

Z.
 08 May 2018 06:51 PM
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geoffsd

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Wasn't the 3m rule only invented when zone 3 disappeared?

Meaning that there was no change as far as allowing sockets was concerned.
 08 May 2018 07:20 PM
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Zoomup

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Years ago Geoff we were not allowed any 13 Amp. sockets in a bathroom. Only a shaver outlet was permitted if it contained a double wound safety isolating transformer. These days we may end up with something like this if we are not careful, (cue link).

WARNING This link is not for the faint hearted or those with no sense of humour. DO NOT TRY THIS AT HOME.

http://www.youtube.com/watch?v=7xMKax9jWao

Z.
 08 May 2018 08:18 PM
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Weirdbeard2

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Originally posted by: AJJewsbury

I must keep on, sorry.

I thought we had decided that it wasn't main bonding ((10sq.mm.) conductor from point to MET) that was required; merely effective connection


Yes, sorry, effectively main bonded (which could be achieved in any number of ways).



And according to this NAPIT article, this can be visually checked:

" (vi) Compliance with Regulation 411.3.1.2
This refers back to the main bonding requirement of Regulation 411.3.1.2 and requires that a check be made to ascertain that all extraneous-conductive-parts of the location are "effectively connected" to the MET.

This check can be done visually or by means of a test instrument."

http://professional-electricia...-extraneous-metalwork/
 08 May 2018 08:42 PM
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geoffsd

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Originally posted by: Zoomup

Years ago Geoff we were not allowed any 13 Amp. sockets in a bathroom.

We still aren't if the room is not big enough - but I don't think anything has changed.
 08 May 2018 10:58 PM
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AJJewsbury

Posts: 17169
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There appears to be a greater risk of electrocution by people using portable electrical appliances in bathrooms via extension leads. This is now being encouraged by the allowance of sockets in bathrooms. The 3 metre rule does little to improve safety. People are encouraged to use appliances in bathrooms perhaps even on the side of the bath. The old rule prohibiting the installation of socket outlets in bathrooms was a good one. That simple rule encouraged people NOT to use portable appliances in bathrooms. That made sense. People knew that keeping portable appliances out of bathrooms was for their safety. There have been more recent cases of electrocution in bathrooms reported by the use of portable appliances like phone chargers than deaths causes by non-compliant or non-existent bonding.

I think what's changed is the new fashion to put baths (and showers) into rooms that aren't traditionally thought of as bathrooms - e.g. in bedrooms. Prohibiting all sockets in a bedroom, because there happens to be roll top bath in one corner is difficult to explain to customers. (Although perhaps it's not really new - thinking of the old tin bath in the kitchen...)

- Andy.
 08 May 2018 11:05 PM
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AJJewsbury

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Why a 5s disconnection time? A distribution circuit suggests a block of flats or perhaps HMO and 5s implies a type D CB protecting the distribution circuit.

I was thinking more of a domestic submain with something like a 63A BS 88-3 fuse protecting it (the sort of thing that often gets thrown in when an extension is added). Or for the two lighting circuits scenario the old circuit being left on a 5A BS 3036.

- Andy.
 09 May 2018 12:17 AM
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gkenyon

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Originally posted by: Weirdbeard2

Originally posted by: gkenyon


Even without electrical "upgrades" to the 17th Edition, considering existing installations to earlier Editions of the Regs, the use of sections of plastic pipe could sever the connection with the supplementary bonding, and permit importing a hazardous touch voltage, for up to 5 s ...




Hi GK, how could the addition of an insulating section in a pipe make it more likely to permit the importation of a hazardous touch voltage?




supplementary local equipotential bonding connection made to one side of the insulating section, but unfortunately, the connection to the protective conductor that carries the touch potential for 5 s disconnection time is on the other ... the plumber or DIY-er inserting the plastic section not aware of this little devilish plot by stray electrons to ruin someone's day.



So, I think the new requirements could be seen as a better attempt at enabling an electrician to be able to "fix" this type of unsatosfactory installation (or at least "report" on it in EICRs - per advice in GN 3, inspections should be done to latest BS 7671).









Which new requirements are you referring to, is it the one due out soon or 2008?
BS 7671:2008, including BS 7671:2008+AMD3:2015 ... we will know more about BS 7671:2018 in a few weeks' time, but not sure it will address this particular issue - but it perhaps could be siad that this issue is a little outside the purvey of standards, as we are talking about bridging two versions of a standard, not implementing one (or the other).

In this context, precedence has already been set, such as the potential (no pun intended) incompatibilities between systems to BS 6651 and BS EN 50310 - a full assessment to the later standard is expected if there are incomaptibilities with conflicting current equipment / system standards.

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 09 May 2018 06:30 PM
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chrispearson

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Originally posted by: AJJewsbury

Why a 5s disconnection time? A distribution circuit suggests a block of flats or perhaps HMO and 5s implies a type D CB protecting the distribution circuit.


I was thinking more of a domestic submain with something like a 63A BS 88-3 fuse protecting it (the sort of thing that often gets thrown in when an extension is added). Or for the two lighting circuits scenario the old circuit being left on a 5A BS 3036.


Andy, thank you for that.

To my mind, "submain" implies a circuit from one DB to another, in which case who makes BS 88-3 fuseholders for putting into DBs, or if you like, fuseboxes?
 10 May 2018 10:05 AM
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AJJewsbury

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To my mind, "submain" implies a circuit from one DB to another, in which case who makes BS 88-3 fuseholders for putting into DBs, or if you like, fuseboxes?

They usually come ready fitted into 1-way unit - e.g. http://www.tlc-direct.co.uk/Ma...se_SP_and_N/index.html - often then Henley blocked into the supply tails.

Hager did do some lower rating fuse carriers for CUs too - e.g. http://www.hager.co.uk/product...riers-bs1361/38531.htm but I've not seen them for a while now. Ditto for the Wylex standard ones - e.g. http://www.tlc-direct.co.uk/Products/WYC30.html - but both still to be found on older existing installations.

Careful design often prefers distribution circuits not to have MCB protection - as you almost certainly loose discrimination with MCBs (and RCBOs for L-N faults) in the downstream CU. MCCBs are a bit big and expensive for domestic use, so HBC fuses still find favour.

- Andy.
 14 May 2018 08:16 PM
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Weirdbeard2

Posts: 519
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Originally posted by: Weirdbeard2

Originally posted by: gkenyon

Originally posted by: Weirdbeard2

Originally posted by: gkenyon

Even without electrical "upgrades" to the 17th Edition, considering existing installations to earlier Editions of the Regs, the use of sections of plastic pipe could sever the connection with the supplementary bonding, and permit importing a hazardous touch voltage, for up to 5 s ...

Hi GK, how could the addition of an insulating section in a pipe make it more likely to permit the importation of a hazardous touch voltage?

supplementary local equipotential bonding connection made to one side of the insulating section, but unfortunately, the connection to the protective conductor that carries the touch potential for 5 s disconnection time is on the other ... the plumber or DIY-er inserting the plastic section not aware of this little devilish plot by stray electrons to ruin someone's day.

Thanks for the reply GK, but it sounds to me that in the example you give, that there cannot be any hazardous voltage difference between these parts as they are both connected to cpcs of the same installation.
So, I think the new requirements could be seen as a better attempt at enabling an electrician to be able to "fix" this type of unsatosfactory installation (or at least "report" on it in EICRs - per advice in GN 3, inspections should be done to latest BS 7671).

Which new requirements are you referring to, is it the one due out soon or 2008?
BS 7671:2008, including BS 7671:2008+AMD3:2015 ... we will know more about BS 7671:2018 in a few weeks' time, but not sure it will address this particular issue -

Thanks again, you threw me when you said new, as it's been 10 years since in general bathroom bonding got dropped in the vast majority of domestic installations, I'm not sure there is an issue that needs addressing?
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