So when actually IS Supplementary Bonding needed?

[JohnW2]

Yet another discussion in another thread has moved me to again ponder the question of when SB IS required in bathrooms etc.

Firstly, there appears to be ambiguity in the regs about what needs to have SB, if/when SB is needed at all …

It is often pointed out that 415.2.1 talks only about SB being applied to “all simultaneously accessible exposed conductive-parts of fixed equipment and extraneous-conductive-parts”. However, 701.415.2, which is the primary reg which (sometimes) requires SB in bathrooms says nothing about exposed-c-ps or “simultaneously accessible”, talking only about CPCs:

… Local supplementary protective equipotential bonding according to Regulation 415.2 shall be established connecting together the terminals of the protective conductor of each circuit supplying Class I and Class II equipment to the accessible extraneous-conductive-parts ….

As I’ve often said, none of my bathrooms have any exposed-c-ps, and I’m sure my house is not unique in that respect. In that situation, 415.2.1 clearly would/could not require SB (since there is nothing to bond), but 701.415.2 would (unless conditions for omission were satisfied, and assuming that were some extraneous-c-ps) require SB to be installed, and connected to all the CPCs.

Moving to the “criteria for omission of SB" (701.415.2), the first point is a rather silly one, which I suspect is an oversight on the part of the authors. The list of ‘criteria for omission is prefaced by “Where the location containing a bath or shower is in a building with a protective equipotential bonding system in accordance with Regulation 411.3.1.2 …. “. With the increasing prevalence of plastic gas and water supply pipes, there must be an increasing number of buildings which cannot have PEB, since there are no extraneous-c-ps (extraneous to the building) to be bonded - but, taken literally, this reg would mean that SB would always be needed in bathrooms in such a case! It would be better if it said “PEB if/when required”.

As for the actual ‘criteria for omission’ (implicit and explicit), if there is main bonding and if (per current requirements) all circuits in the bathroom were RCD-protected, then all we are left with are:

1… (implicitly) for SB to be required, there must be at least one extraneous-c-p (extraneous to the room) - otherwise there is nothing to 'supplementarily bond'.

and, per the reg …

2… All those extraneous-c-ps must be effectively connected to the PEB, the test being the “50/Ia" criterion of 415.2.2

3… That all final circuits in the room comply with the requirements for ADS.

Of those, (3) will obviously be satisfied in an otherwise-compliant installation, leaving us with just (1) and (2) - and I am seriously struggling to think of any real-world situations in which that would not the case …

… in practice, when a pipe enters a bathroom, it will either be connected to the MET via metal pipework and CPCs (hence < <50/Ia) or else will be isolated from the MET by bits of plastic plumbing, in which case the resistance will be sufficiently high (>23kΩ or whatever) for it to be deemed to not be an extraneous-c-p.

So, given RCD protection, main bonding and satisfactory ADS, when, if ever, is SB going to be needed in a bathroom?

Kind Regards, John

 

[EFLImpudence]

It is often pointed out that 415.2.1 talks only about SB being applied to “all simultaneously accessible exposed conductive-parts of fixed equipment and extraneous-conductive-parts”. However, 701.415.2, which is the primary reg which (sometimes) requires SB in bathrooms says nothing about exposed-c-ps or “simultaneously accessible”, talking only about CPCs:

In my opinion, 701.415.2 is carelessly written.
While it does start with "Local equipotential bonding According to Regulation 415.2 shal be established..." it then goes on to contradict this.
The requirement for SB to be connected to the CPCs of Class1 (whether actually required or not) and Class2 equipment is clearly unnecessary - just like CPCs must be run to places where they are not needed.

As I’ve often said, none of my bathrooms have any exposed-c-ps, and I’m sure my house is not unique in that respect. In that situation, 415.2.1 clearly would/could not require SB (since there is nothing to bond), but 701.415.2 would (unless conditions for omission were satisfied, and assuming that were some extraneous-c-ps) require SB to be installed, and connected to all the CPCs.

Exactly.

Moving to the “criteria for omission of SB" (701.415.2), the first point is a rather silly one, which I suspect is an oversight on the part of the authors. The list of ‘criteria for omission is prefaced by “Where the location containing a bath or shower is in a building with a protective equipotential bonding system in accordance with Regulation 411.3.1.2 …. “. With the increasing prevalence of plastic gas and water supply pipes, there must be an increasing number of buildings which cannot have PEB, since there are no extraneous-c-ps (extraneous to the building) to be bonded - but, taken literally, this reg would mean that SB would always be needed in bathrooms in such a case! It would be better if it said “PEB if/when required”.

Oversight or carelessness or ignorance.

… in practice, when a pipe enters a bathroom, it will either be connected to the MET via metal pipework and CPCs (hence < <50/Ia) or else will be isolated from the MET by bits of plastic plumbing, in which case the resistance will be sufficiently high (>23kΩ or whatever) for it to be deemed to not be an extraneous-c-p.

So, given RCD protection, main bonding and satisfactory ADS, when, if ever, is SB going to be needed in a bathroom?

As I implied in the other thread, when the impedance is greater than 1666Ω, although virtually impossible.

 

[JohnW2]

In my opinion, 701.415.2 is carelessly written.
While it does start with "Local equipotential bonding According to Regulation 415.2 shal be established..." it then goes on to contradict this.
The requirement for SB to be connected to the CPCs of Class1 (whether actually required or not) and Class2 equipment is clearly unnecessary - just like CPCs must be run to places where they are not needed.

Yes, agreed.

However, as I said, 701.415.2 is the primary reg concerning SB in bathrooms, and it is clear enough in saying what is needed to comply with that reg, even if it is electrically stupid (with the installation 'as is').

As you said in that other thread, I suppose it could be that they want SB connected to all CPCs (even if those CPCs are not currently 'used') 'just in case' those CPCs come to be connected to some point in the future - even though that does seem to be taking things unnecessarily far. However, as above, whatever the thinking, it's clear enough what the reg is requiring to be done (unless the criteria for omission of SB is required) ... but this all becomes moot if you accept what I go on to say below (since, in practice, SB of any sort will never be required).

Oversight or carelessness or ignorance.

Indeed, one of those.

As I implied in the other thread, when the impedance is greater than 1666Ω, although virtually impossible.

That was my point. Even your "virtually" is questionable, since I really can't think of any credible situations in which the impedance could be greater than 1666Ω yet low enough for the part to still be considered to be an extraneous-c-p.

... which takes be back to my 'bottom line' - namely that in an installation which is otherwise compliant with regs (RCD protection of all circuits in bathroom, main bonding (if required) in place and circuits compliant as regards ADS) there is, in the real world, never going to be a need for SB in bathrooms (in otherwise compliant installations).

Kind Regards, John

 

[EFLImpudence]

However, as I said, 701.415.2 is the primary reg concerning SB in bathrooms, and it is clear enough in saying what is needed to comply with that reg, even if it is electrically stupid (with the installation 'as is').

Well, one cannot comply with all of 701.415.2. That is do what it states while at the same time being in accordance with 415.2.
Therefore, one must, with knowledge, decide which is wrong. I am in no doubt it is, by definition, the unnecessary requirements of 701 which are wrong.

As you said in that other thread, I suppose it could be that they want SB connected to all CPCs (even if those CPCs are not currently 'used') 'just in case' those CPCs come to be connected to some point in the future - even though that does seem to be taking things unnecessarily far.

Exactly. As proved by the Class2 appliance having to have an unnecessary CPC in the first place and to have it unnecessarily connected to the supplementary bonding in the second.
It will be just a conductor running from earth bar to something else via the Class2 item for no reason.

However, as above, whatever the thinking, it's clear enough what the reg is requiring to be done (unless the criteria for omission of SB is required) ... but this all becomes moot if you accept what I go on to say below (since, in practice, SB of any sort will never be required).
Indeed, one of those.
That was my point. Even your "virtually" is questionable, since I really can't think of any credible situations in which the impedance could be greater than 1666Ω yet low enough for the part to still be considered to be an extraneous-c-p.

Quite - but it is the nominal value of 50/Ia for a 30mA RCD.

... which takes be back to my 'bottom line' - namely that in an installation which is otherwise compliant with regs (RCD protection of all circuits in bathroom, main bonding (if required) in place and circuits compliant as regards ADS) there is, in the real world, never going to be a need for SB in bathrooms (in otherwise compliant installations).

I do not see a problem with that.

One might say the same about the derating factor for cables with BS3036 fuses after the last one has been removed - but it will still be the case.

Edit - 3036, of course.

 

[JohnW2]

I do not see a problem with that.

Nor do I, but it ought to considerably simplify (and remove the need for discussion about) the advice we give to people asking about SB, namely ...

... we ought to be able just say that, for all practical purposes, if the installation is otherwise compliant (wrt. RCD, ADS & main bonding), that SB in a bathroom will never be required.

Kind Regards, John