Supplementary bonding?

[odawire]

Met someone the other day, he told me he was a lecturer teaching electrical installation at the local collage, he said that if you have main water and gas bonded you don't need any supplementary bonding regardless of whether you have RCD protection or not. I didn't want to argue with him too much as he is a teacher but I was pretty sure that it only applies if all circuits are RCD protected? Is this some new rule I've missed?

 

[telectrix]

or, like many, he might have been texting on his mobile and not paying attention

 

[Chr!s]

Re: 0.05 Max Impedance for Main Bonding Conductor
As a result of the thread referenced by tony mc, I obtained the following clarification from the IET Standards and Compliance Officer that there is no limit for main protective bonding conductor resistance and that the 0.05 Ohm value quoted in GN3 is only a suitable ball-park figure to prove a connection exists between two supplementary bonding connection points, eg. Between two extraneous conductive parts where the bonding cable cannot be seen for the entirety of the run.

Chris Kitcher’s Practical Guide to Inspection, Testing and Certification of Electrical Installations book, the chapter Testing of Protective Bonding Conductors is completely wrong on this matter and should be totally ignored.

Of course the main earthing conductor for TN-S, TT and PNB can be sized from the adiabatic equation and the main protective bonding conductors can then be greater than half the size of the main earthing conductor with a minimum size of 6mm. Note: Bonding conductors must NOT be sized using the adiabatic equation directly. For TN-C-S where PME conditions apply the minimum size is related to the size of the Neutral conductor as tabulated in BS7671:2008+A1:2011 Table 54.8 eg. Neutral <35mm2 Copper; main protective bonding conductor 10mm2 Copper minimum.

See the following email correspondence below for details.

Questions:
Hi Paul,
I hope you can clarify an issue of much controversy and debate, regarding Main protective bonding and its maximum length.
My understanding is that in general with a TN installation at 230 Volts we need a disconnection time of 0.4 seconds to implement effective ADS, therefore any bonding needs to be sized as required by Regulation group 544, and as such there is no restriction upon length, this is due to the fact we have no limit on touch voltage assuming we meet the prerequisite of ADS.
The only limit I can see on main protective bonding is that of 415.2.2, this is for additional protection and is used in locations of increased electric shock. This is shown in Regulation 701.415.2, where we check the effectiveness of the main protective bonding utilizing 415.2.2.
There is a passage in GN3 related to Continuity of Protective Conductors including main and supplementary bonding Test Method 2, in my opinion the 0.05 ohms is clearly a “ball –park” value for measuring between two extraneous conductive parts to confirm a valid bonding connection, and not to be applied to limit the overall length of the bonding conductor.
I’ve checked in GN8, GN5 and BS7430 and I can see no limitation other than CSA or when additional protection is required.
I’ve also spoken to ECA and they are of a similar mind, that in general no limit is placed on the length of main protective bonding.

Many thanks, Mark.

Answer in reply:

Hello Mark,
I am required to preface my remarks by saying that I have no authority to interpret the requirements of BS 7671:2008, Requirements for Electrical Installations.
The interpretation of BS 7671 is one of the roles of the Joint BSI/IEE Committee JPEL/64. However, within that constraint, I have canvassed the opinions of many members of that committee on your behalf for an “off the record consensus” and therefore hope you will find my comments helpful.

BS 7671:2008 does not have requirements that limit the length of a protective bonding conductor. Chapter 41 is based on the fundamental requirements of BS EN 61140 (Refer to Section 410) which includes reference to the conventional touch voltage limit of 50V. The key technical intent is to meet the requirements for fault protection 411.3 covering protective earthing, protective equipotential bonding and automatic disconnection. Typically you would look to achieve the appropriate disconnection. If disconnection cannot be achieved in the appropriate time then Regulation 411.3.2.6 requires the appropriate supplementary bonding in accordance with Regulation 415.2.
GN3 includes reference to 0.05 ohms but this is more to do with proving there is an actual connection between any two bonding points rather than making any judgement on length. GN3 is currently being updated to clarify this.
Regards
Paul Bicheno
Standards and Compliance Officer,
The IET.

Hmmm, I recognise the email, is that what we call plagiarism?

 

[Geordie Spark]

grammar.

Mine was a Scouser who moved from Scouseland to Geordieland when she was ten years old. She started work at that age, and continued to work until she was 66.

She was a real hard case and the whole family was scared of her.

 

[tidyboiler]

Mine was a Scouser who moved from Scouseland to Geordieland when she was ten years old. She started work at that age, and continued to work until she was 66.

She was a real hard case and the whole family was scared of her.

I know the feeling :rofl:

 

[Geordie Spark]

I know the feeling :rofl:

Waddya tryin' t' say lyke?? :angry_smile:

 

[odawire]

Who knows what this lecturer could have been on about - plastic pipework under a bath, or anything...

Lecturers make cok ups just like the rest of us - but for one to be wrong on something this fundamental is pretty unlikely isn't it - and I don't buy it..

What's more likely is the OP got his 'wires crossed' - and it was the part 'I didn't want to argue with him too much as he is a teacher' that prompted me to reply about speaking up.

Just my opinion though but I think....its a misunderstanding....clear as...

I didn't get my wires crossed, I questioned him a bit on it and he was very sure what he was saying, the reason I didn't push it more was because it was just gonna turn into an argument and I cant be bothered with that.

 

[odawire]

He may have been referring to a bathroom with plastic pipes installed and te student misheard/misunderstood him.

No he wasn't, he was definitely talking about copper pipes, and I'm not a student btw, he was a friend of a friend I was chatting to in a pub.

 

[Amp David]

I bet that was an interesting night out then.

 

[odawire]

I bet that was an interesting night out then.

Well it was early in the evening, things got more wild later when we started discussing the pros and cons of Part P!

 

[telectrix]

when you're out on the beer, the only thing you should be discussing is .........................................










BEER.

 

[Hawkmoon]

No he wasn't, he was definitely talking about copper pipes, and I'm not a student btw, he was a friend of a friend I was chatting to in a pub.

Apologies for thinking you were a student - when you said you were reluctant to argue with him I added 2 and 2 and obviously made 5 lol

So when you questioned him what was his justification for this ridiculous viewpoint - re: not having to supplementary on copper pipes with no RCD?

 

[odawire]

Apologies for thinking you were a student - when you said you were reluctant to argue with him I added 2 and 2 and obviously made 5 lol

So when you questioned him what was his justification for this ridiculous viewpoint - re: not having to supplementary on copper pipes with no RCD?

Well, I was pretty sure he was wrong but I found it strange that a teacher would talk such nonsense. He kept saying "Its a new reg" I said a few times that I was pretty sure he was wrong but he just held up his argument. I know there have been some changes to Part P that I haven't really read up on and thought maybe it was something to do with that. The people we were with obviously weren't particularly interested in what we were going on about and I so I just left it. Maybe he is a technician and not a teacher? Hopefully!

 

[Guitarist]

A lot of people talk absolute rubbish, but say it with such conviction that nobody wants to argue with them. People also lie about being teachers

 

[Geordie Spark]

A lot of people talk absolute rubbish, but say it with such conviction that nobody wants to argue with them. People also lie about being teachers

Aye, well ................. those who can - DO

Those who can't - TEACH

 

[chrisjacob]

What I know is Supplementary Bonding is making use of thinner cable to connect all possible pipes(circuits) in a particular area. The area is typically a bathroom or a room with shower. Sometimes no supplementary bonding required in the bathroom when all the pipes are plastic. This kind of electric installation will probably shows the lecturer point of view.In this case supplementary bonding would not be required.