Bonding gas/water yellow and blue

[Richard Burns]

Even with a plastic incoming water supply, if the installation is completely metallic (copper) it will almost definitely be introducing earth potential via the connection at the boiler. Where the plumber has bonded all the pipes together at the boiler as in the above picture, you can be sure all the metallic pipework will be introducing earth potential as the gas supply pipe to the boiler will be metallic and earthed at the boiler.

So unless you can confirm the pipework is not at earth potential (which it will be if it is an all metallic installation and the boiler supply is correctly earthed) then it will need bonding.

The external earth potential that is being introduced into the property will come from the gas supply pipe which is bonded at the point of entry, this covers the bonding requirements; otherwise you would need to bond the gas pipe every few centimetres because it is connected to another bit of metal just before it that is introducing earth potential and so on. The water pipes are not introducing a new earth potential they are just extending the area over which the already bonded service spreads.

Well I can’t see how say you move a light switch from one wall to another and you complete a minor works certificate PME earthing arrangement.
If the main protective bonding is in 6mm and has been there say all the installations life maybe 30 years, then to say you have to upgrade to 10mm which could be a pig of a job is ridiculous in my opinion

On a minor electrical installation works certificate you sign to say that the safety of the installation has not been impaired by your work.
This implies that there is no requirement to enhance the safety of the installation to the current regulations despite the requirement that earthing and bonding is adequate for the protective measure applied.

 

[Ian1981]

The external earth potential that is being introduced into the property will come from the gas supply pipe which is bonded at the point of entry, this covers the bonding requirements; otherwise you would need to bond the gas pipe every few centimetres because it is connected to another bit of metal just before it that is introducing earth potential and so on. The water pipes are not introducing a new earth potential they are just extending the area over which the already bonded service spreads.

On a minor electrical installation works certificate you sign to say that the safety of the installation has not been impaired by your work.
This implies that there is no requirement to enhance the safety of the installation to the current regulations despite the requirement that earthing and bonding is adequate for the protective measure applied.

I’m interested into what Midwest has previously said about that change to the existing guidance on it.
Tho I suspect upgrading the bonding is more to do with New works carried out.
I’m aware that there’s no statutory law that requires that an electrical installation needs upgrading to the current regulations if you like but that new work must be designed and installed to the current edition of bs 7671.

 

[Midwest]

I’m interested into what Midwest has previously said about that change to the existing guidance on it.
Tho I suspect upgrading the bonding is more to do with New works carried out.
I’m aware that there’s no statutory law that requires that an electrical installation needs upgrading to the current regulations if you like but that new work must be designed and installed to the current edition of bs 7671.

Page 39, first question;

Connections - Spring 2016 - https://issuu.com/redactive/docs/con_spr16_full_lr

This was a change from the previous guidance, quoting thermal damage etc.

Its somewhat vague, as to what constitutes 'new work', that is only implied in the question, not given in the answer.

But I agree with you Ian, in some circumstances it would be ridiculous. Upgrading the bonding, typically water, would be vastly more, than the cost of some new works.

 

[Midwest]

I’m saying testing a piece of metal to see if it’s an extraneous conductive part isn’t the same test to verify supplementary bonding conductors where doubt exists over there effectiveness.
I thought it was quite clear really.

I've been reading GN8 as you suggested, specifically 6.1 - definition of an extraneous-conductive-part. It suggests where a conductive part might be partially insulated from Earth via insulating material, it recommends the use of equation (not 415.2.2) Rcp>(Uo/IB) - ZTL

Now my maths is not good, so using its example, and the use of 10mA (let go threshold), reveals a figure of 22 kohms, so any conductive part above this figure, is not to be considered an extraneous-conductive-part. Conversely, anything under this figure is, and should be bonded.

Is that correct thinking?

 

[Ian1981]

I've been reading GN8 as you suggested, specifically 6.1 - definition of an extraneous-conductive-part. It suggests where a conductive part might be partially insulated from Earth via insulating material, it recommends the use of equation (not 415.2.2) Rcp>(Uo/IB) - ZTL

Now my maths is not good, so using its example, and the use of 10mA (let go threshold), reveals a figure of 22 kohms, so any conductive part above this figure, is not to be considered an extraneous-conductive-part. Conversely, anything under this figure is, and should be bonded.

Is that correct thinking?

Carrying out a 500 v IR test from the MET to the piece of metal if you get 0.02 Mohms or higher then it’s believed not to be extraneous that is it won’t induce an earth potential if you follow the formula 230/ 0.01 amps= 23000 ohms.

 

[Pete999]

Carrying out a 500 v IR test from the MET to the piece of metal if you get 0.02 Mohms or higher then it’s believed not to be extraneous that is it won’t induce an earth potential if you follow the formula 230/ 0.01 amps= 23000 ohms.

Don't forget to factor in the average hunmanbody resistance of 1000 ohms deduct this from your calculated 23000 ohms gives 23000 minus 10000 = 22Kohms

 

[Ian1981]

Don't forget to factor in the average hunmanbody resistance of 1000 ohms deduct this from your calculated 23000 ohms gives 23000 minus 10000 = 22Kohms

Yep your right pete.

 

[Midwest]

So, we've (or I've learnt) two different tests or equations.

One for extraneous-conductive-parts as defined by reg 411.3.1.2 (equation suggested in GN8 6.1), and one for supplementary bonding, as defined by reg 415.2.2.

Sorted.

 

[Richard Burns]

I’m interested into what Midwest has previously said about that change to the existing guidance on it.
Tho I suspect upgrading the bonding is more to do with New works carried out.
I’m aware that there’s no statutory law that requires that an electrical installation needs upgrading to the current regulations if you like but that new work must be designed and installed to the current edition of bs 7671.

What I was trying to say was that the certificate declaration says that you have not made the installation worse in terms of safety. If your work was compliant but the installation as a whole had problems then you would not have made things worse and so could complete the work without upgrading the bonding (for instance).

However this is countered by the requirement to ensure earthing and bonding is adequate for the protective measure employed.
Which takes precedence? Hmm open to interpretation.
Obviously the best thing to do would be to upgrade, but, as you say, it can be ridiculous to do so in some cases.

 

[Deleted member 26818]

22 Mohms is the value which indicates a conductive -part is isolated from Earth (22Mohms if we take account of the resistance of the human body).
0.02 Mohms (20 Ohms) is the value which indicates a conductive-part is reliably connected to Earth and is the minimum resistance in the range which requires bonding.
7.67 kohms is the maximum resistance in the range which requires bonding (6.67 kohms if we take account of the resistance of the human body).
0.05 ohms is the maximum resistance for the connection of a bonding conductor to an extraneous conductive-part.

 

[Ian1981]

Yep your right pete.

22 Mohms is the value which indicates a conductive -part is isolated from Earth (22Mohms if we take account of the resistance of the human body).
0.02 Mohms (20 Ohms) is the value which indicates a conductive-part is reliably connected to Earth and is the minimum resistance in the range which requires bonding.
7.67 kohms is the maximum resistance in the range which requires bonding (6.67 kohms if we take account of the resistance of the human body).
0.05 ohms is the maximum resistance for the connection of a bonding conductor to an extraneous conductive-part.

7.67 would be if your using a 30mA rcd as your threshold if you like.
But 0.02 Mohms is not 20 ohms perhaps you misquoted?
0.05 ohms for a bonding conductor is only guidance at best as only GN3 references this value and not bs 7671 that is it’s not set in stone.

 

[Deleted member 26818]

Yes 30mA, not necessarily RCD protected.
Sorry yes should kohms not Mohms.

 

[Risteard]

Alsways took it as applying to al Earth conductors.
Main earths, bonding, CPCs, etc.

Bonding conductors are not earth conductors though although they are protective conductors.

 

[Midwest]

In reading GN8 6.1 again, it mentions the BS PD 6519, IEC 60479 Guide to effects of current on human beings & livestock, which apparently provides data for ZTL & IB.
For hand to hand contact, it gives a resistance of the human body of 1000ohms, and 0.5mA as the threshold of perception and 10mA as the let-go threshold.

Using those figures (specifically 10mA), gives a figure of 22kohms, suggesting the designer elects to accept the let-go threshold as a safe level. It also says the designer might need to consider variations in resistance & whether a lower limit on the current flowing threw the human body or livestock is necessary.

From that, can I deduce using 30mA as IB, resulting in a figure of 6.67kohms for RCP, is not recommended?

I do have a copy of a 'Connections Technical' that also gives a figure of 6.67kohms, from a calculation using a 30mA RCD. Makes you wonder who to believe!

 

[Richard Burns]

It is up to the designer to decide what level of safety is required for the installation and so then specify the required level of resistance that is acceptable to determine the identification of extraneous conductive parts.
Generally 10mA is used because it is a catch all situation, not too bad not perfect.
If you have lots of large animals then less than 2mA may be the best approach, if there is minimal chance of any accidental contact and everyone in the area is adult then 30mA may be suitable.

 

[Deleted member 26818]

It’s not based on a 30mA RCD, it’s just based on 30mA.

 

[Midwest]

It’s not based on a 30mA RCD, it’s just based on 30mA.

The Connection Magazine example is, if you are referring to my post 114?

 

[Deleted member 26818]

The Connection Magazine example is, if you are referring to my post 114?

Well that’s just nonsensical.
A 30mA RCD can let through much more current than 30mA.

 

[Ian1981]

Yes but for additional protection it must disconnect in at least 0.04 of a second

 

[Midwest]

Well that’s just nonsensical.
A 30mA RCD can let through much more current than 30mA.

Page 48. Quite a good piece, forgot I had it (and read it!), till this thread started.
Connections Magazine Autumn 2014 - https://issuu.com/redactive/docs/con_aut14