Main bondage

[ultinator]

Does main bonding in every circumstance always need to be under 0.05. main bonding in a house where the main water was moved has 10mm cable but resistance of 0.25 ohms!!>>

 

[Geoffsd]

or have I missed something?

No. I said that earlier.

Plus there is no requirement for what would be supplementary bonding between incoming services.


I still maintain that 0.05 is simply a figure deemed to be (as near as dammit) negligible which is what is required.

 

[Geoffsd]

So - 'we' have decided that 0.05Ω isn't a maximum value for main bonding.

However if it's over that there must be a fault.


Anyone see a contradiction?

 

[KAS1]

You have missed something. It's the main bonding conductor not the bonding cable between two pipes that the question was asked regarding if it's resistance has to be lower than 0.05ohms in all cases

Can I ask a question of those who're saying that the 0.05ohms figure should be the maximum resistance reading between 2 different extraneous conductive parts, but not the maximum figure for the bonding cable itself?

How would you see this figure being guaranteed between for example a water pipe and a gas pipe if the figure for the main bonding cables was itself higher than 0.05ohms?

Surely if the figure for the bonding cable is higher than 0.05ohms, then the bonding isn't going to be able to cause the reading between any other ET parts in the house to be lower than 0.05ohms unless this figure was already being achieved prior to the bonding being carried out in the first place.

OK so you could also argue that assuming very high resistance between ET parts in the first place, then both bonding cables being at 0.05ohms themselves wouldn't guarantee that the reading between the ET parts would then be lower than 0.05 ohms, but you can guarantee that in this situation if the reading on the bonding cable is higher than 0.05ohms then the reading for the resistance between the ET parts must also be higher than 0.05 ohms.

So regardless of the precise wording of the regs, you'd need to have a reading of 0.05 ohms or lower on the bonding cable itself in order to stand any chance of this bonding then causing the reading between any 2 ET parts to also be lower than 0.05ohms.

or have I missed something?

 

[Gavin A]

This post from last year probably ought to be in this thread.

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 <35mm[SUP]2[/SUP] Copper; main protective bonding conductor 10mm[SUP]2[/SUP] 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:
[FONT=&amp]
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. [/FONT]

 

[Gavin A]

You have missed something. It's the main bonding conductor not the bonding cable between two pipes that the question was asked regarding if it's resistance has to be lower than 0.05ohms in all cases

last I checked both gas and water both come into the building in pipes. these were the 2 pipes I referred to, and these are the 2 pipes that usually require bonding as main bonding (unless there is no gas)

 

[KAS1]

Why is this not a simple thing for people to get their heads around. It's so simple, the 0.05ohms is just a recommended value between two or more conductive parts ie pipes or steel structure which may or may not be connected together by a BONDING CONDUCTOR and the MAIN BONDING CONDUTOR'S resistance is dependant on the size and length of the cable , which should be determined by calculation and/or the size of the EARTHING CONDUCTOR simple. I REPEAT THE 0.05ohms HAS NOTHING TO DO WITH THE MAIN BONDING CONDUCTOR'S resistive value. The main bonding conductor is from the MET to a pipe or steel structure and a bonding conductor is between two pipes or metalwork that requires bonding to met the requirements of BS7671.

 

[Guest55]

so bonding all extraneous services using a single protective conductor by-passes the whole problem by ensuring zero potential between each point and the 0.05 argument falls flat on its face lol.
;-)

 

[Geoffsd]

And MAIN bonding only goes from entry point to MET.

 

[KAS1]

This thread has been going so long I forget how the Main Bonding Conductor was measured did he say he used a wandering lead which was only 0.05mm in diameter lol

 

[Geoffsd]

Why is this not a simple thing for people to get their heads around. It's so simple, the 0.05ohms is just a recommended value between two or more conductive parts

No it's the recommended value of the main bond - entry point to MET.
There is no requirement to 'supplementary bond' incoming services.
Obviously if they are close one conductor can be used but that is coincidental.

ie pipes or steel structure which may or may not be connected together by a BONDING CONDUCTOR and the MAIN BONDING CONDUTOR'S resistance is dependant on the size and length of the cable , which should be determined by calculation and/or the size of the EARTHING CONDUCTOR simple.

And IF calculated at more than that because of length of run then larger conductor would be required.

I REPEAT THE 0.05ohms HAS NOTHING TO DO WITH THE MAIN BONDING CONDUCTOR'S resistive value.

YES it does because you are measuring from MET to point of bond - i.e. the conductor - as the pipe may have no other connection to the MET.

The main bonding conductor is from the MET to a pipe or steel structure

Quite.

and a bonding conductor is between two pipes or metalwork that requires bonding to met the requirements of BS7671.

Supplementary bonding - yes - for which there is no requirement between such parts unless in a special location, not under the floorboards.

 

[Gavin A]

And MAIN bonding only goes from entry point to MET.

but it goes from entry point of both pipes to MET, therefore both pipes are effectively bonded together via the MET (or sometimes by sharing the same bonding cable to the MET).

btw, this was the post I was referring to, which KAS1 also agreed with.

The 0.05 value is the value below which potential difference should be, between simultaneously accessible extraneous conductive parts. Example of which is a gas pipe and a water pipe next to each other. Test between the two.

Even if both are 10mm bonded individually back to MET, if the difference between the two is greater than 0.05 then they need to be bonded to each other.

The 0.05 figure is not a value to be achieved between MET and bonding point taking into account the value of resistance of the cable.

That is how I understand it, of course, I am open to other more enlightened people's wisdom.

Regards.

my point being that if you do need to achieve a 0.05ohms reading between gas and water pipes as this post suggests, then logically this is not going to be achieved if one of your bonding cables has a reading above 0.05ohms itself, certainly not if it has a 0.25ohms reading.

I've not actually got the 7671 at home to check if the above statement is actually right or not, I'm just questioning how that statement can be squared with statements that the main bonding cables themselves don't need to achieve a 0.05ohms reading as KAS1 in particular has been saying (and KAS1 agreed with the statement quoted). If the statement I've quoted is actually wrong, then fair enough, but the 2 statements seem to be incompatible to me, yet both are being made or agreed with by the same posters.

 

[KAS1]

I rest my case.............................. I understand the requirements I just have to except not all other do..........................

 

[Gavin A]

This thread has been going so long I forget how the Main Bonding Conductor was measured did he say he used a wandering lead which was only 0.05mm in diameter lol

I don't think the OP has answered the question in the 2nd post about whether he zeroed the tester before carrying out the test, which would obviously make the rest of the thread a bit irrelevant if it hadn't been done.

 

[KAS1]

Candidates are still incorrectly stated that the maximum value for the resistance of a main protective bonding conductor is 0.05 ohms. This value is applicable where access to the bonding connection is not possible and a test is made between two extraneous conductive parts (GN3 Page 35). This value is not the maximum permitted resistance of the main protective bonding conductor.

THIS IS ONE OF THE MOST FAILED QUESTIONS IN TEST AND INSPECTION EXAMS when the question has been asked state the maximum value of resistance of a main protective bonding conductor...................I REST MY CASE END OF FINISH

 

[Gavin A]

Candidates are still incorrectly stated that the maximum value for the resistance of a main protective bonding conductor is 0.05 ohms. This value is applicable where access to the bonding connection is not possible and a test is made between two extraneous conductive parts (GN3 Page 35). This value is not the maximum permitted resistance of the main protective bonding conductor.

THIS IS ONE OF THE MOST FAILED QUESTIONS IN TEST AND INSPECTION EXAMS when the question has been asked state the maximum value of resistance of a main protective bonding conductor...................I REST MY CASE END OF FINISH

OK, but how do you achieve that 0.05ohms reading between 2 ET parts if the figure for the bonding cable itself between one of them and the MET is higher than 0.05ohms?