0.05 Max Impedance for Main Bonding Conductor

[loop3]

Here's a good one for you, 18.1 is the resistance milli ohm meter of a copper 1mm csa conductor. Divide this by you earth csa and multiply by 50 for maximum length within the 0.05 ohm restriction eg (18.1/10)x50 for 10mm bonding . Answer is maximum length

 

[loop3]

Ignore the last equation, I got it wrong. Lol

 

[dagrat]

10mm. 27m
16mm. 43m
25mm. 68m
35mm. 95m

These are all maximum lengths of bonding conductors before the 0.05 ohms is exceeded.

 

[loop3]

That's it, 0.0181 divided by your csa and then 0.05 divided by that answer. Got there in the end. Answer is in meters. Or you could look at the table for max lengths :S

 

[loop3]

A good book to read if you're interested in how many of the figures in the wire regs cam about is 'commentary on the iee wiring regs by Paul cook'

 

[tony mc]

This topic seems to be raised on a regular basis and the 0.05 ohms figure seems to be brought up again and again with regards to main bonding conductors!

On the publication of the new GN3 it was believed that this 0.05 figure with regards to main bonding would of been explained.

It now has a paragragh which states Testing bonding conductors and earthing conductors and under this heading it has the 0.05 ohms value but as in the old GN3 this value refers to bonding conductors between extraneous conductive parts where it is not possible to see the clamps.

I am yet to see a value given re bonding conductors min or max as the test is to confirm continuity.

 

[tony mc]

I have just found a post which hopefully will explain this much better than I can!


http://www.electriciansforums.net/i...llations/31857-general-testing-questions.html

 

[MarkieSparkie]

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]

 

[dagrat]

I agree that the 0.05 figure is a misnomer with regards to continuity of bonding conductors and its inclusion in GN3 is misleading. The important point is to check continuity, but also to confirm the readings are appropriate to the length and csa of the conductor. This can only be done by comparison to table B1

 

[brman]

so the conclusion is that the bonding conductor has no absolute limit on resistance, only that should limit touch voltage, help provide adequate disconnection times and not fail in a fault conditions? So the sizing tables are to ensure it doesn't fail but the length will determine disconnection times etc.
Is that right?

is this not covered by post 9 by Joe1979?

A maximum resistance of 0.05ohms for the bonding is considered satisfactory, this figure is given by the amount of current required to blow the 100A service fuse in the required time with a max touch voltage of 50v.

Large currents can flow when there is a fault, 1000A will clear a 100A BS1361 II fuse somewhere in the region of 0.4s which is good as time is important factor. If less than 1000A flows the fuse will take longer to clear the touch voltage between bonding connections will be limited to less than 50v which is good as limiting touch voltages is the important factor, for example it may take up to 5s for the fuse to disconnect a fault current of 630A.

I've not checked the sums but this would appear to show that 0.05 is a good number to use to insure that, in most cases, either the touch voltage and/or the disconnection time is met.
So while the 0.05 is not an absolute limit it does appear to be helpful in ensuring the regs are met.

Mind you, if my meter reads much less that 0.05 I start wondering if I have zeroed it properly..... :lol:

 

[phawk]

Thanks everyone, appreciate the replies. It's been driving me nuts trying to find a justification for the figure. It seemed to be an arbitrary figure that everyone was just expected to know.

Especially thanks to Markiesparkie, who went the extra mile as usual.