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.
