Supplementary bonding

[Loubanez]

Hello all,
Perhaps a few of you could help my understanding of the following query.
The situation I am about to describe is purely hypothetical to help my understanding of the issue.

Let's assume within a domestic bathroom, I have a combination of exposed conductive parts and suspect extraneous conductive parts which can be accessible simultaneously, lets also assume the circuits within the bathroom achieve ADS, and are covered by a 30mA RCD, and that main bonding to incoming services has been carried out. I want to check to see if supplementary bonding is required to the suspect extraneous metal pipe.
Am i correct with the following assumptions:

>25000 ohms: is the resistance of the metal pipe to the exposed metal and of that to the MET. conclusion ; the pipe is deemed NOT extraneous and therefore no bonding required.

<1666 ohms: is the resistance of the metal pipe to the exposed metal and of that to the MET. Conclusion ; the pipe IS deemed extranoues, but the resistance is low enough to achieve a speedy disconnection time, so no bonding is required.

>1666<25000 ohms : is the resistance of the metal pipe to exposed metal and if that to the MET. Conclusion ; the pipe IS deemed extraneous and bonding required.

I have also seen the figure 0.05 ohms when referring to supplementary bonding, I'm not quite sure what this refers to. The minimum resistance required once the bonding is in place?

Thanks in advance.

 

[Midwest]

Useful vid.

And another.

 

[Ian1981]

Think the figure of 0.05 is used so that continuity can be reasonably assumed between any two points of metallic pipes
That figure is used in guidance note 3 but not in bs 7671 that I can recall anyway.

Maximum resistance between bonded pipework and all bathroom circuits 30mA rcd protected would be 1667ohms so the touch voltage doesn't rise above 50v.
50/0.03=1667 ohms

 

[Deleted member 26818]

One of the problems I have with the table from the OSG, is with the column for bonding two circuits together.
My understanding, is that where two circuits have CPCs of different CSAs, then the bonding conductor should be the same CSA as the smaller CPC with a minimum CSA of 4mm2 if not mechanically protected.
i.e. where the CSA of the lighting circuit is 1mm2 and the CSA of the shower circuit is 6mm2, the CSA of the bonding conductor only needs to be 1mm2 or 4mm2 if not mechanically protected.

 

[Ian1981]

One of the problems I have with the table from the OSG, is with the column for bonding two circuits together.
My understanding, is that where two circuits have CPCs of different CSAs, then the bonding conductor should be the same CSA as the smaller CPC with a minimum CSA of 4mm2 if not mechanically protected.
i.e. where the CSA of the lighting circuit is 1mm2 and the CSA of the shower circuit is 6mm2, the CSA of the bonding conductor only needs to be 1mm2 or 4mm2 if not mechanically protected.

The minimum size supplementary bonding conductor size allowed is 2.5mm with mechanical protection with a circuit cpc size of upto 4mm2

 

[Deleted member 26818]

1.0mm2 according to the OSG table, columns 1 and 3.
2.5mm2 is minimum CSA for an earthing conductor, if not part of a multicore cable.

 

[Ian1981]

You are correct my statement is between extraneous part-extraneous part which doesn't comply to this thread
my mistake

 

[Deleted member 26818]

Yes, if it's two extraneous parts, then 2.5mm2, but it would be very rare that there would be mechanical protection.
Some boxed in pipes in a bathroom perhaps?
If there's a CPC involved then it's the CSA of the CPC, but I can't see why it should have to be the larger CPC that determines the CSA.

 

[Loubanez]

Thanks for those videos Midwest, especially the first one, also John ward has some useful videos, and thanks Ian for clarifying 0.05.
Although comments on CSA of conductors wasn't really what I was after.