Hi
Thanks Shakey.
I see what you mean by this not checking the supplementary bonding in this case.
Right then try this!
In that note (page166) which we have already mentioned we are asked to check between the extraneous parts and the main earthing terminal.As you have said this could be 1667 ohms what happens if it is a T.T system with an earth electrode resistance, including earthing conductor of say 100 ohms measured from the main earthig terminal. this would now take us above the max value of in this case a 30 mA RCD.
Thanks Daisy
ok Daisy, i can see your chain of thought, but you may becoming...erm..'confused' between earthing and bonding, as so many people do!!
so for a TT, protected by a 30mA, max Zs of 1667 ohms means it will disconect within the 50V
BUT of course the Zs is Ze (which we know is external) + (R1 + R2)
and of course R2 is the cpc resistance for your circuit from its extremity to the MET
and we know this is earthing, and is the fault path the earth fault current will take, and will take into account the exposed conductive parts
however, as you also no doubt know, the extraneous conductive parts are all about bonding, not earthing, and of course the bonding, (main or supplementary) are not considered to part of the earth fault current path
so consider an earth fault on a bathroom circuit (even on a TT system)
so a fault current is flowing through the exposed conductive part, through the CPC and back to the MET (and when it reaches a max of 30mA the RCD pops)
so lets say you are touching the extraneous conductive part (say a radiator) and the now live exposed condcutive part at the same time
if they were cross bonded they would be at the same potential, and you would be safe.
and if the resistance of the extraneous to the MET was say 1000ohms, then the fault current would have to decide to go the 'easy way' back through the low resistance R2 to the MET or the 'hard way' through YOU AND the 1000ohms extraneous path.
And of course, the exposed conductive part cant get above 50V fault voltage anyway, so you now have a max of 50V AND a 'high' resistance path between you and an electric shock. And of course this only until the RCD trips!
so your 100 ohms TT analogy would ONLY come into play IF there was an earth fault AND the CPC went open circuit
and of course, the definition of fault protection includes the words "under SINGLE fault conditions"
so yes, theoretically, IF the CPC failed, AND your extraneous had a resistance to the MET of 1667ohms AND the Ze through the earth electrode was 100 ohms AND (finally!) there was an earth fault then the fault voltage could rise marginally above 50V before the RCD tripped
BUT
to 'feel' that 50V you would have to be touching the extraneous AND exposed conductive parts at the same time
AND we would now be outside the realms of fault protection (the reason the RCD was put there in the first place) because you would be under multiple fault conditions
hope this has helped
oh, and as with most of my posts
i just made all that up.......