If you mean 'Safety Electrical Connection - Do Not Remove' there should be one.
Main incoming earth bonding
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It's the other way round - tails sized by the fuse.
That someone may have installed larger tails than required does not alter the required size of earthing and bonding conductors and
that someone may have installed larger earthing conductor than required does not alter the required size of bonding conductors.
The earthing conductor would have to be able to handle all of the PEarthFC.
Yeah thats the one
514.13.1 (iii) of the reg states "where seperate from main switchgear"
I always took that as not being in the CU but not sure
Yes, I agree.
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Well, yes you would hope so.
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Always have done, where the MET is external to the CU!
Hi again,
Thanks for all of the input.
If I understand correctly, the work that was done was compliant with regulations and would be sufficient for a satisfactory EICR. Is that correct?
If using 10mm was within regs, is there any reason that you would choose to use 16mm instead? What difference would it make?
Thanks for all of the input.
If I understand correctly, the work that was done was compliant with regulations and would be sufficient for a satisfactory EICR. Is that correct?
If using 10mm was within regs, is there any reason that you would choose to use 16mm instead? What difference would it make?
Lots of 'electricians' are hard of thinking. Because 16mm is required for pme supplies they hear rumours that you have to use 16mm for the main earth and start installing it everywhere without actually learning if it's required.
I've seen TT supplies with 16mm main earth when all that's required is a 2.5mm
I've seen TT supplies with 16mm main earth when all that's required is a 2.5mm
Which shows a huge disparity as our regs only require a minimum of 10mm2 on most services.
So is the advoce in the book that defines what BS1671 says not quite correct or is everybody just playing safe?
Yep, I inherited one, and still have it (why not?
). Mind you, to be fair, it does need to be at least 4mm² if not mechanically protected. One certainly doesn't need to undertake any adiabatic calculation - even 1mm² cable would presumably be more than capable of carrying for an indefinite period of time the greatest current you could get down the average domestic TT electrode - so the limiting factor is the minimum 2.5/4.0mm² requirement of the regs!Kind Regards, John
If you are talking about the earthing conductor, that is not the case.
The earthing conductor (and CPCs) is always calculated by the adiabatic equation.
Table 54.7 is only for use , as it states, "When it is desired NOT TO CALCULATE... ...in accordance with 544.1.3" (the equation).
They do, as is the fashion for 25mm² tails regardless of the fuse rating.
PME regulations merely require a minimum of 10mm² for the main bonding conductor.
BS7671 states as above - paying safe is not really the reason.
That's all true, but neither the adiabatic calculation nor the lazy use of Table 54.7 are totally straightforward in the case to an earthing conductor. In terms of calculation, given that one doesn't know what protective device the fault current will be flowing through, I suppose one has to calculate on the basis of the circuit and device in the installation which gives the largest I²t. In terms of the point DaftPunk made about TT, as I said, an adiabatic calculation is really not necessary if the cable is able carry the PEFC continuously, but if one did want to undertake an adiabatic calculation, I suppose one would have to do it per the characteristics of an RCD?(which would result in a ludicrously small 'minimum CSA', which would be over-ridden by the 2.5/4mm² minima imposed by the regs)
As for those who want to use Table 54.7, that table was designed for CPCs, rather than earthing conductors, so one has to make an executive decision as to what to regard as "the associated line conductor" - I guess that would have to be the incoming supply conductor (in which case it's pretty likely to result in the magic 16mm² figure for the earthing conductor).
Kind Regards, John
That's it, then.
Oh, no.
Why not?
For the earthing conductor, won't it be the main fuse?
The earthing conductor is a CPC.
I don't understand why anyone would want to use 54.7 - it is merely a cover-all figure much like the OSG and 25mm² tails - but you seem to be mixing two statements.
If for CPCs then won't it be the evident circuit line conductor?
If for the earthing conductor then the incoming cable.
54.7 does not have magic 16mm²; there is also an 'S' for line conductors of less than 16mm².
Well, as I said, mathematically it will be the 'circuit (path) and device which gives the largest I²t'. I suppose that we would usually be talking about the cutout fuse, but even that will often pose a problem for the designer, who often will not know for sure what size fuse is in the cutout!
Not per BS7671 definitions it isn't!
Agreed, and I only mentioned it because you, in turn, had mentioned it as an alternative to calculaion.
It will - that's what the Table is designd for.
As I said, that's what I 'guessed' (i.e. assumed)
It's people (including a good few electricians), not Table 54.7, that have "the magic 16mm²" in their heads. I suppose I shouldn't second guess the behaviour of DNOs, but I was assuming that 'S' (for incoming supply) would not normally be less than 16mm² - in which case the minimum CSA for an earthing conductor would, per 54.7, be 16mm² for any incoming cable size up to 35mm².
Kind Regards, John
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