DIY Doctor


Postby buster » Thu Mar 06, 2008 11:09 pm

hi all,
just wanting some info please, is 16mm earth to cu from main correct, then from cu 10mm to water/gas house side. Also all pipework(exept gas) to boiler is plastic until they come down the wall to boiler approx 1m of copper, do i need to cross bond these & go back to cu or dont i need bother bonding at all? thanks
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Postby ericmark » Fri Mar 07, 2008 8:28 pm

Some supply authorities have their own rules and there are two common earthing arrangements for houses. But some basic rules.
Buried not protected against mechanical damage 16mm² Min 542.3.1
Same not buried 4mm² Min 543.1.1 but not less than 6mm² 544.1.1
With TN-C-S then S = √I²t/k to find I you will need the earth loop impedance and incoming fuse rating if one looks at the fastest time shown for a fuse in the tables of 0.1seconds at 1000 amps and an earth loop impedance of 0.23 (That gives 1000 amps) k = 143 then 26.4mm² and 544.1.1 refers to max of 25mm² and since the supply cable is under 35mm² table 54.8 shows min of 10mm²
As you can see one can end up will all sorts, but, in general, 16mm² for main and 6mm² for rest will normally pass. On the new system or with TT system because you have earth leakage trips they don’t really need to be anywhere near that size but I can’t find where it says you can reduce the sizes.
Cross bonding incomers is still done but bathroom is now relaxed in the regs with less required but that will hardly concern you as Part P will not allow you to do anything but straight change in bathroom.

Postby sparx » Sat Mar 08, 2008 7:23 pm

Hi Buster, the short answer is main 16mm2, services 10mm2 as you say, as for boiler there seems to be some confusion mainly from CORGI it seems, there is and never has been any need to cross bond pipes at boiler!!!and certainly should NOT be bonded back to main earth terminal.
If the boiler happens to be adjacent to the bathroom it might be a good place to do cross bonding as all pipes together BUT if you look at pipes going to boiler I am sure you will find they all mount to a common 'gland-plate' of metal which will be earthed via the boiler supply cable,
regards SPARX
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