Armoured Cable connection to CU
- Thread starter mk2phil
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Its interesting the XLPE cables have less armour CSA. Whats the reason for this?
The overall diameter of the bundle of cores is smaller ? ?
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Adam_151: The factor is nowhere near 8.9.
BAS's link shows the relevant tables from the wiring regs and lists the k values used, the factor is more like 2.25 than 8.9. The figures shown under "Required CSA" already take this factor into account.
So as i said earlier any PVC SWA your ever likely to encounter in an domestic install is going to be fine. XLPE seems to be fine up to 6mm in 2 core.
bernard: So presumably the insulation thickness on XLPE is thinner?
BAS's link shows the relevant tables from the wiring regs and lists the k values used, the factor is more like 2.25 than 8.9. The figures shown under "Required CSA" already take this factor into account.
So as i said earlier any PVC SWA your ever likely to encounter in an domestic install is going to be fine. XLPE seems to be fine up to 6mm in 2 core.
bernard: So presumably the insulation thickness on XLPE is thinner?
Possibly. I haven't measured it. ( yet )
Note also that the k values are temperature dependent.
If you're using XLPE but at a max of only 70°C, and you compare the csa requirements at that temperature with the actuals of XLPE cable you'll see you're again OK.
If you're using XLPE but at a max of only 70°C, and you compare the csa requirements at that temperature with the actuals of XLPE cable you'll see you're again OK.
H
holmslaw
..
And all the "equivalents" listed in the table were 15 mm or greater so the SWA would appear to be OK for main bonding.
In fact the main bonding on a PME system with working RCD will never normally have to carry more than the trip current of the RCD so 6 mm is current and impedance wise a massive over size. It is that size to make it mechanically resilient to damage and then to have a suitably long life time resisting corrosion.
the regulations state that the minimum copper equivelent CSA of the bonding conductors, and the note below explains this can either be a copper conductor of the tabulated value or of another metal of an appropiate size affording equal *conductance* as the tabulated value of copper CSA. K values do not relate to conductance, they are constants used in the adiabatic which take into account may factors such as conductance, specific heat capacity, permitted temperature rise and are of only for appraising the size of a conductor in relation to fault currents
Conductivities of different metals relative to copper can be seen here: http://books.google.co.uk/books?id=...epage&q=electromagnetic compatibility&f=false these figures put the factor closer to 10 than 8.9, but bear in mind there are many different grades of steel and it makes sense for it to vary
No, infact currents will circulate in the bonding when when the RCD is turned off, and in a network fault a lot more could flow!
H
holmslaw
..
Indeed. We did that topic to death a few months back; as you say, conductance ratios and 'K values' are very different things, and numerically very different. One of the problems in determining the copper:steel conductance ratio is that there is quite a range of conductances for different types of steel, and I don't think that we really know which is appropriate for SWA armour.
Kind Regards, John
I left out the word non as added in the quote below
Yes I agree that in some very unlikely cases in a PME installation a network fault together with at least one other "fault" could lead to high currents in bonding cables. One scenario is when a metal service pipe is bonded to the CPC and the network neutral bounces forcing the CPC well above ground potential. With the service pipe at true ground potential due to its contact with the ground there will be high current flows in the cable bonding the service pipe to the CPC. The "fault" being the bonding of a service pipe that is not insulated from true ground and presents a low impedance return to the supply sub-station neutral via the ground and maybe the service network pipework if that is also metallic..
Yes I agree that in some very unlikely cases in a PME installation a network fault together with at least one other "fault" could lead to high currents in bonding cables. One scenario is when a metal service pipe is bonded to the CPC and the network neutral bounces forcing the CPC well above ground potential. With the service pipe at true ground potential due to its contact with the ground there will be high current flows in the cable bonding the service pipe to the CPC. The "fault" being the bonding of a service pipe that is not insulated from true ground and presents a low impedance return to the supply sub-station neutral via the ground and maybe the service network pipework if that is also metallic..
Its even worse than that homslaw!
Taking the figure of a 8.9 ratio between steel and copper conductance, that would mean that if you needed a 10mm bond, you need 89mm of armouring to get the same conductance, if you look at the tables for 2C swa you are looking at 95mm before that is acheieved!
Its not going to fly using the SWA to export the bonding, with sizes like that!, either used 3core SWA of 10mm or greater, or a separate 10mm earth.
I don't beleive there is much cost difference between 2C and 3C SWA in the smaller sizes these days, its rather like the situation with B/E and galv conduit, the former used to be cheaper because it is cheaper to make, but lack of demand means its no longer as cheap
Taking the figure of a 8.9 ratio between steel and copper conductance, that would mean that if you needed a 10mm bond, you need 89mm of armouring to get the same conductance, if you look at the tables for 2C swa you are looking at 95mm before that is acheieved!
Its not going to fly using the SWA to export the bonding, with sizes like that!, either used 3core SWA of 10mm or greater, or a separate 10mm earth.
I don't beleive there is much cost difference between 2C and 3C SWA in the smaller sizes these days, its rather like the situation with B/E and galv conduit, the former used to be cheaper because it is cheaper to make, but lack of demand means its no longer as cheap
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