shed supply

[Mr Mark Sparks]

is there anything wrong with this.

20a mccb feeding 16mm 2core swa 80m to shed
dumped the earth from sheath by terminating through compression gland into metal clad cu
10mm to stake
eli to stake 80 ohms

someone has said this is wrong can you guess why.

 

[Deleted member 26818]

Metal clad CU?

 

[Deleted member 9648]

As above...metal enclosure protected by a 20a mcb with a zs of 80ohms.....why would you want to ditch the supply earth??
(Puleeeeeze dont say "you cant export the earth")

 

[Hawk81]

I really hope its not another "cant export a pme earth" answer

 

[Mr Mark Sparks]

im a bit old school so remote buildings on a stake
but can you clarify that is ok to take and use the earth does it not need to be the same size as main earth at origin? in this case 50mm

i was questioned what if the 16mm comes out of the main switch and shorts to the metal cu

but what if the live came out of a cut out on a tt install and touched to earth whats the difference?

 

[RecycleElectron]

To use a metal clad consumer unit on a TT install you have to convert the incommer to Class II by using supplementary insulation, double insulating everything around the incommer . As far as using the supply earth is concerned some SWA is not adequate under some circumstances (varies with size, not sure about 2 core 16) and you have to use 3 core.

 

[Engineer54]

2 core 16mm PVC/SWA/PVC or XLPE/SWA/PVC is fine to use the armour as cpc. You only need 3 cores for PME supplies, if there are services such as a water supply in the outbuilding that need bonding....

You can't get more ''old school'' than me, and i wouldn't dream of TT'ing any supply, unless it was absolutly necessary. TT systems should always be your ''Last'' choice never your First!!

 

[Chr!s]

im a bit old school so remote buildings on a stake
but can you clarify that is ok to take and use the earth does it not need to be the same size as main earth at origin? in this case 50mm

You would size as per table 54.1 table 54.7 and the adiabatic, so if your line conductor is 16mm and you used table 54.7, 16mm required., this may be subject to table 54.1. Or you could use the adiabatic .

i was questioned what if the 16mm comes out of the main switch and shorts to the metal cu

What he is saying is, if the earth is divorced and the installation tt, Dependant upon the ze its likely not to be enough current to disconnect the supply in the required time. This would in turn raise the voltage of the metal cu to that of the nominal voltage, not good.

So two options protect at origin with rcd, or make it so it meets the requirements of class ii between the origin and the rcd. You could use insulated bushes where the cables pass through the metal cu.

but what if the live came out of a cut out on a tt install and touched to earth whats the difference?

Theres no difference

 

[RecycleElectron]

The insulated bush solution should be fine, I've used a small plastic plate with two bushes which extend up to very close to the main switch in the past (female conduit connectors instead of the bush nut are convenient) so that in the (admittedly unlikely)event of the cable coming out it can't get to the CU. All the cables for the incommer need to go through the same hole in a metal enclosure (hence the plastic plate) to avoid any eddy current heating I believe. Terminate the SWA in a small plastic box next to the CU to drop the earth. Seems like a lot of work but may be easier than the alternatives....or perhaps a change to a an insulated CU would be easier thinking about it?

 

[domestic spark]

2 core 16mm PVC/SWA/PVC or XLPE/SWA/PVC is fine to use the armour as cpc. You only need 3 cores for PME supplies, if there are services such as a water supply in the outbuilding that need bonding....

You can't get more ''old school'' than me, and i wouldn't dream of TT'ing any supply, unless it was absolutly necessary. TT systems should always be your ''Last'' choice never your First!!

10mm conductors, 10mm earth 3 core pme this ok?

 

[Engineer54]

Wire it,

Not sure what the minimum PME bonding cable size is these days, ...if it's 16mm then obviously 3 core 10mm isn't going to comply with PME bonding requirements... I'm sure someone here will know!!!