PME head , builder forgot earth cable , 100amp 3phase supply , 10 bedroom house , 10 bedroom house

[Wilko]

the University has several ... substations ... so then there'll be say 3 or 4 300mm2 SWA cables go out to feed other buildings, no separate cpc, then when they enter the building, an earth conductor is took off the armour to an earth bar and then from that you'll have a main protective conductor to the buildings section board, and protective earthing conductors to other services.

I'm guessing DNO can supply an earth to a quality level they feel is ok, but within the installation Table 54.8 would say it needs 50mm copper, which is about 400mm steel. And 4 core 300 is about 320mm steel, so it wouldn't pass muster. Unless I'm talking rubbish

 

[Wilko]

Well I suggest you get a pet rat and train it to run through drain pipes with string tied to it, put it through your duct, then tie a rope to the string, then use rope to pull in earth cable.....job done

I have a method for solving this problem, I've used it a few times now. It involves a Henry, a ball of string and a couple of Tesco bags. Give it a go

 

[Leesparkykent]

thanks for comments guys , i need all phases as massive house with a lift and various kitchens with big appliances etc , but i guess the main switch being 300ma rcd then me using 30ma rcbo units is the way , there is a 6 inch duct howeve its not a straight route and they didnt pull in a draw wire just the cable (bless them )

Unless the RCBO's are double pole or single pole switched neutral then you don't want an upfront time delayed RCD. In your situation I would do as @westward10 says and use the PME for the distribution circuit, Ditch it at load end and make the house TT. Adequately support the tails so there is zero chance of them moving etc and use a four pole main switch incommer kit and RCBO's.

 

[Des 56]

I would plonk for a ferret to solve the problem
Tie a rabbit to the far end with his but in the pipe release the ferret with the string tied to him.
Give him a blast from a heat gun up his rear end to get him going,when the rabbit lets out a terrifying yelp you know you got a ferret hanging on to the rabbits arse
remove rabbit and lever the ferrets teeth out of the rabbits rear and away to go

 

[andysparkfree]

thanks Lee that make sense , and i really do need a pet shop for a ferret / rabbit and a rat lol

 

[Sparkingmad]

PME earth when taken outside of equipotential zone of origin has to be equal to supply conductors. Steel Wire Armour simply doesn’t cut it as a reliable earth. So totally agree use SWA to protect the feeder cable glanded and terminated to DNO earth. I would be tempted to run it through a 300mA time delayed rcd as mentioned above. Install it in insulated enslosure at house with a TT rod or 2 and have every circuit on 30mA rcbo’s. It’s your only option other than digging it up and installing separate earth.

 

[paul291277]

Is it not possible to pull SWA back through duct with rope attached then repull with the CPC?

 

[Leesparkykent]

PME earth when taken outside of equipotential zone of origin has to be equal to supply conductors. Steel Wire Armour simply doesn’t cut it as a reliable earth. So totally agree use SWA to protect the feeder cable glanded and terminated to DNO earth. I would be tempted to run it through a 300mA time delayed rcd as mentioned above. Install it in insulated enslosure at house with a TT rod or 2 and have every circuit on 30mA rcbo’s. It’s your only option other than digging it up and installing separate earth.

Where did you get that it has to be equal to the supply conductors from? Why doesn't armour "cut it" as a reliable earth? So what happens to the 300mA time delayed RCD in the event of a neutral-earth fault on one of the final circuts protected by an RCBO?

 

[westward10]

Must admit I read that several times.

 

[andysparkfree]

just rang the NICEIC help desk as needed to go definitive (lol) apparently the 35mm 4 core swa steel is 84mm which works out more than the minimum of 10mm earth that is required for bonding conductors
so looks like mr KEV 1 N is correct ......... Hail sir Kev 1 N

 

[westward10]

just rang the NICEIC help desk as needed to go definitive (lol) apparently the 35mm 4 core swa steel is 84mm which works out more than the minimum of 10mm earth that is required for bonding conductors
so looks like mr KEV 1 N is correct ......... Hail sir Kev 1 N

That is incorrect, 50.0 four core is the minimum size which will support 10.0 bonding.

 

[Wilko]

just rang the NICEIC help desk as needed to go definitive (lol) apparently the 35mm 4 core swa steel is 84mm which works out more than the minimum of 10mm earth that is required for bonding conductors
so looks like mr KEV 1 N is correct ......... Hail sir Kev 1 N

Not that I'd ever say that Help Desk was anything other than 100% correct, but the current GN8 lists it as 78mm (p169). Anyway, it's very close

 

[Deleted member 26818]

That is incorrect, 50.0 four core is the minimum size which will support 10.0 bonding.

Armour = 50mm2 for 35mm2 3 core SWA.
Armour = 84mm2 for 35mm2 4 core SWA.

 

[The Ghost]

Sure you cant wangle a steel tape through the duct?

 

[Pete999]

yes you right Murdoch , and KEV 1N you have to have a separate earth if you have to install main bonding within an installation

Not if the steel Armour is big enough to do both jobs