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

[andysparkfree]

Hi all , im new here but have always looked for answers over the months

description of my question
there is an external 100 amp 3 phase metered head that this new 10 bed house is to be fed from , the builders installed a 35mm 4core swa as i directed however they forgot about a 16mm earth

now is my only option to treat this a a tt supply or a really stupid question is can i simply make it a pre from the neutral at my distribution board (as it would be the same as whats at the source ) obviously not but it would definatly be easy

now over the last 20 or so years i have not ever done a tt supply (not since collage anyway) do i need to drop earth rod then cover the new 3phase dis board with a 30 ma main switch or can i install 100ma main switch and then protect all circuits with planned rcbos as original plan

also one more thing , was i right in thinking that i was never allowed to take the earth from the pme head regardless to if the builder had remembered the earth ?

so many questions

and thanks

 

[Octopus]

Lesson to learn here - take responsibility AND don't allow the builders to do this - sure let them labour for you but..............

 

[KEV 1 N]

What's wrong with using the cable armouring as the earth or cpc?

 

[andysparkfree]

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

 

[KEV 1 N]

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

Ive seen loads of buildings done this way, example: University colleges, the main building will have the incoming supply to a large section board and its own earth bar for water, gas etc, then SWA cables feed the other accommodation buildings to their own section boards and earth bars ...no separate earth conductor.

 

[The Ghost]

You did say it is a PME supply? So why not use the terminal at the cut-out??? Sorry if I missed something. What or where is the 35mm² SWA coming from and to and how far?

 

[Wilko]

As far as I know, since the installation earth is taken at source (before your swa) then you can't take it again further down the supply.

 

[westward10]

The armour of a four core 35.0 cable may be adequate as a cpc but it is inadequate for pme bonding purposes.

 

[andysparkfree]

Kev 1 N , i covered this issue of using the steel wired as main earth resonantly within a 5 yearly of an office block as after calling the NICEIC help desk and then covering it on my last yearly visit was told the earth cable required if your earthing main water or steel work gas etc etc (he even handed me a few A4 sheets on it ) (the know it all lol)
Vortigern , 4 core 35mm swa from main supply head , its about 35meter stretch (under block paving and slabs not an easy route ) and when you say cut off do you mean at the main switch at the planned new dis board within the house ?

 

[andysparkfree]

westward10 , you worded that better than me !

 

[westward10]

Short of running a separate cpc you are well and truly up the creek.

 

[Wilko]

You did say it is a PME supply? So why not use the terminal at the cut-out??? Sorry if I missed something. What or where is the 35mm² SWA coming from and to and how far?

If I understand correctly it's PME but the builder neglected to run 16mm from MET along with Andy's swa.

 

[paul291277]

Just out of interest (maybe should start a new thread but after being on here for months still can't work out how), is the steel in SWA manufactured to be the correct ratio csa to the cores, bearing in mind it has a higher resistivity etc? Don't know why I have just thought about this but I have blindly used the armour as a cpc before without checking this!

 

[The Ghost]

Ok now I see you can't retrospectively (easily) come from the pme terminal at the main service head (cut-out) But I don't get how you can join SWA into the main cut-out. What about the gland etc. Just ignore me if you think that is dumb. You can even give me a dumb rating I'm a bit short on them.

 

[andysparkfree]

yes

 

[andysparkfree]

goes from supplyers main head via meter to 100amp 3 phase switch fuse (this is where im to gland into )

 

[The Ghost]

Ok so what do you do with the armour of the cable at the suppliers end? Just truncate it there?

 

[westward10]

The easiest thing is to create a TT system at the house. Use the PME earth for the swa cable supply then at the house end gland it off in an all insulated enclosure to separate the PME earth from true earth. Fit rcbos to each circuit.

 

[westward10]

Ok so what do you do with the armour of the cable at the suppliers end? Just truncate it there?

Fit a switch fuse disconnector.

 

[Wilko]

As far as I know, since the installation earth is taken at source (before your swa) then you can't take it again further down the supply.

I think this comes from Reg 543.4.1

 

[The Ghost]

Fit a switch fuse disconnector.

Yeh that makes sense, OP did say that.

 

[Des 56]

You best have that main Rcd as S type
100A 4 Pole 300mA Time Delay RCD | CEF - http://www.cef.co.uk/catalogue/products/142693-100a-4p-300ma-time-delay-rcd

 

[The Ghost]

Don't suppose they put it in ducting by any chance?

 

[KEV 1 N]

I think this comes from Reg 543.4.1

Well if it's in the regs, then it's in black and white....just seems maddnes having a good earth at the DNO cutout, then at the DB end ditching it and then put in earth rod. Again , the University has several of its own substations with massive section boards that feed other large buildings (so technically the substation is the DNO supply) 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.

 

[westward10]

You could always just use two of the phases and use the other core as the cpc.

 

[westward10]

Well if it's in the regs, then it's in black and white....just seems maddnes having a good earth at the DNO cutout, then at the DB end ditching it and then put in earth rod. Again , the University has several of its own substations with massive section boards that feed other large buildings (so technically the substation is the DNO supply) 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.

The armour of those cables may be adequate to support bonding.

 

[andysparkfree]

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 )

 

[KEV 1 N]

The armour of those cables may be adequate to support bonding.

Ah so smaller SWA cables may not be suitable for that. Funnily the cables I've seen used in this way were much bigger that 35mm2, 50mm upwards if I remember correctly.

 

[KEV 1 N]

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 )

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

 

[andysparkfree]

haha KEV 1 N has solved it haha thanks lol ill go to pet shop 1st thing !!

 

[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

 

[andysparkfree]

everything is possible , unless its not lol , I'm going to check when back on site next week to begin 2nd fix , but fairly happy with the 84mm route , however interested in the loop test result at the db so going to make it the 1st install

 

[Pete999]

Short of running a separate cpc you are well and truly up the creek.

Without a paddle, and an earthwire

 

[I2C]

A lot of great answers here. It might be a good idea to speak to the DNO and explain the situation. While the true answer seems obvious, it's still worth talking to the DNO even if it's just to update their records.

 

[davesparks]

Well if it's in the regs, then it's in black and white....just seems maddnes having a good earth at the DNO cutout, then at the DB end ditching it and then put in earth rod. Again , the University has several of its own substations with massive section boards that feed other large buildings (so technically the substation is the DNO supply) 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.

That doesn’t sound like a PME supply, so the requirements for bonding conductors in that situation are likely to be different.

 

[KEV 1 N]

That doesn’t sound like a PME supply, so the requirements for bonding conductors in that situation are likely to be different.

Yeah it's mostly TN-S around the Uni