Shed earthing

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biggles69

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Hello folks.

Please don't shoot me down if this has been covered before, just point me in the right direction.

We recently ran a power supply from a house with PME supply down a very long garden (200ft), via SWA to a large shed.

The 'run' went as follows;

From CU MCB in T & E to junct box, convert to SWA, burried SWA in garden to JB in shed, back to T & E in shed to sub board.

SWA was 3 core 10mm. Used 2 cores as Phase/Neutral and third core as earth. Metal braiding of SWA connected to earth cable in both JBs.

Got respectable Zs, not much volts drop, everything ticketyboo.

Here's the shocker.........

We (the company) have been told this installation is not complient with regs!

What we should do is SEVER the 3rd core we have used as an earth (10mm)at the sub-board and put in an earth rod in to make it a TT sub-system.

The only way to stop us having to do this is to run a seperate 10mm 'bonding cable' (yellow/green) directly from CU earth bar to sub-board.

My boss phoned NICEIC tech desk and they agreed, the installation we have put in does not comply with regs.

Am I being a spaz here, do we seriously have to get rid of a perfecly good 10mm copper earth conductor and use tera firma as the earth return?

Has anyone else heard of this?

Bigs

 
There are dozens of threads on this, some say yes I say no. I personally TT all outbuildings especially pme. DNO do not like you using pme earth on outbuildings and if it was new they would not connect it.

Batty

 
have a search for 'exporting PME (/TNCS)' and you will find many reasons

basics are:

earth may be at different voltage to the earth you stand on

lost neutral means earth will be at 230v (possibly higher) and your RCD wont trip

best thing to do is lose all earth connection form hosue (that includes the armour at shed end) and stick a rod in. and make sure there is a RCD

the JB thing sounds wrong... i really hope you havent done this. otherwise its probably time to call a sparky to do it

 
also, the quote

'The only way to stop us having to do this is to run a seperate 10mm 'bonding cable' (yellow/green) directly from CU earth bar to sub-board.'

is not fully true, even less so since your using a copper conductor as an earth

 
I have heard this before .. but all i do i wack a Rod in , as well.... belt and braces job.
Did we not decide this was a bad idea - mixing earthing arrangements? Taking the falult current down the rod rather than the PME? I forget.

 
Hello folks. Please don't shoot me down if this has been covered before, just point me in the right direction.

We recently ran a power supply from a house with PME supply down a very long garden (200ft), via SWA to a large shed.

The 'run' went as follows;

From CU MCB in T & E to junct box, convert to SWA, burried SWA in garden to JB in shed, back to T & E in shed to sub board.

SWA was 3 core 10mm. Used 2 cores as Phase/Neutral and third core as earth. Metal braiding of SWA connected to earth cable in both JBs.

Got respectable Zs, not much volts drop, everything ticketyboo.

Here's the shocker.........

We (the company) have been told this installation is not complient with regs!

What we should do is SEVER the 3rd core we have used as an earth (10mm)at the sub-board and put in an earth rod in to make it a TT sub-system.

The only way to stop us having to do this is to run a seperate 10mm 'bonding cable' (yellow/green) directly from CU earth bar to sub-board.

My boss phoned NICEIC tech desk and they agreed, the installation we have put in does not comply with regs.

Am I being a spaz here, do we seriously have to get rid of a perfecly good 10mm copper earth conductor and use tera firma as the earth return?

Has anyone else heard of this?

Bigs
The big pivotal question that you have to ask is:-

Has the out building got any extraneous conductive parts that need bonding?

Because during a Neutral supply 'open circuit fault'..

this extraneous part will become the return path for the load of the whole of the main installation..

so your earth wire and metal pipe, (or whatever it is), will be carrying all the amps wot are coming up the Live conductor in the main building!

So the earth will need to be the same CSA, (Or bigger) as the neutral tails!

Reg 542.1.6 (ii) page 126.

The general rule of thumb is You do NOT export PME (TN-C-S) earth arrangements to another building/sub-installation.

but there are occasions where it IS permissible and still quite safe!

Personally I would have done the run in 2 core SWA, (save to cost of 3-core), and made TT at the remote end!

:|

 
I guess we should all do what the Big Book says.... and TT it. then any problems people are covered
But the Big read book does NOT say TT it...

It does say make sure your earthing arrangements can carry the full fault current likely to flow....

Which there are ways to do this without TT.

lets not get into the habit of quoting "Here say" "Non Existent" Regs..

(unless you can back them up with referencing of course! ;) )

various members keep accusing NIC of making up regs....

yet once again..

sparks do an admirable job with the need for NIC input at all! ;) ]:)

 
Did we not decide this was a bad idea - mixing earthing arrangements? Taking the falult current down the rod rather than the PME? I forget.
Correct. I hate seeing this done. You can also end up inadvertently providing a fault path for the entire street\village\town if something were to go seriously wrong.

I used to do some sub contracting for a company and they were obsessed with earthing and rods, think they had shares in green\yellow PVC or something. Everything remotely metallic was connected to everything else and all outbuildings (sheds, garages etc..) had rods driven in so there were mixed paths galore. They all thought it was 'belt and braces' too.

 
Correct. I hate seeing this done. You can also end up inadvertently providing a fault path for the entire street\village\town if something were to go seriously wrong.I used to do some sub contracting for a company and they were obsessed with earthing and rods, think they had shares in green\yellow PVC or something. Everything remotely metallic was connected to everything else and all outbuildings (sheds, garages etc..) had rods driven in so there were mixed paths galore. They all thought it was 'belt and braces' too.
Concur!

:)

Guinness

 
There is nothing in BS7671 preventing you from exporting the earth on a Protective Multiple Earth system.

However, the DNO may not allow it.

In the OPs case, the SWA of the cable should be used as the CPC, and one of the cores should be used as an equipotential bonding conductor.

Using the SWA as a CPC will protect the cable if anyone cuts through the cable with a spade or fork.

If the SWA has not been terminated correctly, then it will require equipotentialy bonding, as it is now an exposed-conductive-part. Which would require the use of at least a 10mm

 
Well spotted folks.

Of course I meant adaptable boxes NOT JBs.

Glad some people are awake.

Out of interest, just how big would the junction box need to be to terminate a 10mm SWA cable?

Ha,ha,ha,ha.

Bigs

 
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