When is a shed an outbuilding and other tales

[cerebus]

Hi there

I a question which is vexing me relating to an outbuilding and I can't suss it our from the regs (16th edition block!)

When replacing a consumer unit and putting in a couple of radials in the outbuilding recently I found when I opened the existing CU that the armoured cable was only delivering live and neutral from the house main CU.

The set up is that the house has TT earth and mcb fusebox which seems to all be overidden by an RCD conveniently situated in the house where it is easily reset (it often trips apparantly).

The outbuilding cct is run from a 30amp mcb at source and is buried underground until it appears at the outbuilding.

My question is should I install a separate TT earth for the outbuilding or have I missed something and this is all Ok (By the way the cu I installed has a 30MA main switch with an mcb for each circuit.

many thanks

 

[ricicle]

Has the outbuilding got a seperate earth rod.I would hope so as there is no earth from the house!

Are you doing this job for payment - how did you carry out your tests and what were the results, particularly the Ze (Zdb) of the outbuilding CU?

 

[cerebus]

Nope
No earth rod. I didn't realise at first as I just assumed (dummy!) that there was a mains earth connection in the CU but nothing!
It's odd because the installation, although quite old, seems to have been done quite competently in every other way. That's what made me question whether thi swould have once been seen as ok

So I guess i better sink an earth rod quick?
thanks

 

[ricicle]

So you didn't test.Are you competant to do this work?

This CU needs to be isolated immediately until the earthing arrangements are sorted out AND tested.

 

[Adam_151]

I'm confused, is the shed earthed at all? is the TT earth from the house taken to the shed via the armour?

 

[cerebus]

I took a loop impedance reading before starting and it was very high which I took to be due to the TT earth at the time. Haven't done anyything except local R1+R2 and IR tests (which were fine) but I didn't want to connect it up until investigating further.
I haven't done a lot of external work before. With an arnoured cable is the steel armouring used to carry the earth direct to a metalclad?

 

[ricicle]

At least you havn't energised yet.If you have fitted a 30mA RCD as the main switch in the outbuilding CU I would leave the SWA earth isolated from the CU at the outbuilding and drive in an earth rod.
You can then re-test and all should (hopefully) be ok.

 

[Adam_151]

With an arnoured cable is the steel armouring used to carry the earth direct to a metalclad?

You're meant to use a banjo as well..

But yeah, if the armouring is of a great enough CSA for the cpc (size using 54G and the adiabatic) then it can be used

Most times it'll be big enough (certainly on a high impedance, rcd'd TT install it should be), [one time where you are very likely to encounter issues is where the armour is used to export an equipotential zone from a TNC-S arrangement to an outbuilding with ECPs (in which case you need to be comparing conductances of the two materials, not just K values) - much recommended to use a core instead if the earth needs to be anything other than just a cpc]

 

[cerebus]

Many thanks chaps.

I am qualified but only recently and painfully aware of what I don't know

thanks for the help and I thin I'll go for driving in a new TT earth rod

 

[cerebus]

You're meant to use a banjo as well

Thanks Adam
I try the theme from Deliverance. That should scare it!