Shed earthing

[HDRW]

I presume I'll get a "Don't do it!" from ban_all_sheds, judging from his ID, but here goes...

I'm planning to run power to my workshop/shed, using 4mm2 Steel Wire Armoured (SWA) cable, and putting a small consumer unit in there, with an overall RCD, and MCBs for lights (6A) and sockets (16A). I'll take the power from a 20A MCB in the house CU.

Now the question I have is: I'll be earth-connecting the house end of the cable's armour, obviously, but should (may?) I also put in a ground stake and connect that to the shed's earth conductors? I realise this is mixing TN - C and TT, which isn't covered in the regs, but does it do any good? It seems to me that having a "real" earth that's connected to the ground near to the shed would give an extra level of safety in case of deteriorated earth conduction back to the house in the future.

Would the Part-P inspection fail due to the unusual earthing?

Incidentally, I was going to run this using an overhead catenary wire, but I've just seen that the regs want 3.5m clearance, and that would look daft, involving at least a 1m mast stuck up above the shed, so it looks like I'll be burying it. I presume the 3.5m spec would be part of the Part P inspection, since it is in the regs?

Cheers,

Howard

 

[HDRW]

OK, why did my message get triple-backslashes in front of every quotation mark (single or double) when I posted it? (I've edited them out)

For example:

I\\\'ll

Cheers,

Howard

 

[ryanj]

The forum software being modified, must have caused that irrating bug.

 

[masona]

Normally happen when copy & paste........

 

[mapj1]

The test is if the earth impedance by it reaches the shed is low enough to reliably trip the breakers if there is a fault.
With an RCD you are allowed so much earth resistance (hundreds of ohms) that the voltage drop dominates the sums on cable length instead.. If your house is PME, be careful that adding a new earth in paralell with the existing, could lead to your new earth wiring carrying a fault current for the whole street, or at least getting hot while trying to, if there is a supply side lost earth/neutral.
It is safer, either to extend the house earth only, thius making the shed part of the equipotential zone (and then bosd any services or metalwork accordingly), or to make a local TT only supply, for the shed earths, and not connect your new rod to the house supply earth at all. If your incoming supply is TNS then adding paralell earths is not such an issue.

If this all seems over the top, givemn that a jo-jo reel extension lead would be perfectly legal, you are right, but even though rare, a broken neutral is possible, and should be considered.
regards M.

 

[ban-all-sheds]

I presume I'll get a "Don't do it!" from ban_all_sheds, judging from his ID, but here goes...

Wrong interpretation of "shed"

I'm planning to run power to my workshop/shed, using 4mm2 Steel Wire Armoured (SWA) cable

Might as well install 10mm²....

Now the question I have is: I'll be earth-connecting the house end of the cable's armour, obviously, but should (may?) I also put in a ground stake and connect that to the shed's earth conductors? I realise this is mixing TN - C and TT, which isn't covered in the regs, but does it do any good?

I assume you mean TN-C-S - a TN-C supply is a bit dodgy.

Anyway - I was always under the impression that if you have a PME supply you should not export the earth to an outbuilding, as there might be a PD between the PME earth and the real local earth at the shed, and being an outbuilding, a distinct possibility of you bridging the two with your body.

So earth the armour at the house end, but not at the shed, terminate the SWA in an insulated enclosure, and get the local earth from a rod. Which should be professionally installed and tested.

Or better still - when you dig the trench for the SWA, lay in a long length of uninsulated copper wire or pipe - this'll make a far better "earth rod" than one just banged in.

 

[ban-all-sheds]

OK, why did my message get triple-backslashes in front of every quotation mark (single or double) when I posted it?

Always happens with posts which come from the "How To" pages...

 

[HDRW]

I presume I'll get a "Don't do it!" from ban_all_sheds, judging from his ID, but here goes...

Wrong interpretation of "shed"

Ah - OK. I did wonder (so what have you got against the Shorts 330 aircraft - known in the trade as a Shed )

I'm planning to run power to my workshop/shed, using 4mm2 Steel Wire Armoured (SWA) cable

Might as well install 10mm²....

Errr - what about the fact that it costs twice as much? And it's a *darned* sight harder to bend!

Now the question I have is: I'll be earth-connecting the house end of the cable's armour, obviously, but should (may?) I also put in a ground stake and connect that to the shed's earth conductors? I realise this is mixing TN - C and TT, which isn't covered in the regs, but does it do any good?

I assume you mean TN-C-S - a TN-C supply is a bit dodgy.

Sorry, brainfade - I meant TN-S. To confirm, the main Earth terminal is a clamp on the incoming mains cable sheath. I don't know what my fingers were doing when I typed that. (TN-C is what they do in France, isn't it?)

Anyway - I was always under the impression that if you have a PME supply you should not export the earth to an outbuilding, as there might be a PD between the PME earth and the real local earth at the shed, and being an outbuilding, a distinct possibility of you bridging the two with your body.

Indeed, this makes it dodgy if you're a radio amateur (I am) and have outdoor antenna masts. Luckily I haven't got PME!

So earth the armour at the house end, but not at the shed, terminate the SWA in an insulated enclosure, and get the local earth from a rod. Which should be professionally installed and tested.

Is this still the advice in the light of having TN-S ?

Or better still - when you dig the trench for the SWA, lay in a long length of uninsulated copper wire or pipe - this'll make a far better "earth rod" than one just banged in.

Really? This is likely to be better than a professionally installed Earthig rod? What would be a reasonable minimum length to use?


Cheers,

Howard

 

[ban-all-sheds]

Ah - OK. I did wonder (so what have you got against the Shorts 330 aircraft - known in the trade as a Shed )

Keep trying... I flew in a Skyvan once. Cargo version - the first and last time I've been a standing passenger in a plane. Lord knows how many CAA regs were broken...

I'm planning to run power to my workshop/shed, using 4mm2 Steel Wire Armoured (SWA) cable

Might as well install 10mm²....

Errr - what about the fact that it costs twice as much? And it's a *darned* sight harder to bend!

Well that's true, but the hardest job is digging the trench, and the day you decide you want a 10kW transmitter you'll be glad you don't have to do it again.

So earth the armour at the house end, but not at the shed, terminate the SWA in an insulated enclosure, and get the local earth from a rod. Which should be professionally installed and tested.

Is this still the advice in the light of having TN-S ?

No - export your TN-S earth. Use 3-core SWA - still earth the armour but don't rely on it for the cpc. How far away is the shed?

Or better still - when you dig the trench for the SWA, lay in a long length of uninsulated copper wire or pipe - this'll make a far better "earth rod" than one just banged in.

Really? This is likely to be better than a professionally installed Earthig rod? What would be a reasonable minimum length to use?

Oh yes better - so much more of it would be deep and permanently wet. And as long as you can make it. Irrelevant now though.

 

[mapj1]

CT
de G7VZY,
4mm is OK, unless voltage drop on full load a concern. See cable selector in 'for reference' section at top of bulletin board, and consider length involved.

PS If connecting your RF earths and Mains ones at any point, choke the mains ones by winding the earth core through some ferrites - I recommend the Radio Amateurs guide to EMC, or past issues of RadComic, if you have any probs.
73 OM