A(not her) shed question

[syl]

I need to get power to sheds - I've done a lot of searching and I will be getting an electrician round to do some of the work, but is there anything I haven't considered?

I have a TN-C-S supply.
I have a high integrity consumer box with a spare non-protected circuit
I have 3 sheds. I'd like a light and a double socket or two in each. I suspect a maximum of only about 10A would be used at any one time, in total across all three sheds, and usually much less (the first shed is a rabbit hutch, the third is an observatory) all won't be using power at the same time.

I perhaps need 10m of cable to get from the consumer unit to where it leaves the property
The first shed is 5m from the house
The second is another 30m
The third is another 5m

The plan:

Use spare non RCD circuit with 32A MCB
Either 10mm T&E in metal trunking or SWA to edge of property
10mm 3 core SWA 450-600mm underground in 50mm ridgicoil trunking
Metal adaptable box in shed connecting the SWA, via T&E, into garage 30ma RCD incomer (with 16A radial circuit MCB and 6A light circuit MCB) and SWA back out to next shed. Same again for each shed.

The 3rd core of 10mm SWA used to export/bond earth (however no pipe work etc in sheds). I believe this is is the right size for TN-C-S if there is external metalwork (there isn't now, but you never know). As 10mm will be needed anyway for a 50m run with lighting based on 22A (I'll not use 22A, but just in case), I may as well run 3 core rather than have a pants TT earth.

One question: I believe I shouldn't use two of the same RCDs in series, hence using a plain MCB at the house. An alternative is to split with a Henley Block into a second CU with a 100ma RCD incomer which would discriminate. Is it worth protecting SWA in this way - any cut to the cable will automatically be earthed through the sheath anyway. Would that just be a waste of money (the parts are going to be expensive enough anyway).

I know it's part P notifiable (new circuit) so I will be getting an electrician in to connect the circuit up. I'll dig the trenches, lay cable and wire up the sheds and pay the electrician to check that over too. Might have to get him to gland the SWA. I'll get one in before I start in case he/she (are there many female sparks yet) wants me to do it differently.

 

[syl]

I meant another, not a not her! Sorry.

 

[bernardgreen]

If you take the supply from an RCD protected MCB in the house CU then a fault in one shed will remove power from all sheds. You could be working in one at night when a fault in another takes out the power plunging you into sudeen darkness. That is a safety hazard.

Take the power from a MCB on the non RCD protected supply to avoid that.
Then each shed has its own RCD and two MCBs, one for lighting and one for power sockets.

Or have the electrician fit Henley blocks and have a switch fuse separate from the CU to supply the SWA feed to the sheds.

Or have a separate CU with 3 MCBs feeding three runs of SWA to the sheds.. That will ensure a serious over current fault in a shed will not black out the other two ( unless the main supply fuse goes out as well )

You also need to decide with the electriciann what "earthing" will be used in the three sheds. If you use ground rods, one each for each shed, then you only need 2 core SWA to feed Live and Neutral to the shed on one fuse in the house. or 6 core if you have 3 separate feeds to each shed.

 

[syl]

Thanks. I was planning on using the unprotected side of the high integrity board. As you say, each shed will have it's own RCD anyway.

The question was whether I should be bothered about not having RCD protection on the SWA cable, or is it a waste of time anyway given that the cable outer is earthed? If thought useful, I could use a larger, even time delayed, RCD in the house on a second CU connected via Henley block as you described. A small trip in the shed should trip the shed RCD first.

Three runs out is expensive in cable and conduit, although I have considered it.

Why would I need 6 core? There would be no advantage over 3 core would there - even if each shed had seperate earth cable (and they weren't all tied together in the adaptable boxes) all of the earths would be tied together at the house anyway?

 

[EFLImpudence]

Thanks. I was planning on using the unprotected side of the high integrity board. As you say, each shed will have it's own RCD anyway.

That will be fine. Unprotected by the RCD.

The question was whether I should be bothered about not having RCD protection on the SWA cable, or is it a waste of time anyway given that the cable outer is earthed?

That is correct.

If thought useful, I could use a larger, even time delayed, RCD in the house on a second CU connected via Henley block as you described. A small trip in the shed should trip the shed RCD first.

Not necessary.

You should really discuss it with the electrician.

 

[bernardgreen]

Why would I need 6 core? There would be no advantage over 3 core would there -

Assuming you are using ground rods at the sheds and therefore not exporting the PME "earth" from the house each shed will need only Live and Neutral. If each shed has its own fuse in the house to protect the SWA then 6 cores need to leave the house. A metre of 4 core SWA is less expensive than two metres of 2 core.

From the house I would use a 4 core of 35 metre length to the middle shed and a 2 core of 5 metre length to the first shed. At the middle shed 2 of the 4 cores feed that shed and the other 2 connect to a 2 core of 5 metre length to feed the third shed.

 

[syl]

Ah, I understand now, sorry - didn't read it in continuity with your ground rod sentence before.

I didn't want to use rods, I'd rather export the earth with the 10mm third core. If people think it useful, I could sink some "extraneous conductive metalwork" (like an earthing rod) and bond to it, but otherwise there won't be any. I'd rather avoid a TT system.

 

[securespark]

Use SWA underground, but why trunk it?

And where you connect up why joint and swap to T&E? Either gland off to a metalclad board or an adaptable box or conduit box butted up to the board(in the case of an insulated board) and take the tails right through without jointing them.

 

[syl]

Trunking in case it ever needs replacing - the main wire to the house blew a few months ago and trunking saved them ripping up the driveway!

The reason for the T&E was because I read it wa a pig to join the SWA directly, and it will need joining to the outgoing (loop) SWA as well in the first two sheds. If it can go straight through (x2) after being banjoed at the daptable box then great.