earthing outbuilding

[weetee1]

Hi all.

I have an out building I am about to take power to from the spare 40 amp non rcd protected breaker in the house consumer unit. THe earthing system is TT.

The shed is 40 metres away from the house and I have got my hands on some 16 sq mm 3 core swa cable and a consumer unit and rcd for the outbuilding. A bit substantial, but you never know what I'll use in the future. I only want to do the job once, though my demands for the forseeable future will be small.

My question regards the earth. Can I carry the earth from the house , using a core, or should I only connect at the supply end and then sink an earth rod at the out building.

I am on top of a hill and the ground is quite rocky and I'm not sure how far I'd get to sink it.

Any ideas would be appreciated.

 

[Jaymack]

I assume that you're using SWA, use the armour as the CPC as a minimum.
Your other options: -
Provide a 3rd core for another parallel CPC.
Install an earth rod or rods, they can be driven at an angle to avoid rock, if feasible.
It would all depend on your existing Ze value - the loop impedance. Ideally this should be less than 100 ohms.
It's preferable to install another rod (or rods) to provide a backup and reduce the value. It's a low cost addition to improve safety.
Ensure that you test the RCD trips OK, preferably using the appropriate RCD tester.
This work would be notifiable to part P.

Regards

 

[weetee1]

thanks for that jay.

Yes, I am using SWA. It is 3 core, so as you suggest I can use the armour and the third core. Should I connect these at both ends? if I am using earth rods at the out building?

I have ordered a loop impedence tester on line and am expecting it soon.

Regards, T

 

[Spark123]

You shouldn't have a non-RCD protected side on the CU on a bog standard TT system.

 

[weetee1]

the c.u. was here when i moved in.

its an mk split load affair. only lights up stairs and downstairs on two 6 amp mcbs on non rcd side. upstairs and downstairs ringmaind on the rcd side on thier own 32 amp breakers.

Lots of spare ways. Gas cooker and gas hot water. All in all low consumption.

Looks like a tidy job. Doesn't look diy. Do you think it is compliant?

 

[Spark123]

If your supply is TT then both sides of a split load CU would normally be RCD protected, the old way was to use a time delayed RCD on one side as a main switch and for the likes of lights, a 30mA device on the other side for the likes of sockets.
That is of course assuming there isn't an external RCD feeding the consumer unit.

 

[weetee1]

hi sparky, thanks for all so far.

No external rcd. So I'll look into obtaining a time delay 30 ma rcd. This will meet the regs? or do I need to have the c.u. changed?

 

[Spark123]

Are you absolutely sure your supply is TT? Can you upload a photo of your CU, supply head etc?
Basically a 100mA time delayed (type S) RCD could have been used in a placcy CU instead of a main switch for the CU to comply with the 16th edition, however we are now on the 17th edition so that might be out of the window!

 

[weetee1]

Hi Spark,

Thanks again.

Trouble on the camera front. Work at sea as an engineer, and dropped it "over the wall", so no pics possible at the moment.

Basically, the live and neutral enter the house from overhead cables with the earth leaving through the wall and clamped to an earth rod in the pavement outside the front door. All the houses in the street are the same. (no idea about thier consumer units though)

I took this to be TT.

Ships electrics are so much more straight forward. Though thank heavens we do carry a "lecky"

 

[Spark123]

If it is a TT system you'd be better having a spark take a look and advise on the best method of resolving this as at the moment it may be dangerous - not enough current can flow in an earth fault to cause the an MCB to operate which is the reason RCDs are normally required for TT systems.

 

[weetee1]

Ill get someone to take a look. Thanks sparky