TT to shed again

[Zeespark]

Hi all, Just need a bit of advice if poss.

I have a 3 core 4mm SWA run to My shed from My house 30 metres up My garden. Its a 16th Ed split load board (spare ways available on either side) Ze is 0.18 ohms TNCS system. The ends or not terminated at all.

I was going to bung an earth rod in at the shed end.

This is where the plot thickens and need your advice.

Me mum wants a second shed for excercise equipment etc, she only wants a couple of light fittings in it. I was thinking of running a 2 core 1.5 swa (the distance between sheds is about 7 metres) and again TT the earthing. Would one single rod be acceptable or would a rod per shed be required??

Many thanks

Andy Guiness Drink

 

[Apache]

4mm? 1.5mm?

some like 10mm or bigger - you wait

You can have as many rods as you like AFAIK

 

[steptoe]

1st things first,

you need to gland your SWA at one end, preferably the source, otherwise you may as well use a bit of flex.

I hope you have read the thread on NOT exporting PME...?

I would run a 2.5 to the other shed, 2 core will be fine, and gland at both ends, depending on Z reading 1 rod may be enough.

not doing any calcs or ****, is your 4mm protected by a 20a RCBO/MCB with 30mA protection? if its RCBO it MUST be DP.

30m is quite a long run, and now it will be 37, so would prob need to be protected accordingly, perhaps 16 or even 10 due to volt drop and Z readings.

any metalwork involved?

 

[Janice]

Zeespark, listen to Steps. What he doesn't know about earthing arrangements isn't woth knowing.

J.

 

[Zeespark]

I have done My calcs Steptoe, I just havent ever worked on TT in a domestic environment is all. All thats going in Me first shed is a few sockets and a strip light.

The question was more a case of needing one or 2 rods in My situation is all

AndyGuiness Drink

 

[Zeespark]

No metal work, conctrete slabs and wooden sheds. I know Janice, steps is the reason why I ask about this stuff without people taking pot shots at me

AndyGuiness Drink

 

[steptoe]

Zeespark, listen to Steps. What he doesn't know about earthing arrangements isn't woth knowing.J.

LOL Janice,

I am NOT an expert on earthing,

perhaps I have just worked on a few more TT and exported a few more systems due to my working on some more countrified systems doesnt make me a be all and end all.

 

[steptoe]

ok, well if your 2nd shed is close to your first shed,(ie 7m) then your sheath on the armour if you use 2.5 (I personally wouldnt use anything less) will be more than enough to cope with the fault current off a 16a MCB and to also ensur a good Z reading.

I would really recommend tho that you gland the SWA at the house and ensure it is isolated at the first shed.

also puzzles me as to why you have used a 3 core when you cant use the earth,? dont be tempted to join it as well as the rod, it will actually make things a lot worse if a fault develops

 

[Zeespark]

No but it does make you a electrician because of it Steps

Andy Guiness Drink

 

[steptoe]

thank you Andy, can I have 1 of your beers,?

bet I get some hammering over this if I ever go to one of these meets.!

Im gonna pretend to be someone else. lol

 

[Zeespark]

No beer steps as I have none . No you wont get a hammering, at least off me, in fact I have been thieving a few pages out of your book actually

AndyGuiness Drink

 

[sparkyt]

ok, well if your 2nd shed is close to your first shed,(ie 7m) then your sheath on the armour if you use 2.5 (I personally wouldnt use anything less) will be more than enough to cope with the fault current off a 16a MCB and to also ensur a good Z reading.I would really recommend tho that you gland the SWA at the house and ensure it is isolated at the first shed.

also puzzles me as to why you have used a 3 core when you cant use the earth,? dont be tempted to join it as well as the rod, it will actually make things a lot worse if a fault develops

Just a quick one- would you find it reasonably accepatable to export tt from a central location to various outbuildings, relying on the one rod at the origin providing zs is low enough and cable calcs ok at outbuildings?

 

[steptoe]

Just a quick one- would you find it reasonably accepatable to export tt from a central location to various outbuildings, relying on the one rod at the origin providing zs is low enough and cable calcs ok at outbuildings?

thats what the DNO does for PME, and I suppose to a lesser extent TNS also.

and they use one rod for countless buildings, thats why you can pull in a fault from a neighbouring property on PME, its an easier path for it to take than going all the way back to the transformer.

there is much more to it than just exporting, each job would have to assessed on its own merits, so sorry, no short answer.

whats the loading?

whats the distance?

whats the ground resistance like?

these are just 3 of the points that need to be taken into consideration when you think about exporting an earth of any kind

(TT/TNS are probably the only two most people will come across that can be safely exported)

the thing with earthing is that your earthing conductor can actually be at a different potential from the actual ground if it has been exported any distance from the source.