garden earthing

[johnad]

just out of interest
if i run power to a garden shed for power tools lighting mower etc,
what would the earthing arrangments need to be
i intend to run 3 core 2.5mm swa into a small metal unit in the shed and was intending to use the 3rd core for earthing (i know the armour is used as an earth) all on a 32a breaker in the cu.
i think i have a tt system as there is no main earth on the cutout.
many thanks.

 

[mapj1]

If the supply is TT, you will have an RCD already (I hope) for the whole house at 100mA or less. There should also be an earth rod outside, in permanently moist location.

You need to have another RCD at the shed of 30mA for the sockets, if the house end is not 30mA protected already (which is poor practice, as ther is no discrimination and a shed fault could cut off the whole house, better is to have 100ma Delayed trip to cover everything, and 30mA to cover sockets that migh feed outside.)
You can use the house earth, and yes earth both the armour and use an internal core, but personally I'd add a rod at the shed in parallel, to guarantee to fire the local trip, even if the earth at the house end was lost for some reason.
I presume you'd size the cable with due regard for voltage drop and current rating, so 2.5mm would be 20A max, 4mm for 32A, fatter if the riun is longer and then either bury it, deep enough to avoid disturbance, or attatch it to a rigid fireproof structure (brick wall perhaps), or suspend it from a steel catenary out of reach.....


just out of interest of course. how far away might the shed be....

 

[johnad]

thanks for the info
the shed is only about 4m from the house
and the swa will be about 8-10m total
thanks

 

[mapj1]

OK, so no voltage drop issues, size the cable on current rating only.

 

[Lectrician]

XLPE SWA would be ok at 2.5 on a 32amp, but normall PVC would not.

Bigger is better!

 

[ban-all-sheds]

There's also the idea that you should not export a PME earth.