I have a big shed and its going to be used as an electronic repair workshop. Going to have some soldering equipment , low wattage led lighting , some low wattage test equipment and a 2kw heater . The cable run by SWA Cable would be 70ft ( 21.33) Meters from consumer unit and I was wondering since the supply in the house is a TN-C-S ( PME ) supply, would the length permit using the CPC earthing from the house to the shed via the armour on the SWA Cable, or would I have to make the shed a TT supply and use a suitable earthing rod which would keep the earthing potential separate from the house earthing . Many thanks
Are you a tradesperson and able to answer this question?
The question can only really be answered by the technical services department of the Distribution Network Operator (DNO) area that you're in. However as a general rule, due to the risk of loss of neutral under fault conditions, it would be better to have a two core SWA running to the shed and an RCD with a local earth rod at the shed end.
22mtrs is not a long run at all and should be perfectly OK to operate as a normal radial supply off of your D-B dependant on earth loop readings of course
This would be notifiable works also to be pre notified to LABC or carried out by a registered electrician to be compliant as per part P regulations as I am sure you already know.
from the information you have above this will be OK run in 3core 6mm SWA protected by a 32A MCB at the distribution board DB2 with 6A+20AMCB's at the workshop. (lighting being negligable option to uprate 20A to 32A) Do not forget the requirement for RCD protection.
If the area has no extraneous conductive parts (Metal water pipes etc) then I would use the earth supply from SWA.
I doubt you will achieve the same earth fault loop reading from the earth rod as you would from a TNCS system.
Have a Google search on the topic, you will find many mixed opinions!
regards
You are not allowed to distribute TNC-S outside the equipotential zone so unless attached to the house sheds must be TT if its a TNC-S supply
The usual way is to earth the armour on the cable with the PME then break this and ~TT it at the shed.
But please get a professional to do it, if you didn't know the answer then you don't have the skill to do the job.
