portacabin

[cliffed]

jus done a new supply to a portcabin,quite a big site main supply 400 amps 3p,came of local db,wiv 3core swa 16mm backed by 63 amp mcb.Installed tt system,done all checks a ok,Ze .26 ohms,very low for tt,but well expected because of site conditions.Went back there recently & on way home thinkin bout the job,an air conditioning unit had been added to the cabin,wiv cable tray from main building attached to cabin, this being fixed to the main steel sructure of the building brings the main earthing system into the cabin i originally had tt.Will main bonding criteria now need 2 be implemented, like 2 no your thinkin on this thxs.

 

[Deleted member 9648]

You should main bond the incoming AC lines to your portacabin MET if they are introducing an earth potential....is the main earthing system TNS or TNCS?....bit of a grey area with portacabins as TNCS is permitted for a fixed building of a construction site,but prohibitted for caravans or 'similar contructions'......Possibly if TNCS you should insulate the tray from the earthing system of your portacabin.
Not wanting to start another Ra debate but a TT Ze of 0.26 ohms seems unrealistic....are you sure there are no parallel paths back to the main earthing system when you measured?

 

[Guest123]

You would need to ascertain wether or not the tray is now an extraneous conductive part. Could it introduce a potential into building 1 from building 2 and vice versa.

 

[DurhamSparky]

I have achieved. RA of less that 1 ohm before, not impossible

 

[Deleted member 9648]

I have achieved. RA of less that 1 ohm before, not impossible

Specially if you whack the rod through a lead water pipe....:yes:

 

[cliffed]

THe site is a big recyling plant area,deep in a quarry wiv various equipment,earhing rods everywhere,making it a very low resistance for earth,the main supply is tncs, tt =.26 zs at db=.26 also . May have 2 insulate tray offa building,but this was installed by others,& a odd chance that i revisited the site & spotted it.