Separating PME & TT earthing for outbuildings

[Msitekkie]

I would appreciate peoples thoughts on fixing this earthing setup to correctly separate PME & TT:

Setup: Workshop at the bottom of a long garden connected to supply via 3 core armoured cable. Earth rod fitted at workshop, but the earth has also been connected to the PME from the supply & the armouring is earthed throughout.
Just to complicate matters further, a No2 Pratley box (Metal) has been used to T off the armoured cable in a garage located between supply & workshop.

More detail: Both outbuildings have individual consumer units & have concrete floors & block walls. Floor in the garage tends to be damp. SWA is partly buried, partly clipped & connected to a consumer unit in the meter box with 40A RCD (not S type unfortunately, but that doesn't affect the earthing & DNO seem happy with CU in the meter box).

Questions/thoughts:
I am aware TT & PME should not be joined, so would begin by disconnecting the SWA earth core at the supply end. Probably also in the Pratley box on the supply side. Leaving the TT earthing connected between workshop & garage.
I am not so sure about the armouring... If it was just the workshop I would just isolate the armouring from the consumer unit there. Would you just do the same with the garage consumer unit and leave it at that?

Additional thoughts:
Would you expect the garage to need it's own earth rod?
Pratley have told me they can supply "non metallic glands" for their box so maybe for the garage consumer unit I could isolate the armouring where it comes out of the Pratley box rather than at the consumer unit end of that short leg (on the wall). Otherwise at the CU end like the workshop.

Summary: So hopefully I would end up with the circuits in both outbuildings TT earthed, but the armouring PME earthed (& isolated from TT) except the short leg on the wall from Pratley box to CU which would be TT earthed.

 

[marcuswareham]

In John's example there are no exposed conductive parts, so it's a different scenario & little risk involved - the foundation earthing mats he mentioned briefly, I understand may become a thing in the next version of the regs - necessary I gather for "Islanding" purposes if you want to take a solar installation off-grid when you have a power cut. I commented on that video asking John to demonstrate the SWA isolation in another viseo, as I haven't seen any of the usual YouTuber suspects do it, but JW hasn't responded (as yet anyway).

David Savery has isolated SWA amour in 2 of his videos, when changing from a TNCS earth to TT, one in his EV charger video and another in his hot tub video, In both case a plastic box was used, As iv said iv used a plastic module enclosure before as it also gives me space for the fault protection RCD,

As mentioned by timhoward using a plastic box does help to provide more seperation between the 2 earthing systems and also makes it harder / less likely they will be connected in the future by mistake, etc

At a friend's house he has the SWA terminated in a plastic junction box, and then some plastic conduit between the junction box and a metal clad socket, the amour stops at the gland and is isolated but the inner cores continue through the conduit to the socket, looks like a nice neat way todo it and without any extra cable joints

 

[pc1966]

I would use a 25mm plastic gland (they IP68 things) as it seals/clamps the cable and allow it to remain isolated. Sure it is not going to deal with the same sort of pull-forces that a SWA gland would, but if the cable is adequately clipped/cleated to restrain it then I don't see any issue.

Allows a metal box if you want, as that would be on the TT earth then, so all that remains is to make sure there is little risk of a sub-main L to E (TT side) fault inside the box.

If you are really paranoid, have the SWA fed from a 100mA delay RCD, then if the rod Ra is low enough (and it ought to be 200 ohms or less anyway) the supply would trip on that fault, and the final circuits on 30mA double-pole RCBOs like the Wylex/Crabtree/Fusebox compact ones would be selective and offer the additional shock protection needed.

Finally if it is a workshop I would look at some sort of emergency lighting in case the power fails for any reason, as being plunged in to darkness with sharp tools all around is not nice!