Can Two Dwellings (Upstairs+Downstairs flat) Share PME Main Earth?

[acvc]

Just been to a subdivided house - now up+downstairs flats. There are two earth rods which go to a single earth block under the floor, then leave the block as two earths up to the C/Us. (Effectively each flat is connected to the two spikes). There is a single mains head with 100A BS1361 fuse which then loops up to another single 100A main fuse (for one flat) and then loops on to another such fuse for the other flat (DNO fitted).

Question: can this supply be converted to PME?
Issues: can a single earthing conductor feed two separate dwellings? I've got distant voices saying in my head that this isn't allowed.

Thanks for any advice.

 

[JUD]

Sounds like you have 3 x 100A fuses in series.

I would say that's a bit of a no no, as there's no discrimination.

Or have I misunderstood

 

[johnboy6083]

sounds like they have looped off the live side of the fuse,so its in paralell. im not sure whether you are allowed to share rods, but im guessing that if you were, then seperate conductors would be needed.

 

[acvc]

I don't think it's three in series, as it goes through the first fuse then to the second - but is looped to the third from the supply/feed side of the second fuse, not from the load side. But even this means there's two 100A fuses in series for each flat.

And it would be a question of whether the shared (first) fuse blew before or after the individual fuses, in an overload situation. So there's the real possibility that an overcurrent/overload on one dwelling affecting the supply to the other. Cheers JUD.

 

[acvc]

Ah, replied at the same time as you johnboy.

Yep, that's the setup. I'd like to just get rid of the whole mess under the floor and PME it. But due to the shared incomer, the PMEd main earthing conductor would be shared up to a block, where it would be split for each flat. I'm sure I've read somewhere that this isn't allowed, but can't find anything about it anywhere. Or is this a figment of my imagination?!

 

[acvc]

By the way, the Ze is good ..... fluctuating around only 7ohms..........

 

[JUD]

You may have read the many threads about not exporting PME to outbuildings.

Never been in your situation, but I found a similar thread on IET forum that may be of help.

IET Forums - Advice needed on EDF TNCS supply

 

[Deleted member 9648]

I would have thought that as there is a single head feeding both flats that is classed as one source of supply therefore a TNCS earth from that source of supply can serve an MET at the intake with a main earth to each flat.

 

[JUD]

sounds like they have looped off the live side of the fuse,so its in paralell....

It's just occurred to me, would this be allowed as there would be no overcurrent protection for the cables that are looped off.

 

[johnboy6083]

yeah its protected by the suppliers cut out. its the same as your tails in a normal cu. you can t off them in a henley block. this is effectively the same thing.

 

[JUD]

yeah its protected by the suppliers cut out. its the same as your tails in a normal cu. you can t off them in a henley block. this is effectively the same thing.

Which brings me back to my first post. As it's feeding another 100A fuse it is wired in series (not parallel) and there is no discrimination between the 2. If the downstream fuse was 60 or 80A then there wouldn't be a problem.

Normally when you split the tails in a Henley block you are feeding other DBs and not a second 100A fuse.

Are you with me?

 

[Deleted member 9648]

The OP concerned whether or not a PME earth can be taken for two flats from a single head.

 

[johnboy6083]

lets say that you have a cable running from a to b, rated at 200a. you then want to tap off that at various points, to houses. you would joint the cable and t off at these points with whatever size cable needed for the load. at the nearest convenient point, you would supply protection to that cable (suppliers cut out).

each of those tap offs are in pararell with the others.

if you had the same set up, but above ground, then do away with the joints, and loop the cable at the cut outs, and that is what it seems like has happened at thes post. the original 200a cable is protected elsewhere( at the sub, or lucy pillar ect)

 

[JUD]

If a 100A fuse is protecting a cable that is feeding another 100A fuse then it is wired in series and there is no discrimination between the 2 fuses.

If a 100A fuse is protecting a cable that is feeding a 60 or 80 fuse then it is wired in series, but this time the 60/80A fuse provides discrimination.

 

[acvc]

Thanks for your interest chaps. Here's a couple of pix i took. The first is the shared mains head. The second shows how the live split, looped on and protected by the fuses. And you can see the neutral split in the adjacent Henley block.

Thanks JUD for the IET forum posting. I'm still going over it.....

I suppose one fact here about sharing the PME earth conductor is that both dwellings have full access to it as it's in the shared entrance hall. So there are no issues about access and safety.