PME and TT?

[Mattja]

Just checked the L-N loop at a fusebox and got a reading of 0.12ohms (great! ), so going to call up and try to arrange to get a PME system put in. Anyway I was just going though 5.4 regs and didn't spot where (assuming it's in there) it says if it's ok to have both PME and TT earth connections (going to the same CU), anyone else familiar with this or found it in the big book?

Basically do I need to get rid of the connection to that earth rod, or just hide it while the guy is on site to install the PME?

Little background on the place: it's currently got a whooping 800 odd (something up there, why count rotten eggs?) earth reading on a TT that as far as I can tell is a rod buried in mostly builders rubble. Sure that's still well under the 1667ohm mark, but of course there's no rcd in there and 850ohms on an rcd is still rubbish. The ground is so bad putting more rods in would be like throwing **** at a fan, long back-breaking ****s at a hard dry fan.

 

[Taylortwocities]

Leave the earth connection to the rod, although it is not going to make a blind bit of difference if there is no RCD. An earth fault wouldn't even trip a 6A breaker. It would liven up all of the pipes in the house, however.

I'd be disconnecting and slapping a dangerous installation notice on the whole shebang.

NO RCD on a TT? That's very unusual, isn't there even an old VoELCB there?

 

[Deleted member 26818]

Why would there be something in the Regs to say this is ok?

 

[Taylortwocities]

Sorry, I'm a bit hard of understanding tonight, can you elaborate please?

 

[Deleted member 26818]

Why would someone expect to find somewhere in the Regs that it is ok to have both PME and TT earth connections (going to the same CU)?

 

[Risteard]

Why would someone expect to find somewhere in the Regs that it is ok to have both PME and TT earth connections (going to the same CU)?

You don't have both TN-C-S and TT earth connections. You have a TN-C-S installation with a local earth rod. (Indeed many other jurisdictions require this.)

However it seems fair to say that there isn't a particularly effective earth rod in this instance.

 

[Mattja]

Why would someone expect to find somewhere in the Regs that it is ok to have both PME and TT earth connections (going to the same CU)?

Well I figured that it's effectively no different to bonding to structural metalwork, so should be just fine to leave it, but on the other hand it's another bit of live metal in the event of an earth fault... although with PME that should only be for a moment. Basically I didn't particularly expect it in there but maybe I'd missed it *shrug*.

@Taylortwocities I think it must've been a DIY job (by someone who noticed no earth and thought earth rods were magic wands you stick down to solve all problems), it's a 6mm copper wire to the rod and everything else is aluminium. At a guess I'd say a sparky has never set foot there before, including during construction!

 

[Deleted member 26818]

There is nothing wrong with having both PME and TT earth connections at the same CU.
Whether someone from the DNO will make an issue of it, is another matter.
Yes it would be no different to bonding extraneous service pipes.

 

[Risteard]

Again, you don't have a TT earth connection with an earth rod in a TN-C-S installation. It is not a TT installation then but simply another point where the PEN conductor is earthed.

 

[Deleted member 26818]

Strictly speaking BS7671 allows there to be different earthing systems within a single installation.
Specifically the installation of TT in a TN system.
However in this instance, the consumer's rod will become just another earthing point for the PME.

 

[Mattja]

Alright yes, it's no longer TT, but for the sake of communicating the situation here it's just a convenient term (although misused, granted), sorry if it bothers you Risteard.

@spinlondon yeah I don't think the DNO will bother if they think there's an earth rod in, so I'm just going to take it out of the fusebox before they turn up, hide the wire while they're there, and stick it back in afterwards.

 

[sparksburnout]

The DNO won't give a ---- about it, they won't go near the CU anyway. I presume you have had them out to check if PME is available? If they have given it the OK beforehand then they will just crack on. However, some of the other guys on here might have more experience in this area as I would have thought if you are going to retain the ROD it may well end up carrying large currents under PME fault conditions and therefore have to be up to spec, so it might be better to just ditch it.

 

[Midwest]

I'm confused, easily done. What is the type of supply? A L-N loop? Is that a prospective short circuit test? Why would a reading of 0.12ohms not be acceptable? I'm led to believe that an Ra above 200ohms is unstable (GN3), so what would be the point of retaining the earth rod? Why bond the earth rod as an extraneous conductive part, with a reading below 1667ohms (if circuits have RCD protection). Genuine questions.

 

[westward10]

I'm confused, easily done. What is the type of supply? A L-N loop? Is that a prospective short circuit test? Why would a reading of 0.12ohms not be acceptable? I'm led to believe that an Ra above 200ohms is unstable (GN3), so what would be the point of retaining the earth rod? Why bond the earth rod as an extraneous conductive part, with a reading below 1667ohms (if circuits have RCD protection). Genuine questions.

I assume he did a L/N test (0.12) to see if it could be PME'd as that would give you some idea of the Ze if PME was put in place. Whole thing rather confusing it ain't just you.

 

[Midwest]

I assume he did a L/N test (0.12) to see if it could be PME'd as that would give you some idea of the Ze if PME was put in place. Whole thing rather confusing it ain't just you.

Wonder what the Ze (as is) was then?

 

[westward10]

Wonder what the Ze (as is) was then?

I can only assume it is the 800 ohm figure he quotes.

 

[Midwest]

I can only assume it is the 800 ohm figure he quotes.

Arrrgh, re read OP again and bit clearer, thought he had DNO earth AND earth rod. See wot he's saying now. Told you I was easily confused

 

[Leyland30]

The earth rod sounds like it's stuffed. Bs7671 suggests anything over 200 ohms will be unstable.
Is a pme available if the system was previously TT?
Abscence of rcd is concerning.

Tread carefully Mattja - let us know how you get on.

 

[Leyland30]

The earth rod sounds like it's stuffed. Bs7671 suggests anything over 200 ohms will be unstable.
Is a pme available if the system was previously TT?
Abscence of rcd is concerning.

Tread carefully Mattja - let us know how you get on.

 

[sparksburnout]

The earth rod sounds like it's stuffed. Bs7671 suggests anything over 200 ohms will be unstable.
Is a pme available if the system was previously TT?
Abscence of rcd is concerning.

Tread carefully Mattja - let us know how you get on.

Mattja..........Mattja!!! Helloooooo!!!!!! I guess hes going to get back to us later?