TT system converted to PME - Earth rod query

[South Spark]

I've just installed two 100A isolators in a farmbuilding and the nearby house. The earthing arrangements seemed unusual.

The power comes in overhead and a single cable drops into the main head. This has PME head and a label on the board stating that there is Protective Multiple Earthing. The building also has an earth rod. ZE here is 0.17.

From the top of the PME head emerges another cable which then goes (presumably underground) to the house where there is another head with a 60A fuse. There ZE is 44.9 - and there is no earth rod visible. Everything there points to a TT system - Old Wylex rewireable fuse board protected by single RCD. Given that there is no earth rod visible and it's possible that the place is earthed via the bonding to the water pipe I've advised that an earth rod be installed.

The question is - by introducing earth rods into a PME system am I creating any problems?

 

[JD6400]

Is it a livestock holding , or just a ex farm

 

[South Spark]

No - no livestock. (is that the 100ma RCD rule?) In effect as far as the farm electrics goes I'm installing an isolator and disconnecting everything else. They're having a car charging point installed (comes free with the car apparently) but the installers won't do it without an isolator present.

The electrics in the farm buildings is a spectacular shambles and basically needs stripping out and weighing in.

 

[JD6400]

No it was more aimed at needing to be TT if it was a livestock holding , if not a livestock holding you will still probably need a 300mA max to cover the agricultural side of things .
It sounds like the house has just got a bad connection possibly and it could be pme as well , I would raise this with the supplier .
Does the house have any pme stickers or any signs to indicate it being pme

 

[JD6400]

Just re read the OP and no it will not create a problem , if anything it will help safe guard against a lost neutral in the very rare advent of it ever happening .

 

[Marvo]

The question is - by introducing earth rods into a PME system am I creating any problems?

As per JD6400 I wouldn't see a reason you can't have local earth rods connected to the MET of a PME installation.

 

[Engineer54]

What type of cable is feeding the farmhouse, and does it conform to PME requirements (eg 3 core SWA, Split Concentric, other)??

I ask because there may well be a lost or loose earth connection to the farmhouse. If you have a 0.17 ohm Ze at the source then the Zdb at the farmhouse should reflect that value....

To be honest the initial DNO's service head shouldn't be feeding another head in another building (farmhouse). The farmhouse should be being supplied via a fused sub-main on the consumers side...

Also if it is a second DNO head, make sure there is not a N-E connection within.

It's actually advantageous to have a local earth rod connected to the MET/EMT, but it really needs to have a value of under 10 ohms to be of any real purpose. In many countries on the Continent it's actually a rule to have a local earth rod when a PME supply is provided....

 

[Farmelectrics]

jd why would it need to be tt system

 

[JD6400]

jd why would it need to be tt system

Hi Farmelectrics

It has not got to be tt , but without fail on all of the agri installs that I have ever come across , the suitable measures that need to be taken in order to allow PME to be used , have not been implemented .
So I know I was being a bit presumptuous to say tt is needed , but apart from it not really being recommended even if you could use PME , the likely hood of being able to use PME would be almost nil in my experience .

 

[Engineer54]

Typical, the OP just disappears without answering questions. Probably leaving the house with a Ze of 44 ohm too..... not part of his remit of fitting a car charging point!!

 

[South Spark]

"Typical" - typical of what exactly? me or OPs in general? Didn't disappear but I have got work to do!

I left the house with a reading of 36 - well below the 200 limit.

"To be honest the initial DNO's service head shouldn't be feeding another head in another building (farmhouse). The farmhouse should be being supplied via a fused sub-main on the consumers side... " I agree, hence the original post.

The cable that emerges from the DNO head looks like a two core braided type. I don't know what exactly.

 

[davesparks]

"Typical" - typical of what exactly? me or OPs in general? Didn't disappear but I have got work to do!

I left the house with a reading of 36 - well below the 200 limit.

"To be honest the initial DNO's service head shouldn't be feeding another head in another building (farmhouse). The farmhouse should be being supplied via a fused sub-main on the consumers side... " I agree, hence the original post.

The cable that emerges from the DNO head looks like a two core braided type. I don't know what exactly.

Why shouldn't the service head have another cable going out from it to another service head? The DNO can install a shared service like this if they see fit, it's unusual these days big nothing wrong with it.

 

[NickD]

The cable that emerges from the DNO head looks like a two core braided type. I don't know what exactly.

Are you sure it's not armoured cable with the armour trimmed way back and the earth is meant to be being provided by the armour?

 

[Engineer54]

"Typical" - typical of what exactly? me or OPs in general? Didn't disappear but I have got work to do!

I left the house with a reading of 36 - well below the 200 limit.

"To be honest the initial DNO's service head shouldn't be feeding another head in another building (farmhouse). The farmhouse should be being supplied via a fused sub-main on the consumers side... " I agree, hence the original post.

The cable that emerges from the DNO head looks like a two core braided type. I don't know what exactly.

Really, Where does it say that, because to the best of my knowledge BS7671 states nothing of the sort, nor does any of it's GN picture books!!

So what you're saying is, that you have a PME 0.17 ohm Ze at the main service head that isn't being utilised anywhere else on the installation. Have you informed the customer of this??

 

[South Spark]

What value ZE do you regard as suitable for a TT then and why?

Yes of course I have informed the customer of this - his energy supplier is making a visit on Monday to see if the farmhouse is suitable fro an upgrade to 100A. Sorry if I didn't make that clear boss, it'll just have to come out of my wages I guess.