PME

[Maceman]

I'm buying a house and having a total rewire. My electrician has asked me to contact Western Power to arrange to have PME installed (he said it could be there but not being used)

Western Power have said that we cannot have PME unless next door have PME also - something about the two systems being linked. They want access to both houses to check the systems so I'm going to ask the neighbour for their consent. Hopefully they have PME already so it won't be a problem. If they haven't and they don't want it, WP say that we will have to have an Earth Spike. . .

My electrician says that this is not a good idea and that if all else fails I should offer to pay for PME installation at my neighbours house (£59 each house)

Does anyone have an opinion on this please? Does this sound right or not??

Thanks!

Maceman

 

[Adam_151]

Sounds right to me, TNC-S (PME) is quite a compromise and bonding is more important than on other systems due to circulating network currents, earth fault level is generally higher than any other system which means undersized CPCs (earths) in late 70's ring mains (they were 1mm instead of the modern 1.5mm) *could* cause problems.

The problem you have, is if you and your neighbour share a metal water pipe... becuase water pipes are bonded to the MET (main earth terminal) then your neighbour will get TNC-S via their water pipe, if their installation isn't really suitable for TNC-S in its own right thats a problem, but delivering it via the water pipe would cause even more problems (not least thats its officially TT, but not)

You could make the system TT (earth rod, and overall time delay RCD) again its another compromise and can be quite a nice system and is actually prefered on installations like farms (animals have bare feet and are more sensitive to small voltage differences than humans, and petrol filling stations (you don't want circulating network currents through tanks full of petrol for obvvious reasons)) on a house while TNC-S is generally preferable, your sparky might be being a tad harsh on TT, modern RCDs are much more reliable than the old ELCBS. Though if your electricity comes in overhead by dodgy looking cables that have been repaired in the past with screw connectors, etc and pass through trees, then I'd give serious thought to TT

FWIW, I consider TNS to be the only umcompromised system, however the installation costs are higher and so they don't install it anymore, and if the street main has no earth conductor, it'd be impossible to install it anyway

 

[Maceman]

Thanks Adam,

I don't think the CPC's should be a problem, as every bit of wiring is being removed and and replaced.

I'm now even more confused though (to be honest I know virtually ziltch about electrics!!)

My main concern is making everything nice and safe - if we go along with the PME are we safe??

Thanks

Maceman

 

[Adam_151]

Does your electricity come in overhead or underground?

In your situation I think I'd probably go for TT (earth road and RCD) to save messing around, yes the time delay RCD adds a bit of cost but probably no more than 2xPME upgrade, and if I was in the position of your neighbour I wouldn't want to feel compelled to have my service changed round if it meant ([for example, and its not going to be that likely in practice] that I'd have to rip a 2.5/1 ring out and install 2.5/1.5) etc*

Have you considered a new PVC water service pipe? then you'd get rid of the link, I'm assuming the gas is plastic?

*Also people like radio amateurs, etc sometimes want TT

The thing to remember is that both TNC-S and TT are compromises in the way that they don't use an earth wire out in the street... TNC-S (PME) uses the neutral wire, TT uses the physical earth, and they both have disadvantages and advantages, but some people are too harsh against TT, probably they remember the old voltage ELCBs too well... but these days we use RCDs, TNC-S also the order of the day since DNOs (electricty companies install it everywhere these days instead of TN-S (the system with a proper earth wire back to the local substation) ... cheaper ya see...) they also sometimes provide it (TNC-S) on old TT supplies where the integrity of the neutral is not so great (they are not known for sticking exactly to the rules!)

 

[Maceman]

(I think electric supply is buried)

I'm starting to understand now (finally!!)

If a water pipe is what links our electrics and I replace it with a pvc one, then I would have a seperate system which could have PME installed??

I'll speak to the neighbour, and my sparky and WP and hopefully get it sorted.

Thanks for your advice - I'm happy to pay experts to do their job, but it's good to have a bit of understanding about what they're doing!!

Thanks again

Maceman

 

[Spark123]

A problem which you may have overlooked is it is fair enough to pay the £59 to upgrade your neighbours (TT?) supply to PME but what about all of the problems this will pose, you may not get off to a good start with them if you end up making your neighbours pay to have their house wiring (bonding etc) modified too.

 

[Maceman]

Nothing is ever straight forward!!!

I think this is going to be more of an issue than I first thought. I think the first step is to go and talk to the neighbour and see if they know what sort of system they have. I think they have also been renovating recently, so maybe they will have upgraded they're electrics already.

I suppose I will have to go along with whatever is most practical.

Thanks for the advice!

 

[Lectrician]

WPD Charge us £18 a time to PME!

 

[securespark]

your neighbour will get TNC-S via their water pipe,

It's not been allowable to source an earth connection from the water pipe since 1966...

The TNC-S connection will come directly from the neutral, which the DNO will bond to earth at the other end.

In my neck of the woods, the DNO's charge 100+VAT to do it, so in my mind, it would be worth paying your neighbour's fee too...

If you have it PME'd the chances are you will have to ensure MEB's are up to scratch.

To my mind, you are better off with PME. It has a much lower Ze reading than TT, and you can do away with an earth rod and TD RCD. It can be a pain in the rear to fit an earth rod and get a reading less than 200 Ohms.

The issue about cpc's in 2.5 being 1mm2 not 1.5 is only really an issue if the Zs is too high for the prot device.

 

[Steve]

your neighbour will get TNC-S via their water pipe,

It's not been allowable to source an earth connection from the water pipe since 1966....

the point i believe adam was making was that the OP's TNCS earth would be bonded down to the water pipe, which could transmit it next door. If next door has a TT supply, this is not the best thing to do, so the DNO wants next door to get TNCS too. . .

. . . Though the flat above my shop has no fewer than THREE earth connections. One to the sheath, one to the exposed earth block on the mainfuse, and an EARTH ROD! (rod looks like it was added later - cable is striped as opposed to solid green)

 

[Adam_151]

your neighbour will get TNC-S via their water pipe,

It's not been allowable to source an earth connection from the water pipe since 1966...

The laws of physics do not read BS7671 craftys got it, if you share a metal water service with someone and you have PME and they don't and everything is correctly bonded then their MET will be connected to the neutral in the street main by way of metal pipe and bonding cables and your PME terminal. This is why the DNO has to avoid giving either PME without giving it to the other

 

[Spark123]

The issue about cpc's in 2.5 being 1mm2 not 1.5 is only really an issue if the Zs is too high for the prot device.

iirc it is also to do with the CPC not satisfying the adiabatic equation when certain protective devices are used (i.e. rewirable fuses).

 

[securespark]

Yes. That as well.