A good guide For 'exported PME' and outbuildings!

[cmac91000]

Thanks for this, very useful and I understand what has been explained about the myths of exporting PME.
I am connecting a garage 45m to the CU in the house on a TNC S and have decided to connect by means of 2 core armoured cable. I shall be adopting a TT system in the garage with an earth rod. The armour will of course only be connected at the house CU.
My question is about discrimination of the RCD's. The MCB for the garage, in the house is protected by a 30mA RCD and the socket circuit in the garage requires a 30mA RCD. What is the best way to achieve discrimination of the RCD's? If I replace the house RCD with a slow 30mA would this contravene the regs on the other circuits protected by it. thanks

 

[Engineer54]

What do you mean by ''Slow 30mA'', do you mean an S type RCD?? How are you going to do that then?? Any other circuits on that S type RCD will not meet BS 7671 requirements!!

Either a non protected MCB in the CU, or Henley Blocked tails with a 100mA S type RCD.

And if there isn't any extraneous services at this garage, then you would be much better off, taking the PME earthing with you from the house!!

 

[mark_]

Can someone please fill in the blanks for me as I'm really interested in this topic but strugling with a few bits.

If you have an outbuilding which you install Db2 with say lighting and power radial circuits as part of a PME system then any copper water bond would have to be connected in 10mm to the DB1 MET.

Now as you have sized your submain to DB2 as 4mm, rather than running a 10mm back to the supply origin you elect to make the out building a TT system. You connect your spike with 6mm and connect to DB2 (all going well) but then there is the copper water pipe. Minimum bonding conductor is 6mm (pretty sure half of line or neutral conductor or 6mm minimum for non PME) but does it matter that this is larger than our line and neutral conductor?


It just seems a bit odd with a 6mm earth and a smaller line and Neutral. If ive got this way wrong please dont bite my head off.

 

[IQ Electrical]

If you don't understand something, just ask, don't bother with the abusive comments.

 

[matt v]

A good guide, am pricing to install heat light and power to out building 35 m from main board. Installing new circuit on none rcd side of split board to 32amp mcb in 4mm armoured clipped and buried to 2nd rcd db in shed with 4 ways, 1 for heater, 1 for sockets, 1 for light and a spare for garden lights heat and power on 20a each and lights on 6a.
Luckily no metal work to bond to complicate matters!

 

[oldAL]

If you don't understand something, just ask, don't bother with the abusive comments.

actually I do,but it seems a lot of people do not understand the differences between TNCS and PME.
they would much rather give out what may well be potentially lethal advice!
it appears more and more frequently that earthing systems seem to be very much misunderstood by far too many people that claim to be competent electricians.

 

[Engineer54]

actually I do,but it seems a lot of people do not understand the differences between TNCS and PME.
they would much rather give out what may well be potentially lethal advice!

it appears more and more frequently that earthing systems seem to be very much misunderstood by far too many people that claim to be competent electricians.

Perhaps you could explain it to us then??

 

[oldAL]

a really easy point to make would be that PME and TNCS are NOT the same thing,
far too many so called 'competent' 'electricians' seem to think they are,
there is a world apart from the 2 systems described by TNCS and PME,
if you actually know what the letters stand for then you should also know what the differences are.
and its not always a good thing to believe what the DNO may tell you or label your cut-out up as,
I know we are supposed to take that as being correct, but I for one am not willing to put someones life at risk for the sake of a sticker on the incoming fuse.

 

[johnboy6083]

PME- protective multiple earthing, which means that the PEN conductor is grounded at many points along its path.

TNC-S, the PEN conductor is split at the service head, to from as neutral and an earth conductor. There is nothing to say that the PEN conductor is earthed on its way from the tranny.
Its good practice to install your own rod, to help stop diverted neutral currents appearing on your installation.

 

[oldAL]

it is, but without a proper knowledge of the systems involved and adequate labelling, it unfortunately very seldom happens,
and that is where a very inherent danger occurs from a very substantial number of sparks.

 

[Engineer54]

PME- protective multiple earthing, which means that the PEN conductor is grounded at many points along its path.

TNC-S, the PEN conductor is split at the service head, to from as neutral and an earth conductor. There is nothing to say that the PEN conductor is earthed on its way from the tranny.
Its good practice to install your own rod, to help stop diverted neutral currents appearing on your installation.

That's exactly what is required in most European countries, and in my opinion should also be in the UK too.


oldAL.

As for PME and TNC-S being different yes your correct, but you'll also find that many of the the DNO TN-S supplied systems are being converted to PME and many cases ARE PME but are still shown at the service head as TN-S.

TNC-S systems that are being supplied by DNO's are in the main TN-S systems that haven't been fully converted. All new cable services being provided by the DNO companies will be PME, where every cable joint will be roded as per PME regulations. Of course you have to take notice of a service cut-out that is labeled PME, you'll soon find out if it's not a full PME system when you do your Ze/KA tests. But in all honesty, i doubt if a DNO company will knowingly provide a earthing point that is not sufficiently safe. That could prove very costly for them, in more ways than one!!

Not sure what you mean by ''putting customs lives at risk'' ...How ?? So what measures do you make, to negate these risks your talking about??

 

[oldAL]

Engineer54, TNS is almost a PME in its very guise, as the sheath is in constant contact with earth, almost, as for actually changing TNCS to proper PME, do you honestly think that will happen overnight, or only as and when they are replacing joints and cables?
I really dont see any UK DNOs digging up streets to spike cables at every junction, which is required by PME.

Im a bit confused by your post about most TNCS are really TNS, thats really not the case in most that I have seen , older building perhaps (pre '70s), but definately for anything post 1980 they all seem to be simple tncs that almost everyone thinks is PME and its very unusual to find a 1990s tncs that is actually PME, in my experience anyway,

 

[Engineer54]

Engineer54, TNS is almost a PME in its very guise, as the sheath is in constant contact with earth, almost, as for actually changing TNCS to proper PME, do you honestly think that will happen overnight, or only as and when they are replacing joints and cables?
I really dont see any UK DNOs digging up streets to spike cables at every junction, which is required by PME.

Im a bit confused by your post about most TNCS are really TNS, thats really not the case in most that I have seen , older building perhaps (pre '70s), but definately for anything post 1980 they all seem to be simple tncs that almost everyone thinks is PME and its very unusual to find a 1990s tncs that is actually PME, in my experience anyway,

Over night?? They have been converting TN-S systems and installing new PME systems for well over 50 years now.

Ican assure you, from a very good good source that there are converted PME systems that are still being shown as TN-S. Probably in the more semi rural areas rather than in the towns and cities, but it's a fact. As and when these old cable sheaths break down to a point where they are no longer capable of providing a stable earth they are converted at the service head. So you can have service heads on the same local network cable showing TN-S and PME!!!


You still haven't told us of these risks your talking about, and what sort of measures you make to negate them??

 

[oldAL]

ok, so you are trying to say that simply by combining the neutral and sheath/earth at the head it is now PME?
that is so unreal it is almost believable, but yet again a statement from someone that doesnt understand how PME works.
how to negate this,?
simple, treat every installation as it is not PME.

 

[andyb]

So if you come across T-N-C-S system, what do you do different to the rest of us oldAL?

 

[IQ Electrical]

So if you come across T-N-C-S system, what do you do different to the rest of us oldAL?

Yes, I was interested in that point.

All this talk of 'additional risks' but I've yet to come across any documented cases to support that phrase and I DO understand earthing and supply systems of all types.

 

[johnboy6083]

if anything, isnt it prudent to assume PME/TNCS, so you can size your bonding conductors accordingly? To assume its TNS, would have you undersizing your bonding.

 

[Engineer54]

ok, so you are trying to say that simply by combining the neutral and sheath/earth at the head it is now PME?
that is so unreal it is almost believable, but yet again a statement from someone that doesnt understand how PME works.
how to negate this,?
simple, treat every installation as it is not PME.

Yes, it is unreal, and not what i said at all, more like you putting words in my mouth....

I think it is You that doesn't understand, it has nothing to do with the sheath. It's the sheath of these old PILC cables that has deteriorated, ...bit pointless connecting to the sheath in that instance isn't it??

TN-S systems that are being and/or have been converted to PME only Rod the Neutral at the joint positions, the sheath is left untouched so to speak, which is why they can still supply a TN-S service head. Have you ever seen a DNO PME'd joint on an old or new cable?? When the TN-S service head is converted to TNC-S/PME the sheath connection is normally removed or isolated....

I think we all know now, what you mean by removing customers risks!! Your one of these scaremongering fools that is convinced that death is just around the corner with a PME connection, ...Right?? You would prefer to put your customers at far greater risk by installing a buckshee single rod TT system with a 200 ohm Ra,because that's what BS7671 calls for. And then totally rely on RCDs remaining in good order, for your earth fault disconnection. And you have the cheek to to say we don't understand about earthing systems. lol, lol!!!

Please don't come back here and say that you know of several instances were a total breakdown of a supplying neutral has occurred, because you haven't!! Your listening to the hear-say of others, that also have probably never experienced one either....

What you Scaremongers forget, is that virtually all of Europe and beyond, have universal PME supplies and have never seen or known of TN-S system's, for general public distribution, that was basically a UK system. You DON'T get any of this PME scaremongering in those countries. So why do you think it should be any different in the UK??

 

[Engineer54]

if anything, isnt it prudent to assume PME/TNCS, so you can size your bonding conductors accordingly? To assume its TNS, would have you undersizing your bonding.

There is no reason to assume anything, you design your installation bonding arrangement according to the DNO supply presented to you...

 

[oldAL]

Yes, it is unreal, and not what i said at all, more like you putting words in my mouth....

I think it is You that doesn't understand, it has nothing to do with the sheath. It's the sheath of these old PILC cables that has deteriorated, ...bit pointless connecting to the sheath in that instance isn't it??

TN-S systems that are being and/or have been converted to PME only Rod the Neutral at the joint positions, the sheath is left untouched so to speak, which is why they can still supply a TN-S service head. Have you ever seen a DNO PME'd joint on an old or new cable?? When the TN-S service head is converted to TNC-S/PME the sheath connection is normally removed or isolated....

I think we all know now, what you mean by removing customers risks!! Your one of these scaremongering fools that is convinced that death is just around the corner with a PME connection, ...Right?? You would prefer to put your customers at far greater risk by installing a buckshee single rod TT system with a 200 ohm Ra,because that's what BS7671 calls for. And then totally rely on RCDs remaining in good order, for your earth fault disconnection. And you have the cheek to to say we don't understand about earthing systems. lol, lol!!!

Please don't come back here and say that you know of several instances were a total breakdown of a supplying neutral has occurred, because you haven't!! Your listening to the hear-say of others, that also have probably never experienced one either....

What you Scaremongers forget, is that virtually all of Europe and beyond, have universal PME supplies and have never seen or known of TN-S system's, for general public distribution, that was basically a UK system. You DON'T get any of this PME scaremongering in those countries. So why do you think it should be any different in the UK??

do they really,? none that I have seen actually do, they simply combine the N E at the head and I dont ever see the street getting dug up to rod the joints.
you are still getting confused and making out that TNCS and PME are the same thing, they are NOT
BS 7671 doesnt ask for 200 ohm Ra , so thats yet another myth you are perpetrating, and to simply bang in a single rod and rely on RCD protection, well thats the forte of all these so-called 'all the kit, full of it' done my time in 5 weeks sort of sparks that have destroyed a proper trade we used to have.
you obviously dont understand earthing by a few of the points I just mentioned, 200 ohm for a TT system, where did you actually pick that number from?
yes, after seeing quite a few installs with either broken neutrals or reversed polarity from localised transformers I dont like TNCS,
because they actually have proper PME systems, unlike the UK which has a poorly planned TNCS system which they are now trying to claim is actually PME to comply with their statutory obligations that come into force next year.