TT to PME?

[JohnW2]

A few years ago in Sutton (nr Macc, not Surrey) I had a terrace on TT wanting PME. UU would not do unless the whole row went PME.

Thinking aloud about these stories we are hearing ...

...firstly, in a case like you describe, it could be that the supply had not yet been PMEd, and they didn't feel that was justified unless the whole terrace was going to take it up.

...secondly, as far as I can see, the postulated hazards of having a PME property adjacent to a TT one only really exist in the absence of adequate main bonding. However, the DNO have no direct control (or probably even authority) in relation to what happens in the consumer's installation and may therefore feel that it's not safe to assume that adequate bonding will be (and will remain) in place.

Kind Regards, John

 

[securespark]

No, in this case, the external supply was grounded already.

But the UU eng. I spoke to wanted all the terraces PME'd.

 

[JohnW2]

No, in this case, the external supply was grounded already. But the UU eng. I spoke to wanted all the terraces PME'd.

Fair enough. Of things I've so far thought of, that only leaves the second of those I mentioned - that the DNO only feels that adjacent TT/PME properties are OK if adequate bonding is (and remains) in place, and they don't feel confident that they can assume that.

Others (e.g. westie) might have some other possible explanations to suggest.

Kind Regards, John.

 

[bernardgreen]

I've never heard such nonsense as that of keeping PME and TT earths seperate as bernard seems so insistent on doing.

It was the DNO who installed my new supply last year who made the point very clear. My CPC ( PME ) must NOT have any connection to the CPC ( TT ground rod )of the adjacent shop ( from which my cottage had been supplied )

Hell the DNO actually deliberately connect the CNE conductor to an earth electrode at virtually every joint they make.

Yes they do in order to try and keep the neutral and CPCs derived from it as close to ground potential as possible. The difference in potential between them being generated by voltage drop along the neutral during the very common occurances of un-balanced loading on phases and occasionally due faults in the network.

It just goes to further reinforce the fact that bernard does not understand earthing and bonding.

I do understand that bonding is to keep all the bonded items at the same potential and "earthing" is either

( PME ) to provide a path for fault currents to reach the neutral without returning through an RCD sense coil and thus an un-balance is created which will trip the RCD

( TT ) to provide a path for fault currents to ground and thus to the star point of the sub station transformer.

If a number of items are bonded and one of them is also "earthed" then electrically all those items are connected to "earth" but the regulations and electricians who stick to those regulations seem to deny those items are earthed insisting they are only bonded.

If the OP upgrades to PME then he's got a proper earth to provide fault protection to the submains allowing him to ditch all the RCDs and the nusiance tripping they could cause, and just locally RCD protect the final circuits as and when neccessary.

A proper "earth" which could pull the steel armour to a voltage above ground via a path from the neutral ( which regulations say should be treated as live conductor ) which has no fuses in it to protect from excessive current.

If an installation has 30 mA or even 100 mA RCD protection fault currents to the CPC and earthed ( or bonded items ) will be limited to that small current because any higher and the RCD will be tripped. So why does the main bonding on incoming pipes have to be cable of such large cross sectional area ? Could heavy current cable be necessary for main bonds because if the CPC (derived from a faulty neutral) rose to a few volts above ground and one of the bonded service pipes had very low impedance to ground then the current in that bond cable could be several 10s or maybe hundreds of amps. ( Remember no fuse involved )

He can even just use PME earth to protect the SWAs and TT the outbuildings if he wants. This is a method I have used on farms before.

Where the regulations require isolation of the SWA ( earthed to the PME CPC ) from the CPC of the TT system.

I've never heard such nonsense as that of keeping PME and TT earths seperate as bernard seems so insistent on doing.

If they were connected then it would be exactly the same as a PME CPC in one house being connected to the TT CPC in the house next door.

 

[bernardgreen]

that the DNO only feels that adjacent TT/PME properties are OK if adequate bonding is (and remains) in place, and they don't feel confident that they can assume that.

Bonding has to be restricted to being inside the properties and not be electrically connected to the bonding in the adjacent property. This is necessary to ensure the TT grounded CPC cannot be connected the neutral derived CPC via the bonding.

The reasoning behind this is that while the impedance of most ground rods is high enough to limit current through bonding they are not confident that the TT ground connection will not be one that could carry extremely high currents from the neutral to ground along a path that has no fuses and would not affect any RCDs. In effect the bonds are not protected and currents through them could be high enough to cause heating of the bond cable and possibly fire.

 

[JohnW2]

So why does the main bonding on incoming pipes have to be cable of such large cross sectional area ? Could heavy current cable be necessary for main bonds because if the CPC (derived from a faulty neutral) rose to a few volts above ground and one of the bonded service pipes had very low impedance to ground then the current in that bond cable could be several 10s or maybe hundreds of amps. ( Remember no fuse involved )

Very true but (for as long as the cable could take it) even a few hundred amps through a few metres of 10mm² bonding conductor is only going to result in a PD of a few volts - so where is the hazard? ... and, in any event, to get 'hundreds of amps' as a result of a PD of 'a few volts' would require a (very unlikely) extraordinarily low impedance from service pipes (or TT electrode) to truth earth - e.g. 500A from 10V would require 0.02Ω.

Kind Regards, John.

 

[JohnW2]

The reasoning behind this is that while the impedance of most ground rods is high enough to limit current through bonding they are not confident that the TT ground connection will not be one that could carry extremely high currents from the neutral to ground along a path that has no fuses and would not affect any RCDs. In effect the bonds are not protected and currents through them could be high enough to cause heating of the bond cable and possibly fire.

In the context of this discussion, that 'reasoning' makes absolutely no sense. A TT electrode itself is not going to draw more than a small handful of amps, even if the MET voltage rose to 230V, so the only possibility for even remotely large currents (very unlikley to be 'hundreds of amps) is via a supply pipe route to ground - and that theoretical risk (of the bonding conductor causing a fire) exists in any PME installation, where it is seemingly regarded as an acceptable (and probably almost non-existant) risk.

Kind Regards, John.

 

[bernardgreen]

Very true but (for as long as the cable could take it) even a few hundred amps through a few metres of 10mm² bonding conductor is only going to result in a PD of a few volts - so where is the hazard?

For a moment forget electric shock hazard and think about hot cables........

I totally agree most ground rods would be hard pressed to sink more than a few tens of amps even if they were connected to a neutral that was at 230 volts. ( network fault, PME neutral at 230, PM CPC at 230, path via linked bondings to the TT CPC ) The rod would be at 230 volts so that would be creating a serious voltage gradient in the ground. ( perhaps the reason why farms are required to have large area ground mats and not just a few rods ).

But one cannot assume the ground rod is not the end of a very low, near zero. impedance circuit to the star point.

 

[JohnW2]

For a moment forget electric shock hazard and think about hot cables........But one cannot assume the ground rod is not the end of a very low, near zero. impedance circuit to the star point.

The goalposts are squirming about all over the place! I think that most people who live in the real world would be happy to make that assumption - not the least because I imagine that crossing a road reprsents a far greater risk! ... and, as I've recently written, it's as true of any PME installation in the presence of metal service supply pipes as it is to the scenario we're discussing. This really is, IMO, getting more than a bit silly!

Kind Regards, John.

 

[xr4x4]

Well i called SSE (scottish and southern energy) earlier, she was most helpful.

She needed to contact another department but they closed at 5pm. She gave me the direct dial number, so i shall call them tomorrow.

As the owner, owns both propertys, i might as well enquire how much it would cost to upgrade both!

 

[RF Lighting]

I've never heard such nonsense as that of keeping PME and TT earths seperate as bernard seems so insistent on doing.

It was the DNO who installed my new supply last year who made the point very clear. My CPC ( PME ) must NOT have any connection to the CPC ( TT ground rod )of the adjacent shop ( from which my cottage had been supplied )

They are EARTHS not CPCs
Did you ask them which regulation requires this?

Hell the DNO actually deliberately connect the CNE conductor to an earth electrode at virtually every joint they make.

Yes they do in order to try and keep the neutral and CPCs derived from it as close to ground potential as possible. The difference in potential between them being generated by voltage drop along the neutral during the very common occurances of un-balanced loading on phases and occasionally due faults in the network.

They are EARTHS NOT CPCs

It just goes to further reinforce the fact that bernard does not understand earthing and bonding.

I do understand that bonding is to keep all the bonded items at the same potential and "earthing" is either

Why the quote marks round earthing? And it's not either PME or TT. There are several earth supply types and it does not matter which the supply type is, they all do the same job.

( PME ) to provide a path for fault currents to reach the neutral without returning through an RCD sense coil and thus an un-balance is created which will trip the RCD

No it isn't. That statement is entirely incorrect, and it just goes to further reinforce the fact that you do not understand earthing and bonding.

There doesn't have to be an RCD involved at all. TN-C-S is exactly the same as TN-S, except the neutral and earth are split at a different place.

( TT ) to provide a path for fault currents to ground and thus to the star point of the sub station transformer.

If a number of items are bonded and one of them is also "earthed" then electrically all those items are connected to "earth" but the regulations and electricians who stick to those regulations seem to deny those items are earthed insisting they are only bonded.

We've been through this so many times and you still don't get it. You are bonding the items. Whether the bonding happens to be earthed or not is not important. You are BONDING, not EARTHING.

If the OP upgrades to PME then he's got a proper earth to provide fault protection to the submains allowing him to ditch all the RCDs and the nusiance tripping they could cause, and just locally RCD protect the final circuits as and when neccessary.

A proper "earth" which could pull the steel armour to a voltage above ground via a path from the neutral ( which regulations say should be treated as live conductor ) which has no fuses in it to protect from excessive current.

If an installation has 30 mA or even 100 mA RCD protection fault currents to the CPC and earthed ( or bonded items ) will be limited to that small current because any higher and the RCD will be tripped. So why does the main bonding on incoming pipes have to be cable of such large cross sectional area ? Could heavy current cable be necessary for main bonds because if the CPC (derived from a faulty neutral) rose to a few volts above ground and one of the bonded service pipes had very low impedance to ground then the current in that bond cable could be several 10s or maybe hundreds of amps. ( Remember no fuse involved )

I've read this maybe fifteen times now and I still dont quite understand it.

Are you saying that in your crazy world you think that all circuits connected to a TN-C-S supply should be supplied through an RCD?

He can even just use PME earth to protect the SWAs and TT the outbuildings if he wants. This is a method I have used on farms before.

Where the regulations require isolation of the SWA ( earthed to the PME CPC ) from the CPC of the TT system.

I've never heard such nonsense as that of keeping PME and TT earths seperate as bernard seems so insistent on doing.

If they were connected then it would be exactly the same as a PME CPC in one house being connected to the TT CPC in the house next door.

No it wouldn't.

I do not use a PME supply on farms due to 4 legged livestock stood on wet ground in contact with and often connected to the installation metal work and wanting to ensure that there is no PD what-so-ever present between the two points in both normal and fault conditions. Entirely different from two houses having their earthing systems connected together by a common utility pipe.

Oh and they are EARTHS not CPCs

 

[securespark]

The only thing I can think of is that it could be some obscure reg of theirs, not ours?

As the DNO said no connection between PME earth and rod earth. I take it they mean an installed one, rather than the GMoE?

There would be a PD between the two.

 

[Adam_151]

I assume the DNO wish to avoid any potential fall out from creating a supply that is technically TT, but in reality behaves like TNCS due to an interconnection and thus would need bonds sized as per TNCS, could not be used to supply farm yard, caravan site, petrol station etc

 

[westie101]

The only thing I can think of is that it could be some obscure reg of theirs, not ours?

Each DNO is free to decide their own policy and Codes of Practice to comply with the various legislation that covers the industry.
Just as Part P it is all non prescriptive and very vague.

Your side of the meter decided to use BS 7671 as a means of complying with Part P and the various legislation, but any electrical contractor is legally free to develop his own ways of complying. (unless he's agreed with the inspection bodies to comply with their rules and BS 7671)

As an example: -
If a person suggests on here placing a cable in a plastic pipe on the surface in a garden he is roundly told that is not permitted.

If we want to place an 11kV cable in a plastic duct on the surface, if we can show that it would be safe it is permitted.

Now if folk were not constrained by adherence to BS 7671, it would be perfectly permissible to do as we could, subject to whoever wished to do it showing it was safe.

So there is no point asking to see the regulations that a DNO imposes, they belong to the DNO and they will have shown compliance within them.
You'll also find that as there is no one size fits all answer, individual engineers are empowered to make suitable decisions on compliance, I certainly do.

 

[JohnW2]

I assume the DNO wish to avoid any potential fall out from creating a supply that is technically TT, but in reality behaves like TNCS due to an interconnection and thus would need bonds sized as per TNCS

Maybe - but, as I've said above, I think I have such an installation ('TT' with Ze of about 0.25Ω with bonds in place), but assumed to be due to water pipes, rather than anything that the DNO has any control over or can do anything about.

Kind Regards, John.