TT to PME?

[westie101]

Have your co. changed tack recently, westie?

Er well looking at the CoP I don't think the refusal of PME to your terrace was actually in accordance with the CoP.

The difficulty arises in that if we convert a network to PME we then have to offer PME to all the customers who need to upgrade the bonding at their expense. In practice most don't want to through cost or ignorance.
The trouble is if we find any where the bonding is inadequate or even non existent, under the regs we should give them an improvement notice. If they fail to abide by this we can disconnect them under safety grounds.

Obviously this would open a huge can of worms!

But going back to your terrace I would suggest it will have been an individual's decision rather than policy.
(subject to the network already being PME (most of it round here is) I would approve it.)

Going back to the original debate.
If the bonding in house TT is connected by whatever means to the bonding in house PME, in effect house TT becomes a PME house by that connection. But without the N/E connection at the cut-out and without the DNO knowing that the bonding is up to PME requirements, that could be seen as providing a PME earth to a non-compliant property. It would also mean that in some cases earthing at house TT could be disconnected and affected by work taking place in house PME.

 

[JohnW2]

If the bonding in house TT is connected by whatever means to the bonding in house PME, in effect house TT becomes a PME house by that connection. But without the N/E connection at the cut-out and without the DNO knowing that the bonding is up to PME requirements, that could be seen as providing a PME earth to a non-compliant property.

Thanks, that certainly corresponds with one of the possibilities I suggested a page or two back, namely:

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.

 

[westie101]

Yep that's it, and as I say we have no definite policy either way, it is up to the relevant engineer.

 

[plugwash]

A long pipe underground connected to many properties would worry me less than a short link between properties.

A long pipe underground is going to be a pretty good earth electrode in it's own right. So even under fault conditions it's unlikely to rise TOO far above local earth potential and therefore unlikely to push too much current down the TT properties bonding system.

OTOH a short link between a TT property and a PME property could push a large portion of the displaced neutral current (or even all of it in a worst case scenario) down the TT propertie's bonding system which is an unknown quantity.

 

[JohnW2]

OTOH a short link between a TT property and a PME property could push a large portion of the displaced neutral current (or even all of it in a worst case scenario) down the TT propertie's bonding system which is an unknown quantity.

The 'links' we've been talking about are generally service supply pipes. Where are you suggesting that this current would be 'pushed down the TT property's bonding system' to - some other service pipe? (you wouldn't get any appreciable current down the TT electrode)

Kind Regards, John.

 

[bernardgreen]

A long pipe underground is going to be a pretty good earth electrode in it's own right. So even under fault conditions it's unlikely to rise TOO far above local earth potential and therefore unlikely to push too much current down the TT properties bonding system.

The voltage can be low but the current high if the earth eletrode is low impedance. 10 volts on 0.1 ohm is 100 amps, so the shock hazard is minimal, but cable damage hazard high.

OTOH a short link between a TT property and a PME property could push a large portion of the displaced neutral current (or even all of it in a worst case scenario) down the TT propertie's bonding system which is an unknown quantity.

Which creates a situation somewhere between the extremes of [1] low current in the link and high voltage on the CPC of the TT property ( shock hazard ) and [2] high current in the link and low voltage on the CPC of the TT ( fire risk in the link over heating )

 

[bernardgreen]

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.

Appling 10 volts to that would result in 40 amps flowing. So if a connection between the supply neutral and the bonds was to occur some very high currents could flow through that connection if ( when ) the supply neutral was not at ground potential.

And before RF jumps in with bonding and earthing are different remember that electric current in the CPC doesn't stop at the point where a CPC and bond are attached to the same bit of metal ( such as a water pipe in a central heating system).

 

[bernardgreen]

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

They explained that in the event of their neutral bouncing ( no longer at ground potential ) my "earth" terminal to which my CPC is connected ( BS7671 ) would no longer be at ground potential while the earth terminal of the TT system to which the CPC of the TT system is connected would still be at ground potential. This would cause a current to flow from my "earth" terminal along my CPC, then through what ever link there was between my CPC and the CPC of the TT system, then through the CPC of the TT system to the earth electrode of the TT system. The link could be anything electrically conductive that was connected to earth in the TT and the "earth" in the PME. If the link was low current carrying capacity the risk of it heating up was significant.

They are EARTHS NOT CPCs

But the CPCs are connected to the "earth" terminal so they are "earthed" even if they have a different name in the regulations. When there were voltage operated RCDs then the CPC really was a CPC and not an "earth".

I use " " around the word earth because in PME systems the "earth" is not earth but a direct connection to the neutral of the incoming supply.

 

[JohnW2]

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.

Appling 10 volts to that would result in 40 amps flowing. So if a connection between the supply neutral and the bonds was to occur some very high currents could flow through that connection if ( when ) the supply neutral was not at ground potential.

Maybe, but I very much doubt it. As I've said, I assume that my installation has effectively become PME. The supposition is that there is a low impedance connection between my MET to my neighbour's MET via the underground water pipe and the main bonding in both properties - that's the most credible way of explaining the very low Ze. If that is the case, then both METs (and the underground pipe) will be be at approximately the same potential, close to that of neutral (no matter how high above true earth that might be) - so your postulated 'very high currents' ought not to flow.

Kind Regards, John.

 

[JohnW2]

They explained that in the event of their neutral bouncing ( no longer at ground potential ) my "earth" terminal to which my CPC is connected ( BS7671 ) would no longer be at ground potential while the earth terminal of the TT system to which the CPC of the TT system is connected would still be at ground potential. This would cause a current to flow from my "earth" terminal along my CPC, then through what ever link there was between my CPC and the CPC of the TT system, then through the CPC of the TT system to the earth electrode of the TT system. The link could be anything electrically conductive that was connected to earth in the TT and the "earth" in the PME. If the link was low current carrying capacity the risk of it heating up was significant.

The goalposts seem to have moved again! The 'links' we've been talking about are (water cooled ) metal pipes, of very high current-carrying capacity. Given adequate bonding in both properties, that would have the effect of turning the TT property into a PME one, and the risks would not be appreciably different from those in any other PMEd property. The fact that the PME 'earth' was connected to an earth rod (yet another 'M'!) at one property would be irrelevant.

Kind Regards, John.

 

[bernardgreen]

The goalposts seem to have moved again!

The goalposts are different for every installation. Only in a row of identical spec built houses will the goal posts for each house will be the same as for the other houses. One has to interprete the regulations to the goalposts that exist at the location and not move the goalposts to fit the regulations.

The 'links' we've been talking about are (water cooled ) metal pipes, of very high current-carrying capacity.

That is the obvious one, which puts the weak point, the potential hot spot else where in the inter system link.

If there are no high current links the un-wanted link could be the CPC in a length of 1 mm T&E to a metal lamp fitting that inadvertantly was also in contact with earthing or bonding in the other property. Or via interlinked fire alarms ( two properties under the same thatched roof need special attention there ) or a data cable. Two TV aerials on the same mast could create a link between the two earthing systems.
.

The fact that the PME 'earth' was connected to an earth rod (yet another 'M'!) at one property would be irrelevant.

In normal use ( no linkage ) the earth rod would sink to ground at most 100 mA before an RCD operated and cut the source of the current. If the rod has to cope with the un-limited current from a bounced neutral via the link then the voltage gradients in the ground around the rod could be high enough to be a hazard.

It has been said that the ground rod system of a farm cannot be allowed to be one more 'M' due to the hazard to animals of the voltage gradients across the farm yard. Surely the same concerns should apply to humans living with an earth rod in their garden.

 

[JohnW2]

The goalposts seem to have moved again!

The goalposts are different for every installation. ...

OK, let's put it another way. Each time one issue is raised and dismissed, another one (maybe a valid one) is then raised.

If there are no high current links the un-wanted link could be the CPC in a length of 1 mm T&E to a metal lamp fitting that inadvertantly was also in contact with earthing or bonding in the other property. ... Or via interlinked fire alarms ... or a data cable. ... Two TV aerials on the same mast ....

All of the 'links' to which you refer would qualify as extraneous-conductive-parts and, as such, should theoretically be main-bonded (with an adequate conductor) where they enter the premises - leaving any bits that might conceivably overheat (if the TT installation has a much lower impedance path to earth than the earth electrode alone) outside of the property. The first of your examples is an interesting one, since one would not normally think of main bonding the CPC of a 1mm² cable - but, if one felt that there was any risk of it being 'inadvertently in contact' with a potential external to the property, then it would qualify as an extraneous-c-p and therefore should be main bonded.

If the rod has to cope with the un-limited current from a bounced neutral via the link then the voltage gradients in the ground around the rod could be high enough to be a hazard.

Another 'goalpost move' (or 'new issue'). Firstly, as I keep saying, no normal domestic earth rod will carry hign currents,let alone 'unlimited currents'. If you connected one to 230V, you might get 5A or so. At 50V, you'd be lucky to get more than about 1A.

It has been said that the ground rod system of a farm cannot be allowed to be one more 'M' due to the hazard to animals of the voltage gradients across the farm yard. Surely the same concerns should apply to humans living with an earth rod in their garden.

The main issues with farm animals are that they have four feet on the ground, often an appreciable distance apart, and that some species are much more susceptible to the effects of small electric currents than are humans. Were this not the case, we would, indeed, have to be much more careful about possible risks to humans (maybe those 'walking on all fours'!) of voltage gradients in the ground in the vicinity of earth electrodes.

We are all different ('viva la difference'!) and I presume you come into the category of highly 'risk averse' people.

Kind Regards, John.

 

[bernardgreen]

We are all different ('viva la difference'!) and I presume you come into the category of highly 'risk averse' people.

I am averse to the growing number of people who avoid thinking and follow the rules with the attitude of " OK so someone died ( or a house burnt down ) as a result of my work but I am not to blame as my work complied totally with the regulations in force when the work was done. " and pass the blame onto the people who wrote the regulations.

I am extremely averse to the few who realise the regulations do not fit a particular situation but still insist on using those in-appropriate regulations knowing that a hazard will be created by following the regulations.

 

[JohnW2]

We are all different ('viva la difference'!) and I presume you come into the category of highly 'risk averse' people.

I am averse to the growing number of people who avoid thinking and follow the rules with the attitude of " OK so someone died ( or a house burnt down ) as a result of my work but I am not to blame as my work complied totally with the regulations in force when the work was done. " and pass the blame onto the people who wrote the regulations. I am extremely averse to the few who realise the regulations do not fit a particular situation but still insist on using those in-appropriate regulations knowing that a hazard will be created by following the regulations.

I'm totally with you on all that - if, in any field, I believe that there is a conflict between regulations and safety, then safety would win every time. However, that's obviously not really what I was talking about - which relates to how much time and effort (and discussion) should be taken in relation to risks which, although theoretically present, are vanishingly improbable. For example, as westie has often said, he is not aware of any fatalities every having occured as a result of supply CNE faults.

Kind Regards, John.

 

[ricicle]

The trouble is you will have to really know your stuff and be able to justify any deviations from the regulations that you have made if you are unlucky enough to be up in front of the beak should a problem occur !