TT to PME?

[bernardgreen]

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.

Do you have an example of that, because that's not what I see people saying.

If one trawls back through the threads on the topic there are many instances of people say that bonding and "earthing" ae separate things. I agree that the purposes of bonding and earthing are diffent but in almost all cases a set of bonded items are also "earthed" so the overall result is all items are "earthed". That would not create any problems if the "earth" used was true ground. But in any system where the "earth" is connected not to a ground rod but is connected directly to the supply neutral there are hazards created. Not for people inside the equipotential zone provied they have no way to make contact with true ground while also touching anything bonded and/or "earthed" during a period when the neutral is no longer at ground potential. I accept this is an uncommon event but it is an event that the people in the house may have no knowledge of.

That said disconnected neutrals seem to be more frequent recently with metal thefts targeting the neutrals at substations.

Consider an extreme situation - a room which is inside a metal box ~~Obviously, having a site office which is live would be hazardous to people and animals outside who are touching the 'earth' or other bits of conductive material which is earthed - not to mention the problem of people getting in and out between the two zones. So we earth the box for their benefit.

If the box was earthed by connection to ground and not "earthed" by connection to the supply neutral then there would be no hazard to people and animals outside the box from touching the box or any service pipes entering the box. There could be a hazard to people or animals if a live to CPC fault inside the box created a voltage gradient around the ground rod to which the box was earthed but awareness of this possible hazard would lead the competent designer to provide a means to detect this situation and then to disconnect the incoming supply.

 

[SimonH2]

In reality there obviously is no way of earthing the cooker without also 'bonding' it to (rendering it roughly equipotential with) anything else connected to the CPC network.

Indeed, and you were correct to dispute my description of earthing. Part of what we call "earthing" is really a special case of equipotential bonding where one of the conductive parts is "earth" (or some proxy for it) - in the case of the metal office, you have to consider the ground people are standing on as a conductive part.

As you say, in the domestic situation, you can't really separate the two functions which share one bit of wire.

However, consider the case of a TT system with a poor earth and relying on an RCD for protection. All the conductive parts are bonded together, but they are not effectively earthed. Without the RCD, a L-E fault would cause significant voltages on <stuff> but the equipotential bonding would mean that that in itself should not be a danger. The lack of a good return path for the fault current means we can't rely on overcurrent protection to disconnect the supply, hence the RCD to sense the fault current (whether directly or by proxy).

For a couple of examples of non-earthed equipotential bonding :

http://www.youtube.com/watch?v=4SX6Ucbb1l8
http://www.youtube.com/watch?v=tA9oBEHRLqQ

 

[bernardgreen]

Do we have an example?

The cottage and shop situation I am in is an example.

If two separate electricians independently re-wired the cottage and the shop to full compliance with BS7671 then the supply neutral would be connected to a consumer's ground rod via two sets of main bonding and the common metallic water pipework.

Another example is the cottage walls which like most rubble and mortar walls in old buildings have varying degrees of conductivity to the ground. How does one under BS7671 bond a wall to prevent extranious ground potential entering the equipotential zone. ( Rubble being stones of random size and shape )

The regulations change as situations come to light in sufficient numbers to justify a new edition being written. In effect an admission that for some suituations the previous edition was not perfect. Thirty years ago a meter installer refused to connect our supply to caravan and building site because we had two RCDs in the system. Now the sense of splitting supplies across more than one RCD is not only permitted it is strongly reccomended.

 

[JohnW2]

In reality there obviously is no way of earthing the cooker without also 'bonding' it to (rendering it roughly equipotential with) anything else connected to the CPC network.

Indeed, and you were correct to dispute my description of earthing. Part of what we call "earthing" is really a special case of equipotential bonding where one of the conductive parts is "earth" (or some proxy for it) ...

Exactly - which is why I am surprised that many people get so emotional when it is suggested that 'bonding' (seeking to equalise potentials) is often an important part of what 'earthing' achieves.

As you say, in the domestic situation, you can't really separate the two functions which share one bit of wire.

Again, exactly. I can't really think of any situation (even non-domestic ones) where this is not true, at least qualitatively. Of course, the 'earthing' conductor may not be 'adequate' in bonding terms, but it nevertheless will be providing a degree of bonding (hence at least helping to reduce PDs).

However, consider the case of a TT system with a poor earth and relying on an RCD for protection. All the conductive parts are bonded together, but they are not effectively earthed. Without the RCD, a L-E fault would cause significant voltages on <stuff> but the equipotential bonding would mean that that in itself should not be a danger.

A third 'exactly'!! If one considers a TT installation without an RCD (of which there used to be countless, and undoubtedly some still remain), then this all becomes (or should become!) much clearer. In that situation, providing a load with a CPC connection clearly does not fulfil the definition (which I think we've all agreed) of "earthing", since it's very unlikely that any protective device (i.e an OPD) will operate in the case of an L-CPC fault, even in the presence of this connection to a CPC. However, the CPC in such situations is a significant safety measure - because it provides a degree of 'bonding' (approximate equipotentiality). If there is an RCD, then it also provides a crucial 'earthing' function, since it faciliates operation of the RCD under some 'L-E' (specifically 'L-CPC') fault conditions, but that doesn't alter it's functionality in terms of bonding, particularly when there is no RCD.

Kind Regards, John.

 

[EFLImpudence]

in the domestic situation, you can't really separate the two functions which share one bit of wire.

I don't really agree with that insofar as, as you said before, the earthing (essential) is to ensure the opd operates and the bonding (if necessary) equalises potential .

So, all exposed parts will be earthed and if necessary supplementary bonded by another piece of wire to extraneous parts.

If the exposed part was not part of a fitting or appliance requiring earthing it would not be an extraneous part and so not need bonding (if still there).


Is it not improbable that the (likely 1mm²) cpc affords satisfactory bonding to, say, a light fitting in a bathroom when 15mm. pipes and a 10mm² conductor may not?

 

[EFLImpudence]

Do we have an example?

The cottage and shop situation I am in is an example.

If two separate electricians independently re-wired the cottage and the shop to full compliance with BS7671 then the supply neutral would be connected to a consumer's ground rod via two sets of main bonding and the common metallic water pipework.

Is that a problem? - if I have understood correctly.

Another example is the cottage walls which like most rubble and mortar walls in old buildings have varying degrees of conductivity to the ground. How does one under BS7671 bond a wall to prevent extranious ground potential entering the equipotential zone. ( Rubble being stones of random size and shape )

I do not know the situation.
Will this not be achieved by the main bonding?

The regulations change as situations come to light in sufficient numbers to justify a new edition being written. In effect an admission that for some suituations the previous edition was not perfect. Thirty years ago a meter installer refused to connect our supply to caravan and building site because we had two RCDs in the system. Now the sense of splitting supplies across more than one RCD is not only permitted it is strongly reccomended.

Again, I'm not sure I understand completely.
Were the two RCDs against the regulations?
Was the meter fitter correct ?

 

[JohnW2]

in the domestic situation, you can't really separate the two functions which share one bit of wire.

I don't really agree with that insofar as, as you said before, the earthing (essential) is to ensure the opd operates and the bonding (if necessary) equalises potential .

As I recently wrote, think about a TT installation without RCD protection (bad and non-compliant, I know, but far from unknown). In that situation, do you not agree that having CPCs, and having them connected to exposed conductive parts, would make a useful contribution to safety?

If you do agree, then why is that? It can't be because the CPC provides a useful 'earthing' function, in the sense we're defining it,since is not going to result in any OPD operating (conceivably on a 6A circuit, not on any circuit with a >6A OPD) in the presence of an L-CPC fault - so it's surely because it helps to minimise pds between exposed parts (and between exposed and extraneous parts) - which is, by our definitions, 'bonding'.

I think you are probably focussing too much on the two specific types of bonding (main and supplementary) described in, and required by, the regs, without recognising that useful degrees of bonding (equalisation of potentials) occur, often 'incidentally', as a result of other conductors.

Kind Regards, John.

 

[RF Lighting]

 

[JohnW2]

<clutching_at_straws>

I really don't understand why so many people seem almost frightened of the idea (and/or frightened to acknowledge the idea) that conductors other than main or supplementary bonding conductors can play a useful role in helping to equalise potentials between accessible parts - i.e. have a partial 'bonding' functionality.

Kind Regards, John.

 

[RF Lighting]

Because they don't.

If an earth fault occours with your metal kettle, and for what ever reason the OCPD doesn't operate, the installation MET will rise to 230V.

The MET is a ECP, as is the water and gas supply etc.

As all ECPs are bonded together, they all remain at equipotential.

The CPC does not form part of the bonding.

 

[JohnW2]

Because they don't. If an earth fault occours with your metal kettle, and for what ever reason the OCPD doesn't operate, the installation MET will rise to 230V.

It will certainly rise to some level - provided that there is some conductor which is attempting to equalise the potential between the metal kettle body and the MET. What is the word that we use to describe the function of seeking to equalise potentials?

The MET is a ECP, as is the water and gas supply etc. As all ECPs are bonded together, they all remain at equipotential.

Indeed, the things which the BGB calls 'main bonding conductors' would make the MET and supply pipes etc. pretty equipotential.

Because The CPC does not form part of the bonding.

Obviously not part of the 'main bonding', which has a specific defined meaning. However, if you just have your 'main bonding conductors' keeping the MET & service pipes etc. roughly equipotential, what exactly do you think it is that makes exposed conductive parts (of metal kettles etc.) equipotential with them? 'Equipotential bonding' which did not extend to exposed parts of appliances etc. would be pretty useless, wouldn't it? Do you really think that the conductor which makes the exposed parts of an app[liance equipotential with the MET, supply pipes etc. "does not form part of the bonding"? If so, perhaps you had better remind me what you think "(equipotential) bonding" means.

Kind Regards, John.

 

[bernardgreen]

EFLImpudence wrote:

Is that a problem

Yes it is if the neutral to my cottage goes open circuit then the rod in the next door shop and all metal conected to it is raised to the same voltage as my disconnected neutral. If any appliance or lamp is on in my property when my neutral goes open circuit then my neutral goes up to live and so does my MET. No protective devices will act on this fault.

Cottage walls are naturally damp and therefore conductive, therefore they import the potential of the ground they are standing on into the equipotential zone. It has been suggested that a metallic damp proof course bonded to the MET would solve this problem. The simpler and much safer option is to use TT instead of PME

The meter installer was correct in that he followed the regulations in force at the time. His boss had the common sense to see that using two RCDs, one for the building site and a separate one for the caravan meant in the event of an accident on the site that tripped the site RCD there would still be lights in the caravan to render first aid, call 999, make hot sweet tea etc.

 

[westie101]

Yes it is if the neutral to my cottage goes open circuit then the rod in the next door shop and all metal conected to it is raised to the same voltage as my disconnected neutral. If any appliance or lamp is on in my property when my neutral goes open circuit then my neutral goes up to live and so does my MET. No protective devices will act on this fault.

Cottage walls are naturally damp and therefore conductive, therefore they import the potential of the ground they are standing on into the equipotential zone. It has been suggested that a metallic damp proof course bonded to the MET would solve this problem. The simpler and much safer option is to use TT instead of PME

This would only apply if the neutral is totally isolated from earth, the connection to the TT electrode would serve to lower or eliminate the problem depending on the earth resistance of it and other associated metal. (in fact it has been discussed on here the idea of even with a PME supply having an earth rod)

I still feel that the theory is overtaking the facts, again we've dealt with a few cases of lost neutral on a single phase supply, apart from minor shocks (which are bad enough in a way) there have been no serious effects.

The simpler and much safer option is to use TT instead of PME

A rather sweeping statement don't you think. Does it refer to this case or in general?
If in general I'm sure that the autors of BS7671 & ESQCR will have debated it and if it were the case would have made that clear, rather than requiring the DNOs to provide an earth terminal for new supplies which generally will be PME and to maintain any existing arrangements.

I have. today, approved a job to make a PME terminal available to one of a row of terraced cottages all fed from an underground system and then mural wiring. I suspect, like many, it would have originally used the water service for an earth which will now be plastic.
I have absolutely no qualms about doing this as I see no major safety issues at all.

Remember that in risk assesement terms there are two factors, the risk and the likelyhood of that risk occuring. If by using historic data it can be shown that the likelyhood is very low, even thought the thoretical risk is high the overall "score" is low ,so does not really warrant any action in terms of being reasonably practicable

 

[bernardgreen]

This would only apply if the neutral is totally isolated from earth,

Which has been known.

the connection to the TT electrode would serve to lower or eliminate the problem

the earth resistance and resistance of the appliance would form a voltage divider with 230 volts across it and the CPC connected to the mid point

I still feel that the theory is overtaking the facts,

theory can predict the hazards that faults will create, better theory and prevention than post incident recovery

again we've dealt with a few cases of lost neutral on a single phase supply, apart from minor shocks (which are bad enough in a way) there have been no serious effects

.There were newspaper reports of several fires in meter boxes and damage to appliances with connections to ground ( other than the CPC ) when the neutral ground link was stolen from the substation supplying the area

The simpler and much safer option is to use TT instead of PME

A rather sweeping statement don't you think. Does it refer to this case or in general?

In this specific case.

Remember that in risk assesement terms there are two factors, the risk and the likelyhood of that risk occuring.

The third factor is cost....Removing the earth conductor from the network reduces cable cost by 20% Which is more than the cost of replacing appliances damaged when the combined neutral+"earth" bounces due to a network fault or metal theft.

PME was frowned on with overhead supplies using two wires in case the neutral snapped leaving the live connected and no "earth"

If by using historic data it can be shown that the likelyhood is very low, even thought the thoretical risk is high the overall "score" is low ,so does not really warrant any action in terms of being reasonably practicable

Provided historical facts are not outdated by, for example, increasing metal thefts disrupting the network.

 

[westie101]

There were newspaper reports of several fires in meter boxes and damage to appliances with connections to ground ( other than the CPC ) when the neutral ground link was stolen from the substation supplying the area

Which is a whole different scenario than the loss of the neutral on a single phase service which very rarely damages appliances.

Loss of neutral on a 3 phase network is a different case where it is possible to get over 400V live to neutral which is what causes the appliance damage not the fact that they are connected to earth, this applies whether it is PME or not and would not be stopped by having TT earthing either.
In the case of cable faults the loss of neutral on a 3 phase system does ususally, I agree, only occur in PME systems but may not be restricted to them.
Overhead networks are more vunerable but have more earth rods so I have never seen appliance damage from a lost neutral.

Some of the bigger cases in this DNO were on non PME networks where the neutral was just left floating, in a lot of cases PME can mitigate this by having multiple earth rods.

PME was frowned on with overhead supplies using two wires in case the neutral snapped leaving the live connected and no "earth"

Really, certainly not since 1973 but I've never heard mention of this within the industry.

You mention about cost, that is what the term "reasonably practicable" is about.

Again the reality is that the loss of a neutral on a single phase service is not a major issue.