Need to earth back boxes? Earth sleeves etc.

[JohnW2]

That is really going to depend on the definition of "energise". .... But when you intentionally supply something which in normal operation creates a potential difference between the cpc and true earth so that you intentionally cause current to flow in the cpc......

OK - so you appear to be saying that the BS7671 definition of a live conductor (live part) does, or may, include CPCs. Is that correct? ....

I'm sure it is.

Fair enough - so you agree that the authors of BS7671 did not intend that the definition of a 'live conductor' should include CPCs? ...

So maybe those who claim a definition for "live conductor" which ends up making a cpc one should reconsider their definition rather than falsely claim that cpcs are not intended to carry current during normal operation.

Maybe. I certainly don't know where 'intended to carry current during normal operation came from.

However if, as above, you think that the BS7671 definition of 'live conductor' does/may include CPCs, yet are sure that such was not the intention of the authors, are you suggesting that they simply got the definition wrong?

Kind Regards, John

 

[John D v2.0]

If the Neutral is to considered as Live conductor then any conductor connected to the Neutral will have the same potential as the Neutral.

Hence if the Neutral is to be considered as a Live conductor then the MET, CPC and all items bonded to the MET have to be considered as being Live as well.

If your neutral is connected to the met within your installation, you have a fault that needs rectifying.
Just because the supplier of a TN system connects them together somewhere doesn't mean the supplier provided earth is a live conductor.
That's why you can't make your own pme by using the neutral, the electricity supplier has some addition requirements to make their supplied earth not a live conductor.

 

[JohnW2]

If your neutral is connected to the met within your installation, you have a fault that needs rectifying.

I suppose that depends on what you mean. In a TN-C-S installation, the (only) Earthing Conductor is that which goes from the MET to the incoming Neutral (at the cutout).

That's why you can't make your own pme by using the neutral, the electricity supplier has some addition requirements to make their supplied earth not a live conductor.

Interestingly, the BS7671 definition of a live conductor specifically excludes PEN conductors - which presumably means that the DNO's neutrals in TN-C-S supplies are not live conductors.

Even more confusing ... my incoming neutral is, I'm told, capable of providing a PME earth, but I'm not 'taking advantage' of that. Does that make the incoming neutral 'live' or not?!!

I'm not quite sure how much thought has gone into all this terminology!

Kind Regards, John

 

[John D v2.0]

In a TN-C-S installation, the (only) Earthing Conductor is that which goes from the MET to the incoming Neutral (at the cutout).

I was under the impression that connection belongs to the supplier and isn't part of the customer's installation. And the connection only exists because the electricity supplier has made guarantees on their side of the supply. Eg multiple earth connections. And the customer has promised to bond with sufficient size cables.
Happy to be wrong on that though.

Although that raises the question whether the supplied pme earth would need to be treated as live if bonding was omitted. I would have thought so.

 

[bernardgreen]

If your neutral is connected to the met within your installation, you have a fault that needs rectifying.

The Neutral block in the cut out is connected to the MET That is not a fault.

 

[John D v2.0]

Assuming you have pme then yes, but that's in the supplier's side not in your installation.

My point I just made on the other thread, is that your point is correct if all the conductors have zero impedence. If not, then the topology of the bonding and earthing with respect to neutral becomes important.

 

[JohnW2]

I was under the impression that connection belongs to the supplier and isn't part of the customer's installation.

Well, that's just a bureaucratic/contractual question, although it would be a little odd to have an installation which, because of that bureacracy, did not have an Earthing Conductor. Indeed, as I have said, I have been 'offered' a PME earthing terminal, with no obligation to use it - so if I chose to connect it to my MET, I presume the Earthing Conductor I used to do that would, again bureacratically, be part of my installation?

And the connection only exists because the electricity supplier has made guarantees on their side of the supply. Eg multiple earth connections.

As above, not a 'connection' but a 'provision for a connection'. However, as I often say, I was very disillusioned when the late-lamented westie told us that the 'M' of PME often referred to just one extra earth connection (at the far end of the main) in addition to the one at the transformer!

Although that raises the question whether the supplied pme earth would need to be treated as live if bonding was omitted. I would have thought so.

If there were something that needed bonding, I don't see how it could be permitted to omit it. In any event, I would have thought that whether (something connected to) a PME earth is "treated as live" is merely a matter of definitions, and would not be affected by the presence/absence of bonding.

I have already observed that the BS7671 definition of a 'live conductor' expliocitly excludes a PEN conductor - which seems rather odd!

Kind Regards, John

 

[John D v2.0]

I'm not so much interested in the terminology and official definition, more than the reasoning why a provided earth would be safe to be uninsulated (within the equipotential zone) whereas a neutral wouldn't necessarily be so has to be protected from direct contact.

Tellingly, in the situation where the earth and neutral are connected close to the property the important of the main earth conductor and equipotential bonding having a low impedence (relative to the incoming neutral) is amplified.

What situations would the neutral be unsafe to touch? I'm not sure whether to include single fault situations or just normal situations, I feel single faults should be included.

 

[JohnW2]

I'm not so much interested in the terminology and official definition, more than the reasoning why a provided earth would be safe to be uninsulated (within the equipotential zone) whereas a neutral wouldn't necessarily be so has to be protected from direct contact.

I think you know the answer to that, certainly with TN-C-S. If the two are connected together within the property, then there surely can be NO rational 'reasoning' which says that one may be bare (including 'earthed' exposed-c-ps, let alone mere 'conductors'!) whilst the other (close to the origin) requires 'protection against direct contact'!

With TN-S, there is a much less direct/local connection between N and 'E', so not so surprising they they should be regarded differently.

HOWEVER, with TN-C-S, once one gets a significant distance from the origin of the installation (where N and E are joined) there could be quite appreciable N-E pds, so the fact that N then needs to be protected from direct contact makes some sense.

Kind Regards, John

 

[John D v2.0]

with TN-C-S, once one gets a significant distance from the origin of the installation (where N and E are joined) there could be quite appreciable N-E pds, so the fact that N then needs to be protected from direct contact makes some sense.

Agreed with most of your post, but given the limits on volt drop, how could there be an appreciable volt change on neutral? Perhaps where there is an overload of around 4x rated current on a long circuit it could just about get there?

 

[JohnW2]

Agreed with most of your post, but given the limits on volt drop, how could there be an appreciable volt change on neutral?

Well, for a start, the numerical 'limits' are actually only recommendations - and, as you go on to say, appreciable overloads do occur.

However, during fault conditions, when the potential of neutral could rise (only very briefly one hopes) to anything up to around half line potential - that certainly would count as "live". Although one could argue that much the same can be said of CPCs and exposed-c-ps, which one is allowed to touch, the real problem would arise if one simultaneously touched a 'bare neutral' and an exposed-c-p during the period before an L-N fault was cleared. With an L-E fault, although the exposed-c-ps will briefly rise to high potential, there shouldn't be any 'true earth potential' around to touch simultaneously

Kind Regards, John

 

[John D v2.0]

simultaneously touched a 'bare neutral' and an exposed-c-p

although the exposed-c-ps will briefly rise to high potential, there shouldn't be any 'true earth potential' around to touch simultaneously

I agree there wouldn't be true earth potential, but I think there could be potential closer to earth then the case of the faulty appliance. The met would be closer to earth than the case of the appliance with the fault.

That's the topology point for me, as in the first quote above it wouldn't really be much different from touching the exposed case of a faulty fridge at the same time as a sink bonded back to the met independently. The earths come together at the met, and the neutral and earth come together slightly earlier, at the supplier's cutout.

That also explains why the need for supplementary bonding on the bathroom, to improve the topology to make it even more favourable, even though the electrons could have taken the "long way round"

 

[JohnW2]

I agree there wouldn't be true earth potential, but I think there could be potential closer to earth then the case of the faulty appliance. The met would be closer to earth than the case of the appliance with the fault.

Indeed. It's inevitable that the case of an appliance with an L-E will be higher (probably much higher) than, say, the case of another appliance which has a separate CPC route back to the MET (or, as you go on to say, some other earthed object, like a sink). That is probably a good reason for not having Class I appliances in close proximity served by different circuits since, if they are on the same circuit, sharing the same CPC back to the MET, they are going to remain at essentially the same potentials. That's one reason why I'm not keen on two or more 20A radials supplying sockets in one kitchen.

That also explains why the need for supplementary bonding on the bathroom, to improve the topology to make it even more favourable, even though the electrons could have taken the "long way round"

Indeed. In practice, there is unlikely to be any Class I items in a bathroom, and far less likely that there would be two 'touchable' ones on different circuits, so the main issue relates to 'non-electrical earthed objects'. Whether it is sensible to restrict these concerns to bathrooms (I'm not sure that 100V+ across one's body is desirable even when 'wet naked bodies' are not involved) and/or whether it is sensible to remove the requirement for SB when there is RCD protection (given that the other two criteria will nearly always be satisfied) is perhaps a different matter - although I would be no fan of returning to the 'bond everything' days!

Kind Regards, John

 

[Draughtsman]

Sry to interrupt, but does this look ok? The top cable between the bricks is 75mm below top of floorboard.

 

[mattylad]

How dare you!

Well! interrupting their discussions that are disagreements within "your" thread..... lol
Whatever next!

Whats the separation between the cables and the heating pipes?