Gas bonding

[JohnW2]

Where do the neutrals come into this? All the earths are on a common bussbar.

I would have thought it unlikely that the flats would not all share the same neutral.

However, if they didn't, and if the flats were TN-C-S, then bernard would presumably be right. In that situation each of the flat neutrals (which could, if 'different', be at appreciably different potentials) would be, directly or indirectly, connected together via bonding conductors - so, as bernard said, substantial currents could then flow in those conductors. What is this 'common bus bar' connected to?

Kind Regards, John

 

[bernardgreen]

Where do the neutrals come into this?

Assuming the flats have PME electrical supplies then the "earth" in each flat is derived from the incoming Neutral. The Main Earth Terminal is connected to the Neutral.

All the earths are on a common bussbar.

Are they ?

 

[SUNRAY]

All the earths are on a common bussbar.

Are they ?

Yes, why do you think of questioning this factual statement?

Each flat has a SWA from meter cupboard in lobby to CU in flat. In the flat I have worked in it enters CU using an insulated gland with no connexion to armour.
Main earth in meter cupboard is connected with a thick green/yellow (120mm²ish) to a copper bussbar (30x4mm or so) which runs up the riser.

25mm² from earth bar to CU (quoted from post 5)

There is no E/N link at CU.

 

[JohnW2]

...There is no E/N link at CU.

Interesting. Are you saying that the supply to the building is TN-C-S but the supplies to each flat are TN-S?

Kind Regards, John

 

[bernardgreen]

Yes, why do you think of questioning this factual statement?

Beause a vertical common busbar serving three flats stacked vertically would be an expensive way to achieve "Earths" to the flats

 

[SUNRAY]

Interesting. Are you saying that the supply to the building is TN-C-S but the supplies to each flat are TN-S?

Kind Regards, John

You have raised an interesting question, which I'm sure must have been discussed on here before.

I'll assume the incoming supply to the building is TN-C-S but I know there are several earth rod pits connected to the main earth bar (As opposed to the riser bussbar) so it is unlikely but feasible it could be TT.
I'd say each flat does look like TN-S as the earth arrives independently of the N, however the same is true of TT.


I'd assume that in this situation the whole block would remain the same as the source, the only time I personally have been aware of a change has been not exporting the earth of a TN-C-S or TN-S to outbuildings and changing it to TT.

 

[JohnW2]

I'll assume the incoming supply to the building is TN-C-S but I know there are several earth rod pits connected to the main earth bar (As opposed to the riser bussbar) so it is unlikely but feasible it could be TT.

Even if there are additional local earth electrodes connected, if the earthing system ('of the building') is also connected to the incoming neutral, then it is TN-C-S. Connecting addition earth rods to a TN-C-S earth (as 'the 18th' threatened to require) does not stop it being TN-C-S (and nor does adding a local earth electrode to a TN-S supply stop it being TN-S).

I'd say each flat does look like TN-S as the earth arrives independently of the N, however the same is true of TT.

The big difference between TN-S and TT is that the ('separate'/'independent') earthing conductor is connected to the neutral of the supplier's transformer in one case but (only) to a local TT electrode(s) in the other. As you imply, one cannot distinguish between TN-S and TT on the basis that the installation's earth is derived from a conductor which is not locally connected to the (location's - e.g. flat's) incoming neutral.

I'd assume that in this situation the whole block would remain the same as the source ...

... except that it would not be in the situation you describe. If the earth arrives in a flat via a conductor separate from the L and N conductors, then that flat's installation cannot be TN-C-S (no "C").

... the only time I personally have been aware of a change has been not exporting the earth of a TN-C-S or TN-S to outbuildings and changing it to TT.

I don't personally see any difference. In both cases the local earth is derived from a conductor other than the neutral arriving at the location - so it cannot be T N-C-S. In one case that ('separate') conductor is connected to the supplier's neutral somewhere, and in the other case it is connected (only) to a local earth electrode.

Kind Regards, John

 

[SUNRAY]

It's just occurred to me I'm trying to consider this block of 12 flats in the same way I'd treat one property with submains which of course it is not.

Beause a vertical common busbar serving three flats stacked vertically would be an expensive way to achieve "Earths" to the flats

These are, or rather were designed to be, 'exclusive' properties and silly things are over spec'ed such as separate cable trays in the risers for different services, cables are cleated to tray with bolts, all stopcocks/valves are stainless steel, pipes are chrome plated or treated with 'chrome aluminium' paint, doors have 4 hinges etc.

 

[JohnW2]

It's just occurred to me I'm trying to consider this block of 12 flats in the same way I'd treat one property with submains which of course it is not.

Yes, if it were a single property (i.e. single electrical installation) with submains, then I presume one would regard what was on the ends of the submains as being part of the overall installation with a particular supply type (e.g. TN-C-S), even if, for example, what appeared at the end of a sub-main was not, in itself, TN-C-S (since there was no CNE).

With separate dwellings, I imagine (but am not certain of conventions) that one would consider each dwelling to have its own separate electrical installation, with whatever 'earthing arrangement' arrived at the dwelling - and, as I said, that would not really be TN-C-S if the earth arrived as a 'separate conductor'.

Kind Regards, John

 

[bernardgreen]

that one would consider each dwelling to have its own separate electrical installation,

which would means each dwelling has it's own "Earth". Unless these "Earths" are all derived from the same source there should not be any form of connection (*) between the "Earths" from two or more dwellings.

(*) bonding, earthing, via metallic pipework that is common to two or more dwellings and bonded in each dwelling,

 

[SUNRAY]

which would means each dwelling has it's own "Earth". Unless these "Earths" are all derived from the same source there should not be any form of connection (*) between the "Earths" from two or more dwellings.

(*) bonding, earthing, via metallic pipework that is common to two or more dwellings and bonded in each dwelling,

I have to totally disagree with this in the strongest possible term.
In the case of these flats there is a common earth in the form of a copper bar and I think there is no issue with the interconnexions.

In the case of a row of old cottages where the water rising main runs through the terrace (and then back down into the ground to feed some more cottages and water troughs in the fields), and the electrical mains supply runs along the front of the properties as L&N.

Each CU is TT with their own spike and bonded to the common steel water pipe.

 

[bernardgreen]

I have to totally disagree with this in the strongest possible term.
In the case of these flats there is a common earth in the form of a copper bar and I think there is no issue with the interconnexions.

In the case you are talking about you are probably correct to think that there is no issue with the interconnnections adjacent to the meters.

Advice has to be general and not restricted to one situation.

In the case of a row of old cottages where the water rising main runs through the terrace ~~ Each CU is TT with their own spike and bonded to the common steel water pipe

Metallic water mains that are extremely low impedance to ground should not be bonded to an "Earth" that is derived from the Neutral if there is any risk at all of the Network Neutral being lost.

https://www.fire-magazine.com/castleford-explosion-injures-firefighters-footage-released

Metal thieves removed the copper Neutral straps / cables between the substation and the houses. It is almost certain that bonding cables or connections to water pipes over heated when carrying the current that the Neutral would have carried.

 

[SUNRAY]

Advice has to be general and not restricted to one situation.

Absolutely and I wasn't trying to.

Metallic water mains that are extremely low impedance to ground should not be bonded to an "Earth" that is derived from the Neutral if there is any risk at all of the Network Neutral being lost.

https://www.fire-magazine.com/castleford-explosion-injures-firefighters-footage-released

Metal thieves removed the copper Neutral straps / cables between the substation and the houses. It is almost certain that bonding cables or connections to water pipes over heated when carrying the current that the Neutral would have carried.

Which just highlights to me yet again that it's high time we stopped this ridiculous and highly dangerous system of using a current carrying conductor as a safety device.

 

[bernardgreen]

Which just highlights to me yet again that it's high time we stopped this ridiculous and highly dangerous system of using a current carrying conductor as a safety device.

Two wires are cheaper than three, and the accountants run the business

 

[JohnW2]

which would means each dwelling has it's own "Earth". Unless these "Earths" are all derived from the same source there should not be any form of connection (*) between the "Earths" from two or more dwellings.

I am somewhat struggling to think of a situation in which the 'earths' of each dwelling within a building would not be 'derived from the very same source'. What sort of situation were you envisaging?

In the building we are discussing, the earth to the 'earth' of the various flats clearly do very much derive from the same source.

Kind Regards, John