Exporting TN-C-S to outbuilding - again!

[bernardgreen]

The problem of connecting a PME "earth" to the earth of a TT system is that when the PME "earth" is no longer earth potential ( due to a neutral fault ) the TT earth will be pulled up to the same potential. This could be a hazardously high voltage. There will be some current flowing into the TT ground rod which does not pass through any RCD or MCB and is therefor limited only by the impedance of the circuit. It is possible that the PME "earth" could be pulled up to 230 volts via equipment in the PME property if the break in the neutral is between house and street cable. Hence the TT ground rod would be at 230 volts above ground. A rod impedance of 100 ohms would result in up to 2 amps flowing Voltage gradients around the rod would be hazardously high. If the ground rod was a metallic service pipe with a very low impedance ( or other bonded pipes provided very low impedance routes to ground ) then the neutral current of the PME property would be returning to the sub-station via the TT system earthing arrangement. An electric shower in the PME property would send 40 amps through the earthing of the TT property, There would be no obvious indication of this potentially very serious fault.

 

[JohnW2]

The problem of connecting a PME "earth" to the earth of a TT system is that when the PME "earth" is no longer earth potential ( due to a neutral fault ) the TT earth will be pulled up to the same potential. This could be a hazardously high voltage. There will be some current flowing into the TT ground rod which does not pass through any RCD or MCB and is therefor limited only by the impedance of the circuit. It is possible that the PME "earth" could be pulled up to 230 volts via equipment in the PME property if the break in the neutral is between house and street cable. Hence the TT ground rod would be at 230 volts above ground. A rod impedance of 100 ohms would result in up to 2 amps flowing...

That's exactly what I've been saying, although I was a little more generous in talking about 3A (earth rod ~75&#937

... If the ground rod was a metallic service pipe with a very low impedance ( or other bonded pipes provided very low impedance routes to ground ) then the neutral current of the PME property would be returning to the sub-station via the TT system earthing arrangement. An electric shower in the PME property would send 40 amps through the earthing of the TT property,

All true - but, as I wrote before, the theoretical (extremely small) risk of that sort of thing happening is at its greatest when the other property has an (intact) PME (not TT) earth - so I don't understand why you are focussing on the situation of the 'earth' of a PME installation being linked to the earth of a TT one.

Kind Regards, John

 

[bernardgreen]

High impedance TT ground rod = hazard of electric shock from TT CPC and/or from two point connection with the ground around the rod ( potential gradient )

Low impedance TT ground rod ( pipes etc ) = risk of fire from overloaded earth and bonding cables.

 

[JohnW2]

High impedance TT ground rod = hazard of electric shock from TT CPC ...

Only if you don't have an equipotential zone.

...and/or from two point connection with the ground around the rod ( potential gradient )

If you're talking about (2-legged) human beings, I think this is scraping the barrel. Has that ever happened to a significant extent (to human beings)?

Low impedance TT ground rod ( pipes etc ) = risk of fire from overloaded earth and bonding cables.

Indeed - but, as I said, the risk of that (very unikely) sort of thing is theoretically higher with TN installations than with TT - which is why I don't understand why you're 'singling out' TT installations.

Kind Regards, John

 

[viewer]

Head in lion's mouth time.
I suggest that the conundrum of "is there - isn't there" a path through water pipes is best dealt with by fitting a plastic section; just as in the bonding of baths conversation of last week.
To prevent links between PME and TT - this is what I did at home where there are three sheds:-
The house is PME
The SWA armour and CPC are connected to the MET
At the first shed the armour and CPC from the house are linked together but not to anything else.
The first shed now has L & N into a mini CU but no CPC and so now has its own TT rod installed (and tested).
The armour of the outgoing SWA and CPC are linked to the shed's own MET and TT rod.
The SWA now goes to the second shed
Again the armour and SWA CPC are linked but not connected to anything else
Again the L & N into the second shed and a TT system is created.
This is repeated to the third shed.
In this way each shed has a separate earthing system. The connecting SWA has a link to a proper earth so any super garden fork would create a fault back to the last proper earth.
The mini CUs each have a 30mA RCD and I know many would challenge this, but I've never had a nuisance trip or difficulty with differentiation.
As always, I'm willing to be shot down, but think that this should over come the suggested problems of deciding whether to export/TT etc

 

[JohnW2]

I suggest that the conundrum of "is there - isn't there" a path through water pipes is best dealt with by fitting a plastic section; just as in the bonding of baths conversation of last week.

Yes, if one wants to avoid a possible connection through water pipes, that's the way to do it. However, if there are also gas pipes, there is a problem - since plastic sections in them (inside houses) are not allowed.

To prevent links between PME and TT - this is what I did at home where there are three sheds:- ....but think that this should over come the suggested problems of deciding whether to export/TT etc

Yes, what you describe is a way to avoid exporting the PME earth - but what you write doesn't actually help in deciding whether or not one wants to avoid exporting the earth.

In terms of 'not exporting the PME earth', per se, there is obviously no need to isolate the three TT earths from each other. From that point of view, could could connect the armour/CPC of the SWA runs between shed at both ends - it's only the one from the house to the first shed that needs 'isolating'. Depending on the distances involved, and various other factors, you might even consider having just one TT rod for all three sheds.

Kind Regards, John

 

[THRIPSTER]

Hello BAS, are you back and with Regulations yet? In one of your earlier posts, you categorically stated that extraneous pipes can not be used to link MET's - have you changed your mind given OMS' s comments in the Other Place - would be interested to hear your views.

Regards

 

[ban-all-sheds]

Yup, back, but had forgotten about this outstanding Q.

BAN, as OMS has pointed out on the IET forum - have a look at 543.2.6 - you might change your mind.

I'd not paid attention to what the pipe was, and hadn't appreciated that it was under your control, and therefore not at risk of being removed without you knowing.

If you were sure you would be keeping it, you'd be on much firmer ground, but you'd need to think about 543.2.6 (i) wrt copper pipe exposed to the elements, with joints in etc.

 

[JohnW2]

I'd not paid attention to what the pipe was, and hadn't appreciated that it was under your control, and therefore not at risk of being removed without you knowing. If you were sure you would be keeping it, you'd be on much firmer ground, but you'd need to think about 543.2.6 (i) wrt copper pipe exposed to the elements, with joints in etc.

I'm a bit confused by 543.2.6. It relates only and specifically to the use of extraneous-conductive-parts as protective conductors, but how can that work in terms of definitions? - i.e. how can something which is 'liable to introduce a potential' be used as a protective conductor?

More generally, in terms of 543.2 as a whole, it also seems a bit odd that one is allowed (with conditions) to use an extraneous-c-p as a protective conductor, but seemingly one is not allowed to use (with appropriate conditions) pipework wholly contained within a location (hence not qualifying as an extraneous-c-p) for the same purpose. Can you think of any logic in that?

Kind Regards, John

 

[EFLImpudence]

I'm a bit confused by 543.2.6. It relates only and specifically to the use of extraneous-conductive-parts as protective conductors, but how can that work in terms of definitions? - i.e. how can something which is 'liable to introduce a potential' be used as a protective conductor?

Why not? The potential will have been equalised by the main bonding.

More generally, in terms of 543.2 as a whole, it also seems a bit odd that one is allowed (with conditions) to use an extraneous-c-p as a protective conductor, but seemingly one is not allowed to use (with appropriate conditions) pipework wholly contained within a location (hence not qualifying as an extraneous-c-p) for the same purpose. Can you think of any logic in that?

I'm not at home with the BGB but -

You would not want to.
You would be creating a hazard by 'earthing' isolated perts which may then require 'bonding' elsewhere.
Isolated parts cannot (not possible) be 'bonded'.

 

[JohnW2]

I'm a bit confused by 543.2.6. It relates only and specifically to the use of extraneous-conductive-parts as protective conductors, but how can that work in terms of definitions? - i.e. how can something which is 'liable to introduce a potential' be used as a protective conductor?

Why not? The potential will have been equalised by the main bonding.

It just seems conceptually odd. For a start, if it's being used as a protective conductor, then it presumably must be connected to the MET (atthe end remote from where it would be main bonded), in which case the main bonding is essentially redundant (the CSA of the pipe will almost certainly be greater than that of the bonding conductor). However, maybe it's not as odd as my bones think it is!

More generally, in terms of 543.2 as a whole, it also seems a bit odd that one is allowed (with conditions) to use an extraneous-c-p as a protective conductor, but seemingly one is not allowed to use (with appropriate conditions) pipework wholly contained within a location (hence not qualifying as an extraneous-c-p) for the same purpose. Can you think of any logic in that?

I'm not at home with the BGB but - You would not want to.
You would be creating a hazard by 'earthing' isolated perts which may then require 'bonding' elsewhere. Isolated parts cannot (not possible) be 'bonded'.

I suppose one of my problems is in understanding what 534.2.6 is all about - i.e. why/when one would want to use an extraneous-c-p (let alone a pipe which didn't qualify as an extraneous-c-p) as a protective conductor - and, indeed, what that actually would mean in practice.

Of course, in practice, if people follow the word of the regs, they may well be 'main bonding' things which are not actually (per the reg's definition) extraneous-c-ps (e.g. if they bond a water pipe on the house side of an 'insulating section' or insulating meter) - which you may regard as creating a hazard (even though merely 'obeying the regs'), because it represents "...'earthing' isolated parts which may then require 'bonding' elsewhere". Whatever, it seems as if 534.2, as written, allows the water pipe to be used as as a protective conductor (whatever that means in practice) if that insulating section is not present, but not if it is present (albeit it's still main-bonded, per word of the regs). Is that logical?

Kind Regards, John

 

[EFLImpudence]

I suppose one of my problems is in understanding what 534.2.6 is all about - i.e. why/when one would want to use an extraneous-c-p (let alone a pipe which didn't qualify as an extraneous-c-p) as a protective conductor - and, indeed, what that actually would mean in practice.

The pipe(s) can be used as supplementary bonding.
E.g. wire from pipe to pipe under the sink and wire from electric shower to light cpc.
Thus the cold water pipe is used as a supplementary bond between the cold water pipe and the shower bonding the hot water pipe to the light cpc.

Of course, in practice, if people follow the word of the regs, they may well be 'main bonding' things which are not actually (per the reg's definition) extraneous-c-ps (e.g. if they bond a water pipe on the house side of an 'insulating section' or insulating meter) - which you may regard as creating a hazard (even though merely 'obeying the regs'), because it represents "...'earthing' isolated parts which may then require 'bonding' elsewhere".

In that case the pipe is unlikely to be isolated because of other connections - boiler and gas pipe - and should be main bonded anyway.

Whatever, it seems as if 534.2, as written, allows the water pipe to be used as as a protective conductor (whatever that means in practice) if that insulating section is not present, but not if it is present (albeit it's still main-bonded, per word of the regs).

It doesn't apply in this example but if some pipework were actually isolated by various lengths of plastic pipe then it would be undesirable to 'earth' it.

Is that logical?

If done properly, yes.

 

[JohnW2]

I suppose one of my problems is in understanding what 534.2.6 is all about - i.e. why/when one would want to use an extraneous-c-p (let alone a pipe which didn't qualify as an extraneous-c-p) as a protective conductor - and, indeed, what that actually would mean in practice.

The pipe(s) can be used as supplementary bonding.
E.g. wire from pipe to pipe under the sink and wire from electric shower to light cpc. Thus the cold water pipe is used as a supplementary bond between the cold water pipe and the shower bonding the hot water pipe to the light cpc.

Fair enough. However, if, for whatever reason, the pipework did not qualify as an extraneous-c-ps, then 534.2.6 would theoretically not allow it to be used for supplementary bonding in the manner you describe - even though, electrically, it makes no difference whether the pipework is an extraneous-c-p or not. Even if you do, I don't find that very logical!

In that case the pipe is unlikely to be isolated because of other connections - boiler and gas pipe - and should be main bonded anyway.

True (but see **). However, in that case it would be the gas pipe that needed main bonding. If you are going to argue that anything electrically connected to a main-bonded extraneous-c-p becomes an extraneous-c-p itself by virtue of that connection, then every CPC in the premises would qualify as an extraneous-c-p!

[** not all houses have piped gas. I don't, and were it not for the fact that there's about 4 feet of underground metal pipework involved, my incoming LPG supply pipe would not qualify as an extraneous-c-p. Give or take a bit of diuscussion (depending of route of pipe etc.), many oil installations don't create any extraneous-c-ps]

It doesn't apply in this example but if some pipework were actually isolated by various lengths of plastic pipe then it would be undesirable to 'earth' it.

Agreed but, as I said, some people feel 'forced' to do that 'undesirable' thing because 544.1.2 appears to require it! I agree that this is likely to become moot if (as will usually be the case) the pipework is earthed by other routes, but that's hardly an excuse (there must be at least some premises with no wet central heating, no immersions etc.).

Kind Regards, John

 

[EFLImpudence]

Fair enough. However, if, for whatever reason, the pipework did not qualify as an extraneous-c-ps, then 534.2.6 would theoretically not allow it to be used for supplementary bonding in the manner you describe - even though, electrically, it makes no difference whether the pipework is an extraneous-c-p or not. Even if you do, I don't find that very logical!

It doesn't make any difference electrically but safety-wise it is undesirable to 'earth' that which does not require 'bonding'.

However, in that case it would be the gas pipe that needed main bonding.

Indeed, but as it is usually not possible to determine the NON-extraneousness of the water supply because of the connections to other services then it would seem the requirement is to main bond it as it will not, in any case, be detrimental and as it IS connected to other parts it is not isolated and so may require supplementary bonding.

If you are going to argue that anything electrically connected to a main-bonded extraneous-c-p becomes an extraneous-c-p itself by virtue of that connection, then every CPC in the premises would qualify as an extraneous-c-p!

Not that it DOES but, with the water pipe, we cannot be certain it does NOT but if you can be certain (measured before connection) then do not main bond but as above it may still require supplementary bonding in certain locations .
It would be unlikely a cpc is extraneous in its own right but they, too, require supplementary bonding in certain locations.

not all houses have piped gas. I don't, and were it not for the fact that there's about 4 feet of underground metal pipework involved, my incoming LPG supply pipe would not qualify as an extraneous-c-p. Give or take a bit of diuscussion (depending of route of pipe etc.), many oil installations don't create any extraneous-c-ps]

Should the water service be the only one and you can definitely determine that it is NOT extraneous then, obviously, it will not require main bonding but then (Edit) if not connected to anything electrical nor will any parts of it require supplementary bonding therefore to use these parts AS supplementary bonding would be silly.

 

[JohnW2]

It doesn't make any difference electrically but safety-wise it is undesirable to 'earth' that which does not require 'bonding'.

I think you're somewhat talking around in circles. You've said (and I would agree) that the water pipework is nearly always going to be 'incidentally' earthed, even if it does not require main bonding (electrical speaking - but goodness knows about 544.1.2!) because it is not an extraneous-c-p (because of an 'insulating segment' where it enters the property). However, if you take 534.2.6 as written, such pipework cannot be used as a protective conductor for supplementary bonding in the manner you described - since, although earthed, it is not an extraneous-c-p. That still doesn't seem logical to me!

Indeed, but as it is usually not possible to determine the NON-extraneousness of the water supply because of the connections to other services then it would seem the requirement is to main bond it ....

Maybe, but if "it is not possible to determine the NON-extraneousness of the water supply pipe" then, per the word of 534.2.6, I don't think you can use it as a protective conductor for supplementary bonding in the manner you have described, since that reg only allows you to do that if it IS extraneous (it doesn't say 'bonded', it says 'extraneous'). If you don't know whether it is extraneous or not then, in terms of the way it's written, you cannot 'enjoy' the provisions of 534.2.6.

Kind Regards, John