Armoured Cable Query

[JohnW2]

As far as the installation is concerned, the earth electrode at the outbuilding counts as an extraneous-c-p - which means that, with TN-C-S, a bonding conductor with a CSA of at least 10mm² would have to be run to the outbuilding if the TN-C-S earth were being 'exported' to there.

So use 10mm² 3-core SWA to the outbuilding. Then you've got a choice of using one of the cores or the armour, or both. Since, as is often observed, the difference between an earthing connection and a bonding one is just naming, there's no reason why a conductor cannot have two names at the same time.

[ I've attempted to tidy up your abomination of quoting ]
Sure, but (unless ≥10mm² is needed anyway), that only makes sense if there are other ('conventional') extraneous-c-ps entering the outbuilding. If not, it surely makes more sense to not export the TN-C-S earth but, instead, to simply rely on the local ('TT') earth electrode (now connected just by a 'earthing conductor', which is not also a 'bonding' one!) at the outbuilding?

Kind Regards, John

 

[bernardgreen]

If you have a TT system ( either for the main house or for a outbuilding fed from the main house ) then all the earth electrode has to do is sink at least 30 mA to ground without the potential on the rod ( relative to true ground ) rising above 50 volts. ( equates to approx 1600 ohms )

It doesn't really what the electrode is or how many there are provide that condition can be met 356 days a year and during the driest of dry weather when the ground dries out.

1600 ohms meets but in reality it should be much lower. Around 200 ( 6 volts at 30 mA or 20 volts if the protective device is a 100 mA RCD.

It goes without saying that all circuits of a TT installation should ( must ) be protected by one or more 30 mA RCDs and the incomer main swith should be a 100 mA RCD as back up should a 30 mA fail to operate when it should.

Or am I doing to much free thinking for the likes of cable installers.

 

[JohnW2]

If you have a TT system ( either for the main house or for a outbuilding fed from the main house ) then all the earth electrode has to do is sink at least 30 mA to ground without the potential on the rod ( relative to true ground ) rising above 50 volts. ( equates to approx 1600 ohms )

Indeed so (assuming the required RCD protection), but that doesn't mean that the regs would be happy with a 0.1mm² (or whatever) earthing/bonding conductor!

Kind Regards, John

 

[EFLImpudence]

I'm starting to wonder if you're not simply saying that if there is a "low impedance extraneous-c-p" entering the outbuilding, then it may well be more convenient to not export the supply earth to the outbuilding - but, instead, to rely entirely on a local TT electrode. If so, we 'all' know that, but it's not what we're discussing here!

Yes, I am saying that.

I apologise if it is not the discussion but I am trying to counter the argument that there is no hazard or prohibition to connecting an electrode to the supply earth by raising reasons why that may not be the case.

If the earth electrode is just an extraneous-c-p then remove it.
If it is required to 'bond the ground' then (in Britain) for TN-C-S a large csa bonding conductor will be required or just TT the location but then it must not be connected to the supply.

 

[PBC_1966]

I have to say that I got a big shock (not an electrical one!) when I discovered (from westie) what the 'M' of PME often meant. I had previously imagined/assumed that it meant that there really were 'multiple' connections to earth, relatively close, spread along the run of a network main. However, westie said that it very often means that, in addition to the connection of neutral to earth at the transformer, there is just one other connection to earth, at the far end of the main. Whilst I suppose that, literally speaking, two counts as 'multiple', that certainly disillusioned me!

I know the old PME regulations stipulated an earth electrode at the transformer, an electrode at the farthest distribution point of each branch, and other electrodes spaced at regular intervals along the route of each branch "so far as practical." In the original application of PME that typically meant an electrode every so many poles along the route.

There was also a requirement that the resistance to earth be sufficiently low to operate protective devices on the HV side in the event of a breakdown between windings in the transformer, and that the resistance to earth from the neutral should not exceed 10 ohms at any point along its route. Those requirements went right back to the Electricity Supply Regulations 1937. What the DNO's specifications are now, I have no idea.

Over here, it must be remembered that in general we use a larger number of smaller transformers to feed a given number of homes, so in the case of a small group of isolated homes there may be limited opportunity for providing additional earth electrodes beyond the one at the transformer itself and those at each service entrance.

That's changing a little in newer subdivisions, though. My street, for example, was developed about 25 years ago and has all underground utilitites with a couple of relatively large pad-mounted transformers for the entire street (although still nowhere near the scale of British-style huge transformers feeding multiple streets on a 3-phase wye system). If I walk a block or two away I'm into neighborhoods developed mostly from the 1950's to the 1970's which still have all overhead distribution and smaller pole-mount transformers feeding anything from a couple of homes up to a maximum of about half a dozen homes each.

It's not uncommon though for a neutral to be extended around multiple transformers in a neighborhood, creating an extensive multiple-grounded neutral system even though there might be dozens of transformers involved. It's when you get to relatively isolated properties (or small groups of properties) where there's little opportunity for anything but an electrode on the single transformer plus the electrodes at the service entrance at each house. But then that's really not much different from the situation in parts rural Britain where a transformer feeds just one or two cottages out on their own.

 

[ban-all-sheds]

I've attempted to tidy up your abomination of quoting

I've no idea how I came to make such an abomination. Or why I didn't notice it at the time. When I returned just now I saw it and thought I'd fallen for the old quote-instead-of-edit trick, but no.

I'll put it right for the benefit of latecomers.

Sure, but (unless ≥10mm² is needed anyway), that only makes sense if there are other ('conventional') extraneous-c-ps entering the outbuilding.

My suggestion that when there are such ecps then use 10mm² in order to not have to use local TT and guard against contact with the armour.

If not, it surely makes more sense to not export the TN-C-S earth but, instead, to simply rely on the local ('TT') earth electrode (now connected just by a 'earthing conductor', which is not also a 'bonding' one!) at the outbuilding?

IMO it never makes sense to discard a perfectly good DNO earth in favour of TT unless you absolutely have to.

 

[PBC_1966]

The NEC here stipulates minimum grounding/bonding conductor sizes based on the supply conductors, e.g. with #2 supply conductors you can use #8, with 3/0 supply conductors you need to go up to #4 (assuming all copper). However, there is also a specific code which states that where a conductor is running only to a made electrode (a rod, plate etc. and not structural metalwork or existing metallic services) then it never needs to be larger than #6.

Size conversions for above:
8 AWG = 8.36 sq. mm
6 AWG = 13.3 sq. mm
4 AWG = 21.1 sq. mm
2 AWG = 33.6 sq. mm
3/0 AWG = 85 sq. mm

 

[JohnW2]

I'm starting to wonder if you're not simply saying that if there is a "low impedance extraneous-c-p" entering the outbuilding, then it may well be more convenient to not export the supply earth to the outbuilding - but, instead, to rely entirely on a local TT electrode. If so, we 'all' know that, but it's not what we're discussing here!

Yes, I am saying that. ... I apologise if it is not the discussion but I am trying to counter the argument that there is no hazard or prohibition to connecting an electrode to the supply earth ...

Fair enough - and that is the matter I've been discussing recently, but ....

... by raising reasons why that may not be the case.

I'm probably being dim, since I still can't really understand what those 'reasons' are.

Also, don't forget that I'm talking in general, not specifically about supplies to outbuildings - since the belief that many seem to have that it is hazardous and/or prohibited to connect any local earth electrode to a TN-C-S earth (something we've been told is actually compulsory in at least some parts of US).

Kind Regards, John

 

[JohnW2]

f not, it surely makes more sense to not export the TN-C-S earth but, instead, to simply rely on the local ('TT') earth electrode (now connected just by a 'earthing conductor', which is not also a 'bonding' one!) at the outbuilding?

IMO it never makes sense to discard a perfectly good DNO earth in favour of TT unless you absolutely have to.

Given RCDs, I'm not sure that I necessarily agree. I certainly don't feel thgat there are any particularly strong reasons for taking that view.

In my installation, I have not "discarded a perfectly good DNO earth" (since I don't have one!) but I have declined an offer to be provided with a TN-C-S earth (which, in my case, would probably only involve adding a bit of G/Y to the existing DNO neutral Henley block) - primarily because I don't pereciev that it would afford appreciable benefits.

Kind Regards, John

 

[JohnW2]

I have to say that I got a big shock (not an electrical one!) when I discovered (from westie) what the 'M' of PME often meant. ....

I know the old PME regulations stipulated an earth electrode at the transformer, an electrode at the farthest distribution point of each branch, and other electrodes spaced at regular intervals along the route of each branch "so far as practical." In the original application of PME that typically meant an electrode every so many poles along the route.

That's what I had always assumed was the 'norm' here, until I was disillusioned! From what I've been told it seems that, in reality, that "so far as practical" is very often deemed to not be the case - so that there are frequently just two connections to earth, at either end of what can (over here) be a pretty long neutral.

Kind Regards, John

 

[PBC_1966]

Also, don't forget that I'm talking in general, not specifically about supplies to outbuildings - since the belief that many seem to have that it is hazardous and/or prohibited to connect any local earth electrode to a TN-C-S earth (something we've been told is actually compulsory in at least some parts of US).

Probably all of the United States - The provision of an earth electrode at each service entrance is an NEC rule, and every state adopts the NEC as the basis of its electrical code.

While states (and individual counties & cities) sometimes amend the NEC to suit local requirements, I've never heard of anywhere making an amendment which relaxes a long-standing fundamental rule such as this one, so while I can't say with absolute certainty that there might not be somewhere within the nation where it's not required, I very much doubt it. State or local amendments more commonly impose extra requirements over and above NEC requirements rather than relax them, e.g. the City of Chicago requires all wiring to be in conduit and does not permit NM-type cable (equivalent of British T&E).

 

[JohnW2]

The NEC here stipulates minimum grounding/bonding conductor sizes based on the supply conductors, e.g. with #2 supply conductors you can use #8, with 3/0 supply conductors you need to go up to #4 (assuming all copper).

IT's conceptually similar here for PME supplies. The minimum CSA for earthing/bonding conductors is then determined by the CSA of the supply neutral, ranging from a minimum of 10mm² for supply neutrals of 35mm² or less (i.e. virtually all domestic and many commercial) up to a maximum of 50mm² for a neutral supply conductor of 150mm² or larger.

However, there is also a specific code which states that where a conductor is running only to a made electrode (a rod, plate etc. and not structural metalwork or existing metallic services) then it never needs to be larger than #6.

That seems to make sense, for reasons we've discussed. However, presumably mainly because we don't envisage connecting electrodes to TN-C-S earths, there is no similar provision in the UK.

Kind Regards, John

 

[EFLImpudence]

I'm probably being dim, since I still can't really understand what those 'reasons' are.

The reason is that the conductors may be of insufficient csa for bonding a TT installation in an outbuilding.
Therefore connecting the electrode AND associated extraneous-c-ps to a TN-C-S earth can be hazardous if large currents flow through the small conductors and low impedance e-c-ps.

Also, don't forget that I'm talking in general,

Surely you cannot do that after complaining that people are not knowledgeable enough to work it out but rely on one size fits all solutions.

not specifically about supplies to outbuildings -

...but they are one example of where it may, indeed, be hazardous and by implication prohibited.

since the belief that many seem to have that it is hazardous and/or prohibited to connect any local earth electrode to a TN-C-S earth

Fair enough. That blanket belief is not correct.

(something we've been told is actually compulsory in at least some parts of US).

Presumably with large enough cable to be safe.

 

[PBC_1966]

(something we've been told is actually compulsory in at least some parts of US).

Presumably with large enough cable to be safe.

Here are the relevant cable sizes for copper (aluminum or copper-clad aluminum is different):

Supply conductor       Grounding electrode conductor (minimum)
2 or smaller               8
1 or 1/0                   6
2/0 or 3/0                 4
Over 3/0 - 350 MCM         2
Over 350 - 600 MCM        1/0
Over 600 - 1100 MCM       2/0
Over 1100  MCM            3/0

Converted to metric sizing (sq. mm):

Supply conductor  Grounding electrode conductor (minimum)

33.6 or smaller          8.36
42.4 or 53.5            13.3
67.4 or 85              21.1
Over 85 - 177           33.6
Over 177 - 304          53.5
Over 304 - 557          67.4
Over 557                85

But as I noted earlier, where the electrode conductor is running only to a driven rod (the typical 8 ft. earth rod used here) it does not need to be larger than 6 AWG (13.3 sq. mm).

 

[JohnW2]

I'm probably being dim, since I still can't really understand what those 'reasons' are.

The reason is that the conductors may be of insufficient csa for bonding a TT installation in an outbuilding. Therefore connecting the electrode AND associated extraneous-c-ps to a TN-C-S earth can be hazardous if large currents flow through the small conductors and low impedance e-c-ps.

I really don't see that the fact that someone might connect them with cables of an inadequate CSA is a credible reason for 'forbidding' (or regarding as 'potentially hazardous') the connection of an earth electrode to a TN-C-S earth (which is what we are talking about). By the same logic, you would 'forbid' sockets, cooker, shower circuits etc., on the grounds that someone might wire them with 1mm² cable.

In any event, in what you say above, the connection to the earth electrode is not the issue. Very few domestic earth electrodes would have an impedance anything like as low as 20Ω, and even at 20Ω, a 1mm² conductor would be more than adequate (CCC-wise). Is the connection to other (possibly low impedance) extraneous-c-ps that could be of inadequate CSA, and hence potentially hazard, but that has got absolutely nothing to do with whether is is allowed/safe to connect an earth electrode to a TN-C-S earth.

(something we've been told is actually compulsory in at least some parts of US).

Presumably with large enough cable to be safe.

Sure - but, as above, for any real-world domestic earth electrode, 1mm² ought to be more than enough (CCC-wise).

Kind Regards, John