Drop bare earth in trench ?

[JohnW2]

I have seen people bury old copper cylinders before.

me too.

Kind Regards, John

 

[JohnW2]

I have only fitted earth rods in sandy soil of Point of Aye, and the requirement was 8 Ω, rods were 1.2 meters long, and needed between 2 and 6 rods to get the reading,

As I've said/asked, I wonder why (given RCDs) anyone would feel a need for a TT electrode with anything like as low an Re as that in a domestic installation. Even 8Ω is not quite low enough for ADS even with a B6 MCB.

no where else have I ever used the equipment to measure an earth rod, I have in domestic never had the room to put out the probes to measure, so have simply compared it with the DNO earth, using earth loop impedance, which is a combination of DNO's earth, volt drop on cables, and your earth, so your earth is always below what actually measured.

If one measures the loop impedance using a 'disconnected' earth electrode (and L at the origin of your installation), the DNO's earth does not come on to it, so the impedance one measures will be the sum of the earth rod's impedance and the impedance of the L conductor back to the DNO's transformer. However, since the latter will be negligible in comparison with any credible domestic earth roid, the measured loop impedance will be very close to the impedance of the rod.

...., only once have I ever come across a problem, a radio ham had used earth tape for his transmitter, and the electrician wiring up his shack had bonded the aerial earth to the DNO earth and the PEN was lost, and it melted the 4 mm² earth cable. But that is only one in 50 years in the trade.

One occasionally hears such stories. Maybe I'm missing something, but what I don't really understand is why, when people install very low impedance earths for "RF reasons", they don't put in series with it a capacitor which would have high reactance at 50Hz, but without having any appreciable effect at the RF frequency of interest.

Kind Regards, John

 

[SimonH2]

Yes, a significantly larger area, but at a significantly lower depth - which, as I said, might become an issue during dry conditions.

In our sort of soil, I doubt it will get sufficiently dry, even in the worst of droughts, to become ineffective - perhaps a bit degraded, but not ineffective.

And I did have in mind that (depending what I put in) it could also serve as the CPC to the shed supply, giving me more options in terms of what cable I pull through the duct later - does anyone make concentric with the outer layer all blue insulated ? I can't think of any reg that would prohibit that.

True, but I would personally be inclined to just use a suitable cable!

I am thinking about using a suitable cable - not something that's suboptimal in one or more ways to suit the regs.

does anyone make concentric with the outer layer all blue insulated ?

Why would you want to use such a cable?

Because it might be the optimal cable to use.
I think you have probably made a few assumptions which may, or may not, be appropriate. You haven't asked about the use of this shed (i.e. loads expected), or the supply main fuse rating, or if there's any extraneous conductive parts (there are at the moment, but that may or may not change), ...

To start with, when I say "shed", I intend to be able to fire up the stick welder without worrying about tripping the supply - yes it will be part "shed", part "man cave". So that means a decent supply.
Our main fuse is 63A, so that puts an upper bound on practical supply size - no point going to the point where there's no discrimination with that, nor larger than I can reasonably foresee using.
Without having done all the checks/calculations, it looks like 6mm² cable would suffice supply wise - possibly 4mm². If I still have extraneous conductive parts then I need 10mm² CPC to the house - so either a separate CPC or upgrade the whole cable to 10mm². If I'm running a separate CPC, then no point duplicating it with a superfluous one in the cable. If I'm running a separate CPC, then it's not such a big leap to making it an earth electrode too - though that does increase the minimum CSA. Yes, quite a few IFs involved

 

[JohnW2]

In our sort of soil, I doubt it will get sufficiently dry, even in the worst of droughts, to become ineffective - perhaps a bit degraded, but not ineffective.

That's obviously possible - I have no knowledge of your soil or 'situation'. In any event, as I keep saying, in reality even a seriously 'degraded' TT earth is adequate to ensure RCD operation - which is why I questioned why you were thinking beyond the usual/simple earth rod.

I am thinking about using a suitable cable - not something that's suboptimal in one or more ways to suit the regs.

When I said "suitable cable" I was referring to one which, itself, contained an adequate CPC/bonding conductor, if such was required, so that no additional CPC/bonding conductor would be required.

I think you have probably made a few assumptions which may, or may not, be appropriate. You haven't asked about the use of this shed (i.e. loads expected), or the supply main fuse rating, or if there's any extraneous conductive parts (there are at the moment, but that may or may not change), ...

Well, the main assumption I made was that, given it's all about your considering using a much 'better than normal' local earth, you were contemplating TTing the shed/workshop/whatever. If that's the case, then I don't see that any of those other considerations you mention are relevant.

Without having done all the checks/calculations, it looks like 6mm² cable would suffice supply wise - possibly 4mm². If I still have extraneous conductive parts then I need 10mm² CPC to the house - so either a separate CPC or upgrade the whole cable to 10mm². If I'm running a separate CPC, then no point duplicating it with a superfluous one in the cable. If I'm running a separate CPC, then it's not such a big leap to making it an earth electrode too ...

As above, IF (another one ) you are going to TT the shed/whatever, then you wouldn't need any sort of CPC / bonding conductor going back to the house, would you?

Kind Regards, John

 

[SimonH2]

I wasn't thinking of TTing the shed - just supplementing the DNO earth.
There's also an element of experimentation - seeing what I get. Might have an impact on what I do at church - where driving rods could be "difficult" due to underlying rock

 

[JohnW2]

I wasn't thinking of TTing the shed - just supplementing the DNO earth.

Oh, in that case you can ignore most of what I've said

However, if you are talking about "just supplementing the DNO earth", then I wonder even more about why you are considering a 'better than usual' local earth, particularly if it is a TN-C-S supply - after all, the lower you get the impedance of your local earth, the more you will probably get involved in the discussions about 'melting CPCs in the event of a 'lost TN-C-S PEN'!!

... but I suppose this leads to another question - why not TT the shed (which, as I said, makes a good few of the potential issues go away)? Have you considered, and dismissed, that possibility and, if so, why?

Kind Regards, John

 

[SimonH2]

Good questions - none of which I have answers to yet. To a certain extent, putting ducting in is a means of kicking decision time down the road a bit
I suppose in one way I just "don't trust" an earthing system that relies on electronics (an RCD) for fault protection.
But then, having read (and been involved in) the various "lost PEN" discussions, I'm not so sure I trust the DNO earth either (or not as much as I did)
Of course, if I did TT the shed, then the ideal cable to use for its supply would be concentric with blue insulated outer cores - you did ask why anyone would want such a cable

 

[EFLImpudence]

Of course, if I did TT the shed, then the ideal cable to use for its supply would be concentric with blue insulated outer cores - you did ask why anyone would want such a cable

I think you are getting confused.

On the one hand, why would the outer cores have to be sleeved insulated and if they were how would that affect ADS if damaged and the need for RCD protection and would it be allowed - and

on the other hand, why not just use armoured cable?

 

[JohnW2]

Good questions - none of which I have answers to yet. To a certain extent, putting ducting in is a means of kicking decision time down the road a bit

Fair enough.

I suppose in one way I just "don't trust" an earthing system that relies on electronics (an RCD) for fault protection.

Many installations (like mine) have no choice. However, you could dramatically increase the degree of trust you could have in such a situation if you did what many people call 'pointless' and put two RCDs in series!

But then, having read (and been involved in) the various "lost PEN" discussions, I'm not so sure I trust the DNO earth either (or not as much as I did)

Quite. Swings and roundabouts. However, for what it's worth, I would say that you should be to some extent comforted by the fact that both failure of an RCD (and I mean real failure, not just the trip threshold creeping up to slightly more than 30mA/whatever) and "lost PEN" ate pretty (probably very) rare occurrences.

Of course, if I did TT the shed, then the ideal cable to use for its supply would be concentric with blue insulated outer cores - you did ask why anyone would want such a cable

Yes, I did ask, but I'm still not sure why you would consider such a cable to be 'ideal' (or necessarily even 'acceptable') , even if the shed were TTd.

Kind Regards, John

 

[SimonH2]

On the one hand, why would the outer cores have to be sleeved insulated and if they were how would that affect ADS if damaged and the need for RCD protection and would it be allowed - and

Because of the requirement for neutral cores to be both insulated and sheathed (or double insulated) unless the entire cable run meets the requirements for single insulated cables (i.e. IPX4 or mumble mumble). Split-con has bare CPC but blue insulated neutral, plain con seems to just have bare wires on the assumption that it's only use would be CNE (combined neutral and earth) in a DNO setting.

on the other hand, why not just use armoured cable?

Because for a given size it's a lot thicker, stiffer, and generally less easy to work with.

 

[JohnW2]

Because of the requirement for neutral cores to be both insulated and sheathed (or double insulated) unless the entire cable run meets the requirements for single insulated cables (i.e. IPX4 or mumble mumble). Split-con has bare CPC but blue insulated neutral, plain con seems to just have bare wires on the assumption that it's only use would be CNE (combined neutral and earth) in a DNO setting. ... Because for a given size it's a lot thicker, stiffer, and generally less easy to work with.

Well, most people would use SWA, despite its size and thickness.

More to the point, as EFLI and myself have both hinted/asked, would buried (in duct) straight con actually be 'allowed', even if the neutral conductors were insulated? As far as I can see, unless it were adequately mechanically protected (by more than just plastic ducting), it is not acceptable to bury any sort of cable unless it has an earthed armour or other metallic 'sheathing', is it? What you propose doesn't sound all that much different from the "T+E in hosepipe" which is advocated/advised in some DIY books I have dating back to the 70s

Kind Regards, John

 

[JohnW2]

More to the point, as EFLI and myself have both hinted/asked, would buried (in duct) straight con actually be 'allowed', even if the neutral conductors were insulated? As far as I can see, unless it were adequately mechanically protected (by more than just plastic ducting), it is not acceptable to bury any sort of cable unless it has an earthed armour or other metallic 'sheathing', is it? What you propose doesn't sound all that much different from the "T+E in hosepipe" which is advocated/advised in some DIY books I have dating back to the 70s

I should perhaps have added that, if you/we decided that such an approach is acceptable (which I rather doubt), then, rather than hunting for the (quite probably non-existent) variant of straight con that you're talking about, why not use 'standard' flexible cables - which are available as 2-core and 3-core (and many-core) in sizes as large as you are likely to want/need ... such as ...

I do, however, doubt the acceptability of burying anything without an earthed metallic covering in plastic ducting (assuming that's what you're thinking of).

Kind Regards, John

 

[flameport]

Concentric cable can't be used within installations as it's a combined N&E conductor. Can't be used with the outer as a neutral either as the wires are not insulated, and all live conductors must be insulated, which includes line and neutral.

Split con can be used within installations as the neutral is insulated, but it's not an armoured cable and can't be used where an armoured cable would be required. It generally costs more than SWA and isn't something to be recommended or used.
The same applies to a mythical concentric cable with all insulated neutral conductors and no CPC.

 

[JohnW2]

I've already indicated that I don't think Simon's idea of using straight con is sensible or (without complicating it even further) compliant, but for the benefit of other readers ...

Concentric cable can't be used within installations as it's a combined N&E conductor.

That presumably wouldn't apply if, per Simon's suggestion, it were being used just as a 2-core (L+N) supply to a TTd outhouse - and I presume that the requirement to 'run a CPC to every point in the installation' would not apply in that situation.

Can't be used with the outer as a neutral either as the wires are not insulated, and all live conductors must be insulated, which includes line and neutral.

Simon obviously understands that, since he is asking about a hypothetical variant with insulated neutral conductors (which I suspect doesn't exist).

Split con can be used within installations as the neutral is insulated, but it's not an armoured cable and can't be used where an armoured cable would be required. It generally costs more than SWA and isn't something to be recommended or used. The same applies to a mythical concentric cable with all insulated neutral conductors and no CPC.

Quite so, as I have already pointed out. I really don't think that it is worth him expending time and effort trying to find a way of avoiding using SWA.

Kind Regards, John