Drop bare earth in trench ?

[SimonH2]

At some point I plan to install some ducting to a shed which will replace what's left of the greenhouse that didn't survive the storms. I'm thinking that it would be an opportunity to bury around 15-20m of earth conductor in the bottom of the trench. Other than the cost, any reason why not to do it ?
Anyone got any guestimate of what sort of impedance I might get ? It's "ordinary" soil - not sandy, not clay, and I guess should be around 18" deep. I'll partially backfill with soil (i.e. cover the earth electrode) before dropping the duct in with sand bedding. TN-C-S supply.
And what would you use ? flat copper "tape" ? bare stranded copper ? something else ?

 

[JohnW2]

At some point I plan to install some ducting to a shed which will replace what's left of the greenhouse that didn't survive the storms. I'm thinking that it would be an opportunity to bury around 15-20m of earth conductor in the bottom of the trench. Other than the cost, any reason why not to do it ?

I can't think of any, but the first question which comes to mind is "Why"? If you're contemptating TTing the shed, why not do as everyone else does and just use an earth rod?

Anyone got any guestimate of what sort of impedance I might get ? It's "ordinary" soil - not sandy, not clay, and I guess should be around 18" deep. I'll partially backfill with soil (i.e. cover the earth electrode) before dropping the duct in with sand bedding.

I suspect that, despite the length, the impedance might (at least 'at times') be pretty disappointing - in my experience, earth rods generally don't have very low impedances unless they go well into the subsoil. My greatest concern would probably be the 'at times' issue - because, at, at such a relatively shallow depth, it could well 'dry out' during periods of relative 'drought'. Having said that, the impedance really only has to be low enough for an RCD to work, so that 'almost anything' would probably be adequate!

Kind Regards, John

 

[flameport]

Surface area of the electrode is the main factor, so flat tape will be much better than a round wire.

 

[JohnD]

The metal you use will be subject to corrosion.

Some buildings have a mat of conductors laid in the ground for a really good earth. They used to use thick copper strip as used on lightening conductors, but maybe stainless steel grid would be cheaper. Or copper pipe. Connection points will be most prone to corrosion damage. Stranded wire would be very unsuitable due to the thinness of individual strands.

There are probably standards published by BT, who would be the biggest users.

 

[JohnW2]

The metal you use will be subject to corrosion.

It will, but no more than when the same metal (or metal coating) is used for conventional earth rods. As you go on to say, copper would seem to be the most obvious material.

Kind Regards, John

 

[Mottie]

When I had my own electricity supply added to my workshop about 35 years ago, the LEB told me where to dig a hole on my side of the wall facing the street and they dropped me round a big metal spike. My mate who worked for the LEB at the time told me to hammer it into the ground for the earth. I did that and he checked it and said it wasn’t great so he gave me another one that screwed on to the original and I hammered that in too. Must have gone down at least 6 feet. Still wasn’t great but he said 'that will do'!

The interesting bit was watching a bloke work on a three inch thick mains cable, splitting it open, separating the individual 1” thick cables inside and soldering my cables onto them while it was live. Fook that - one slip and that would be it!

 

[ericmark]

This is what was done where I worked at the Point of Aye gas terminal, there was a large physical size resistor about the size of a small van between the TN-C-S supplied earth and the site earth, to limit the current in the earth cable should there be a loss of PEN, and this is the problem, with a simple earth rod, between 2 and 6 at 1.2 meters were required to get 8Ω and 230/8 = 28.75 amps which is not really a problem for a 6 mm² cable, but with an earth mat you could get 0.8Ω so 287.5 amp, and this would melt the cable under fault conditions.

 

[mattylad]

IF you have a trench, then look into the condudisc - Flameport can tell you more about how good they are.

 

[EFLImpudence]

You could always use the house earth.

 

[SimonH2]

When I had my own electricity supply added to my workshop about 35 years ago, the LEB told me where to dig a hole on my side of the wall facing the street and they dropped me round a big metal spike. My mate who worked for the LEB at the time told me to hammer it into the ground for the earth. I did that and he checked it and said it wasn’t great so he gave me another one that screwed on to the original and I hammered that in too. Must have gone down at least 6 feet. Still wasn’t great but he said 'that will do'!

When you think about it, you've put in less than 2m of a relatively small rod - so not much contact area. That's why I'm thinking that utilising the trench would give me significantly larger contact and dispersal area.

This is what was done where I worked at the Point of Aye gas terminal, there was a large physical size resistor about the size of a small van between the TN-C-S supplied earth and the site earth, to limit the current in the earth cable should there be a loss of PEN, and this is the problem, with a simple earth rod, between 2 and 6 at 1.2 meters were required to get 8Ω and 230/8 = 28.75 amps which is not really a problem for a 6 mm² cable, but with an earth mat you could get 0.8Ω so 287.5 amp, and this would melt the cable under fault conditions.

I have a family member who works for a DNO. He once gave dad and myself a tour of the local 132/33/11kV substation. He did mention that they use a large earth grid across most of the site - which I suspect will be in part to avoid the potential (no pun intended) to have dangerous voltage gradients where people might be working. And yes, a big neutral earthing resistor between a transformer star point and the earth grid.
He also mentioned that they have had problems with the earth grid being ripped out by thieves - along with various other metal thefts.

IF you have a trench, then look into the condudisc - Flameport can tell you more about how good they are.

I've heard of those before. I suspect Conducrete would be a better fit for this project. I do find myself wondering if it's one of those "if you have to ask the price, you can't afford it" kind of things.

You could always use the house earth.

It was really a case of - I'm digging a trench, so apart from the cost of materials there's almost no effort in adding a significant earth electrode. A couple of things make me think of this :
* That there was at one point an expectation that it would become a required thing for BS7671 - so putting one in now would avoid needing to try and drive a rod later.
* Discussions regarding lost PENs. SWMBO keeps an eye on someone's cottage while they are away - and one day they had no power. I didn't have any tools/meters with me, but following the overhead wires back, I'm fairly certain it was the PEN that was broken at the top of the pole - 3 poles along and after at least one other property was connected. We have underground services, but still scope for it.
* At some point I guess we'll end up with an EV of some sort - and hence a charge point.
* 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.

 

[JohnW2]

When you think about it, you've put in less than 2m of a relatively small rod - so not much contact area. That's why I'm thinking that utilising the trench would give me significantly larger contact and dispersal area.

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

However, you haven't really answered my "Why?" question (why do you want an unusually low TT earth impedance?). As I said, if it's lowe enough to facilitate RCD operation (which almost anything would be) that would seem adequate - and if it gets too low, then we get into these discussion about "melting bonding conductors in the event of a 'lost TN-C-S PEN'!

It was really a case of - I'm digging a trench, so apart from the cost of materials there's almost no effort in adding a significant earth electrode. A couple of things make me think of this :
* That there was at one point an expectation that it would become a required thing for BS7671 - so putting one in now would avoid needing to try and drive a rod later.

Having been dropped at the eleventh hour from the original version of BS7671:2018, it's now back with Amd2 - but only as a recommendation, not a requirement. However, even if it ever becomes 'required'#, hammering in an earth rod in the vicinity of a garden shed is never going to be much of a mission - and there's nothing about the 'recommendation' that suggests they would require an impedance any lower than any other TT electrode (i.e. <100Ω).

* At some point I guess we'll end up with an EV of some sort - and hence a charge point.

In the vicinity of your hypothesised garden shed?

* 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

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

- does anyone make concentric with the outer layer all blue insulated ? I can't think of any reg that would prohibit that.

Why would you want to use such a cable?

Kind Regards, John

 

[Mottie]

When you think about it, you've put in less than 2m of a relatively small rod - so not much contact area. That's why I'm thinking that utilising the trench would give me significantly larger contact and dispersal area.

Well it’s been fine since 1988 so I assume it’s worked. Until something goes wrong……

 

[JohnW2]

Well it’s been fine since 1988 so I assume it’s worked. Until something goes wrong……

I don't know for sure, but I would not be surprised if my TT rod (which, like yours, is still fine) is the 'original', installed when the house first had an electricity supply, which was best part of a century ago. It was certainly looking 'well old' (as my offspring would describe it!) when we inherited it in 1987.

Kind Regards, John

 

[ericmark]

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, 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.

But under 200 Ω is accepted, common for single rod 60 Ω, and as long as more than 8 Ω the current will not exceed 30 amp so no problem, the problem is when below 8 Ω, one could go for a 100 years with a 4 Ω reading and no problem, 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.

 

[securespark]

I have seen people bury old copper cylinders before.