Cable size

[uksparks]

Hi,

I am going to be connecting an out door power supply, away from the building, its all metal casing.

The supply is 10mm, I am using a 3 core SWA for part of the run, the inside part was going to be in twin and earth, but then I was just thinking, going on table 54.8 section 544.1, am I looking into this too much or does it need to have a 10mm earth as its extraneous metal which is fixed into the ground?

In which case I was just thinking of running 3x 10mm inside the building so the earth is 10mm too to them comply with min size of bonding conductor.

I am right to make it 10mm throughout the entire run aren't i?

 

[davesparks]

4 rods? I'm not suggesting for one min this is going to be acceptable, but sub 200 ohms is deemed as stable, so it would be easy to satisfy that requirement, but it's satisfying the other part of the equation.

Not that old pile of horsecrap again! nowhere does any regulation state the 200 ohms is acceptable or stable, it only says that above 200ohms it may not be stable!

There is an equation to calculate the maximum Ra for earth electrodes used for EV charging points and I believe teh result comes out at around 2ohms.

 

[uksparks]

Unless you are specially ordering 8' rods from someone like furse then you'll be using the same 4' long rods we all use no? A single 4' long rod will not provide a stable resistance to earth, you'll need at least 8' depth to get stability so will need at least 2 joined (i recommend solder/braze the joint to stop it rattling loose when you wallop it in)

Add to this I have never yet managed to get a single figure Ra with a single rod, and even if I did I wouldn't trust that to remain stable.

Im just at a loss as to know where to go with this installation, I have no planned for this extra work of rods, it's not be factored in, an oversize on my part.

 

[uksparks]

Not that old pile of horsecrap again! nowhere does any regulation state the 200 ohms is acceptable or stable, it only says that above 200ohms it may not be stable!

There is an equation to calculate the maximum Ra for earth electrodes used for EV charging points and I believe teh result comes out at around 2ohms.

Thats the equation I am looking for.

 

[davesparks]

No I missed that one.

4 rods equals 4 rod pits which equals more cash...

Not necessarily, 4 rods interlinked by conductors brazed to them would not require any earth pits technically, but you would normally have one at the rod which is connected to the cable which runs back to the MET. 4 Rods makes a nice crowsfoot arrangement with the earth pit installed at the rod which the others are spurred out from.

 

[davesparks]

Im just at a loss as to know where to go with this installation, I have no planned for this extra work of rods, it's not be factored in, an oversize on my part.

If I remember correctly, in the last thread you had about this job you said that the other quotes were significantly higher than yours didn't you?

I think you might just have stumbled upon the reason your quote was so much lower.

 

[uksparks]

There's were £4000 more min, I'm oonly talking a few earth rods here.

 

[uksparks]

Not necessarily, 4 rods interlinked by conductors brazed to them would not require any earth pits technically, but you would normally have one at the rod which is connected to the cable which runs back to the MET. 4 Rods makes a nice crowsfoot arrangement with the earth pit installed at the rod which the others are spurred out from.

So the tops of the rods, are you suggesting they are just left covered in mud? Seems wrong to me.

 

[spark 68]

4 grand more ?
I wonder if they are/were going this same route ? and if they know something about the soil resistivity that you don't ?

At the moment I would be looking seriosly at all methods, Archy's suggested route is looking better by the minute

Seriously though UKsparks you should have researched this a lot more before quoting, this is not a dig btw, we are here to try and help you.

It might be a steep learning/cost curve but hopefully you might have learned something by the end of it

 

[uksparks]

I've had a thought about this, I'll connect 4 rods together with 16mm which will be cheap enough, are you saying its acceptable to just put the 16mm in the ground unprotected? Seems a bit wrong, is there a proposed depth or can it just be under the surface?

 

[uksparks]

The cost of running another earthing conductor in is not a problem, it's just the issue of the rods really, I appreciate your help on this, it's very interesting and had opened my eyes.

I think they were just rip off merchants to be honest.

The rods are cheap, it's more the hassle of putting them in.

 

[davesparks]

The best thing for linking rods together is bare copper conductors, it helps to lower the Ra as it then acts as horizontal electrodes.

But bare copper conductors are not cheap!

 

[davesparks]

I've had a thought about this, I'll connect 4 rods together with 16mm which will be cheap enough, are you saying its acceptable to just put the 16mm in the ground unprotected? Seems a bit wrong, is there a proposed depth or can it just be under the surface?

Don't go fixing on a specific number of rods until you have actually got some in and tested them.
You may very well find that 2 8' rods will be adequate, but you could also need 4/5/6 8' rods.
Generally speaking you don't gain much more reduction in Ra going deeper than 8' and you don't get stability by only going 4'

 

[spark 68]

I've had a thought about this, I'll connect 4 rods together with 16mm which will be cheap enough, are you saying its acceptable to just put the 16mm in the ground unprotected? Seems a bit wrong, is there a proposed depth or can it just be under the surface?

UK I have no idea if 4 rods are too many or not enough, you will have to try what Dave suggested earlier.

Put one long proper rod (not a PXssy twig), measure it, then add another spaced at least 1.5 the length of the first rod away, rinse and repeat until you get a low enough Ra, check out the calc Richard Burns did to get the desired reading you need. That is if you are still going with this method

Alternatively look at all of the other options including Archy's earlier suggestion.

 

[uksparks]

Im off to sleep now and will sleep on it, I'll be in touch gain tomorrow.

Thanks everyone.

 

[HandySparks]

So the tops of the rods, are you suggesting they are just left covered in mud? Seems wrong to me.

I think that this would be dependent on the method of connection to the rod. davesparks' option of brazing would count as maintenance-free, but it's hard to think of any other method that would be acceptable buried.