What Rods should read

[malcolmsanford]

Here is what a rod should read and what our old friend E54 advocates.

Fair enough it is not a single rod but 15 19mm dia x 1800mm length rods in a network around the building all joined by a 70mm conductor. So a bit expensive and also would pay havoc in Mr Browns raised flower beds!!!

It is a functional earth for data and our remit was less than 1ohm, so pretty pleased with that

 

[Bobster]

Give it 12 months and it'll probably be lower still!

 

[uksparks]

That's impressive.

I am putting a load of rods in tomorrow, I have got 8x 5/8 rods to play with, so will see where it leads me, I have not got a massive amount of space to play with, but enough to hopefully get a good reading.

Why did you join them with 70mm just out of interest? Not exactly cheap stuff to buy.

And what spacing did you have between the rods roughly?

 

[malcolmsanford]

You made me get the drawing out now lol.

There in fact 21 (I missed one) dotted around the building They are roughly 7-10 meters apart depending on basically where it was convenient to site a pit. I wanted to get each rod approx valuation of between 10-15 ohms.

The 70mm conductor is not cheap I agree but I spec it as I needed to hit that 1 ohm target. I was thinking of using some lightening tape but I decided against that as the conductor lays bare and I wasn't sure how it would react in the soil.

I have yet to find any sort of realistic calculations on getting the "right" results for this. I guess you can take soil resitivity and conductor sizes etc, but the bottom line is suck it and see. I may have been able to get away with 50mm conductor, but it it hadn't worked, then there is a lot of red faces on the site, especially mine lol

 

[uksparks]

I have a load of spare 25mm I might use for the ones I am doing tomorrow, its over kill but it will help.

Were they 4ft rods coupled together then or did you get one big rod?

I was thinking of putting concrete pits in around this outbuilding as there about a 12" gap between the footings of slab and the grass where its been cut back and its about 18" deep, they are now just back filling the entire thing with gravel, so it might just end up being in plastic end cap things that you put on the top of the rods as you can just move the gravel out the way to access it. I don't know yet, id rather put pits in, ill have a look tomorrow to assess what I can do.

 

[malcolmsanford]

The rods are 19mm diameter by 1800mm long so they are 3/4" x 6ft rods approx. Strip the PVC off the cable and lay it bare in the trench.

I prefer pits but you have to counteract with cost. On industrial as I was on, for the 30K we spent on getting this 1ohm reading, it now allows 10+ million pounds of IT equipment to be installed, so in the scheme of things pennies, but in a smaller job it would add up.

What are you trying to achieve value wise

 

[uksparks]

Jeeees, thats a lot of money!

blimey 3/4 rods too!!

Well ideally I would like to get a sub 1 ohm, but will see, its only a large summerhouse thing, but I would rather not accept 200ohms, I banged a rod in my garden last week, two 5/8 rods connected end to end and got 42 ohms, I think with 6 I might stand a fair chance of getting a good reading, but who knows! Ill find out in the morning.

The cost does start to creep up indeed, the pits are the best part of £30 each, so its a little hard to explain to the customer I need the best part of £100 of concrete pits when they don't even understand what a rod does.

 

[ifindoubtdont]

This is a very informative thread for me as I haven't involved with any rods yet, but time will come. You read all the bumf and look at the drawings and think, yes I sort of understand it. But this is just like being on site without getting cold and wet. Thank you Malcolm and Uk great work.

 

[uksparks]

As a person with clear capability of getting a good Ra, you know when you couple rods, what's the best wY to overcome the fact that your coupler has just made the hole too big and only the bottom rod is touching earth?

 

[telectrix]

the earth will settle to the rod in time.

 

[Bobster]

Hence the give it 12months comment.

 

[uksparks]

I had a feeling you would say that, it's probably worth giving the ground a tap with a sledge hammer for good measure anyway.

 

[telectrix]

or just pee down the rod.

 

[UKMeterman]

Why does an IT system need a functional earth RA of less than one ohm?

 

[Marvo]

Great result and very impressive if you only required 2 meters to get 10-15 ohms per rod.

We've tried resistivity sampling in the past but even that can be hit and miss. For us the best way to predict how many rods and what depth is required is to knock in one or two sample rods and take readings every meter as they go in. We've even taken to doing this at the quotation stage if a sub-ohm earth is specified.

@ UKSparks, I'd suggest rather than spending 30 pounds per earth pit you might just use a length of plastic drain and cut a slot each side for the wire coming in and going out. You get 225mm PVC pipe and end caps (8-inch in old measurements), you could make numerous rod covers from a single length of pipe.