Connecting earth rods

[homealoan]

Hi, I'm getting a ze of 150ohms from an earth rod. To bring it down to a more acceptable level I thought of banging another one in and looping them together. Does anyone know if there is a specific or recommened distance that the rods have to be from eachother? I seem to remember hearing somewhere that they should be a rod's lenght apart??

Thanks

 

[Lectrician]

150ohm is acceptable, but lower is achievable.

I would look for suitable soil more than a suitable distance.

Earth tapes are usually more effective than earth rods, but mean digging a trench. Laying a hard drawn copper conductor (50 or 70mm really) into a trech can give very low readings. this is what the DNO do......

We did a job a while ago which envolved a 100m trench for a 400mm submain, and also the installation of a 630amp genset and automatic transfer switch. We run a 50m length of hard drawn bare copper in the same 100m trench as the submain. It gave us a reading of around 30ohm. This was the grounding poing for the genset.

 

[Gees]

Hi, I'm getting a ze of 150ohms from an earth rod. To bring it down to a more acceptable level I thought of banging another one in and looping them together. Does anyone know if there is a specific or recommened distance that the rods have to be from eachother? I seem to remember hearing somewhere that they should be a rod's lenght apart??

Thanks

I would think that ANY distance is sure to lower the ze. It cannot make it higher....

 

[ban-all-sheds]

You'd think wrong.

 

[Taylortwocities]

The best method is to install a longer (extension piece) or use a thicker rod.

If you use a second rod it must be at a distance more than the other rod according to this article http://www.keison.co.uk/furse/furse20.htm

BS7430 apparently says 2.2 times the driven depth between each rod . So it looks like the further apart the better.

There is a lot of scary formulae on this. If you can't get to sleep have a gaze at
this!


Gees, re

I would think that ANY distance is sure to lower the ze. It cannot make it higher....

It would seem that it can make it higher, as they interfere with each other if they are too close.
Have a peek at http://ecmweb.com/mag/electric_bringing_grounding_down/ and scroll down to "Ground Rod Spacing"

What fun, hey?

 

[plugwash]

There is a lot of scary formulae on this. If you can't get to sleep have a gaze at
this!

Not that scary really but anyway they are based on some rather flawed assumptions like assuming ground resistance is constant.

Have a peek at http://ecmweb.com/mag/electric_bringing_grounding_down/ and scroll down to "Ground Rod Spacing"

What fun, hey?

Two rods close together may be considerablly worse than two rods a long way apart but it should still be better than a single rod.

 

[holmslaw]

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[ricicle]

there are areas (central london) where one long rod is better

Gotta mind out for the tube trains though !!

 

[flameport]

It would seem that it can make it higher, as they interfere with each other if they are too close.
Have a peek at http://ecmweb.com/mag/electric_bringing_grounding_down/ and scroll down to "Ground Rod Spacing"

The total resistance cannot be increased by adding rods, regardless of where those rods are. That website actually states

The overlap increases the net resistance of each rod

which is very different from increasing the combined resistance of the set of rods.

Two rods located very close together will be virtually the same as a single rod. As the distance between the rods is increased, the total resistance measured when the rods are linked together will reduce.

 

[homealoan]

Thanks for all the replies and useful links. Looks like I've got some reading to do!

 

[holmslaw]

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[Taylortwocities]

More like 150 years.
The first London underground railway was opened in 1863....

 

[holmslaw]

..

 

[Taylortwocities]

More like 150 years.
The first London underground railway was opened in 1863....

and earth rods had not been invented.

You must be joking. From the earliest days of communications the mass of earth was used as a signalling and return path for systems. Maxwell and Tesla used this as part of their discussions and later patents.

Right from the beginning wireless required substantial earths (including rods) and aerials. Ditto telegraph and telephone. The ringing current for many lines (party/shared service lines) rang between one leg of the transmission pair and earth.

The earth facilities were often provided by earth rods.

 

[holmslaw]

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