Earth Electrode Query

[Jabbajaws]

Would it be permissible under bs7671 to use two earth rods bonded together, in order to make one?

I have read the section by Lenny on Earth Electrode resistances, and note the diagram where multiple rods are used. (1.2m spacing for two rods).

As l am pretty much An Earth Rod Virgin at mo, id like to know as much as poss, before l get my first rod to install.

Why can't l put the two rods right next to each other and bond them together, to make one lower resistive earth rod, than to space them out?

Any advice appreciated. :wink5:

 

[telectrix]

you start with 2 rods, screwed together to make a long one (2m). if the Ra is too high, then you bang in another. as you rightly say, spaced 1.5x the rod length. too close together and they tend to cancel each other out. not sure of the theory here, but i'm sure it's on another thread.

 

[Jabbajaws]

Thanks Telectrix...

 

[spark 68]

Yes it is permissable within BS7671, even desirable in some cases to use more than one earth rod.

A deeper thicker single rod will always be better and more stable than two or more thinner shallow rods, see E54's many posts on this subject.

Where it is neccessary to use multiple rods, they should be spaced not less than the distance buried (depth), as they will influence each other.

There is no real point in adding two rods close to each other, as there will be little or no improvement than a single rod would give due to the influences mentioned above, to see real improvement (lower Ra) they need to be spaced apart (see above).

 

[Jabbajaws]

Just to clarify guys, in the event that l do need to use multiple rods, do l bond the rods together to treat them as one rod, or do l run separate parallel runs from the MET?

 

[telectrix]

either way is acceptable. make sure you install a decent earth pit for each.

 

[spark 68]

Just to clarify guys, in the event that l do need to use multiple rods, do l bond the rods together to treat them as one rod, or do l run separate parallel runs from the MET?

You can link them together with a single continuous conductor back to the MET, or use two seperate conductors from each rod back to the MET.

However Tel's suggestion of using deeper multi-part rods, screwed together, as a single rod is the best way in most cases.

 

[Jabbajaws]

Thanks once again guys...

 

[MarkieSparkie]

Just to clarify guys, in the event that l do need to use multiple rods, do l bond the rods together to treat them as one rod, or do l run separate parallel runs from the MET?

You can run a single earth conductor from the MET to the first electrode and then to each subsequent electrode in a 'series string', it is considered good practice to this with one continuous cable. The limit to this method is about six electrodes, beyond this it is usual to split the field of electrodes into multiple strings.

 

[Engineer54]

Just to clarify guys, in the event that l do need to use multiple rods, do l bond the rods together to treat them as one rod, or do l run separate parallel runs from the MET?

Why do you think, that you will need multiple earth rod positions if this is a domestic set-up??

Do not make the mistake of using multiple single 1m rods over coupling them together, you could well end-up with an unstable system. You need depth for stability, which should also give you better Ra values into the bargain, so always, always go for depth over multiple short rods. The only exception is where it is ''KNOWN'' that soil depth, before hitting rock will not support coupled rods.

In such cases, it may be a better idea to use bare hard drawn copper conductors to inter-connect the separate driven earth rods. Id also suggest positioning the earth electrodes in a ''Crows Foot or Star'' configuration. The first rod should be complimented with a decent flush earth rod pit, the other rods can be left buried with the conductor to rod connection protected by a good covering of amalgamating tape. Although far better to braise or exothermically weld such buried connections... Drive the buried rods as deep as you can, even if it means excavating a hole of some sort, as i say, depth is the key to a decent TT system... And Yes, the multiple rods should be treated as a Single earth electrode...

What others have been stating about distance between rods is correct, although it won't make much of difference to exceed 1.5 times the buried depth. Each rod creates a ''zone of influence'' that should never be over lapped. The problem is, without a dedicated earth tester you won't know if you have overlapping zones of influence between two or more rods.... (The 1.5X distance formula, will generally keep you clear of over lapping problems)!!

 

[ackbarthestar]

Here's a little more info

Installation and measurements of earth electrodes - Electrical Installation Guide

The interesting factor here, not specifically mentioned in the link, (although distances between the rods are mentioned), are the zones of influence between each of the installed earth rods and why there is an optimal distance. which would indicate the fall off potential so that there is no overlapping gradient between each of the electrodes

 

[Chr!s]

I would ensure your spacings are 2.2 times the length of the Rod, providing you have the space, this should ensure no overlapping.

 

[Engineer54]

I would ensure your spacings are 2.2 times the length of the Rod, providing you have the space, this should ensure no overlapping.

It would serve no useful purpose spacing rods at these sort of spacings on a multi rod installation... 1.5X the driven rod depth is ample.

 

[telectrix]

Well, that spacing has worked for thousands of years between the klingon and romulan empires, so it should be ok for the humble earth rods.

 

[ackbarthestar]

I've just had a chat with my wholesaler and he said that it would be difficult to drive in an inter-galatic earth rod, anyway they didn't sell them and suggested B&Q

 

[telectrix]

they are cheaper at the borg on line shop. they come ready integrated as a collective combination of rods.

 

[ackbarthestar]

But then that would be a special order - No point in any measurements, since Resistance is Futile

 

[Chr!s]

It would serve no useful purpose spacing rods at these sort of spacings on a multi rod installation... 1.5X the driven rod depth is ample.

One, it's not the depth, it's the length of the rod which determines the size of the hemisphere, two, the 2.2 is the correct spacing, at 1.5 there will be overlapping of the hemisphere's. Come on E54 get current lol

 

[telectrix]

well they're going to overlap when the borg take over anyway.

 

[ackbarthestar]

Enter The Starship Part P Enterprise in the Galatic Federation....To boldly go where no other earth electrode has gone before.....Well
that's the end of any sense from this thread....