high earth loop impedance

[stillp]

But John, they do contradict themselves. They don't refer to avoidance of overlap of the spheres of influence, they state that each rod should be outside the SOI of the other.

 

[riveralt]

They contradict themselves here:- (From Wiring Matters)
"Where it is necessary to drive two or more rods and
connect them together to achieve a satisfactory result,
the separation between rods should be at least equal
to their combined driven depth to obtain maximum
advantage from each rod"

There is no contracdiction, and holmslaw is right. The IEE report quoted by riveralt said:

.... provided that each is located outside the resistance area, also known as the sphere of influence, of any other. Generally, the resistance area is deemed to be fulfilled by a separation distance equal to the driven depth of the rod.

If the radius of the resistance area is equal to the depth of the rod, then avoidance of overlap of resistance areas requires a separation of twice the rod depth (assuming both are same depth).

Okay John, in the 11 days left to 'D' let me pick your brains because the way I read the article that is not what is being said - and the articles do appear to be contradictory. I cannot cut and paste the link but the article I am referring to is Wiring Matters Winter edition 2006 (issue 26)- Ricicle's article is Wiring matters Winter 2011 (issue 40).

In the article I quote the author states that the distance between the two rods S is equal to the driven depth D - and goes onto re-enforce this by stating that rods in excess of 2 times the distance is of little benefit.
There are two drawing highlighting the S = D proposal. The first is just two rods separated where S = D and the second is where three rods are separated where S = D but place in a triangle.

 

[JohnW2]

But John, they do contradict themselves. They don't refer to avoidance of overlap of the spheres of influence, they state that each rod should be outside the SOI of the other.

You're right. Since common sense suggests (at least to me!) that there should be no overlap between the two 'resistance areas' (or 'spheres of influence') I 'read' something that was not there! Apologies for having wrongly suggested that holmslaw was right!

Kind Regards, John.

 

[holmslaw]

..

 

[JohnW2]

Okay John, in the 11 days left to 'D' let me pick your brains because the way I read the article that is not what is being said - and the articles do appear to be contradictory.

You're right. As I just wrote to stillp, I read what I thought/think it should say, rather than what it did say. My apologies.

However, does not your view of common sense correspond with mine, in feeling that what should be required is 'no overlap' of the resistance areas?

As for what the 'D' which happens in 11 days represents, I doubt that many of us believe that is means 'disappearance'. However, I really do need to regain some time for the rest of my life activities, so I'm going to try to stick to my promise to myself that, once the 12 months is up, 'D' may at least stand for 'diminished participation'!

Kind Regards, John.

 

[riveralt]

However, does not your view of common sense correspond with mine, in feeling that what should be required is 'no overlap' of the resistance areas?

Indeed there should be no overlap and that was what was being demonstrated in figure six (Wiring Matters No. 21 (2006) - thanks holmslaw). There the author was clear that the perception that the wider the gap between earthing rods the better the results is a fallacy. Once the additional rod was outside the sphere of influence it could be placed in position - this did not need to be at least 2 rod driven lengths apart and could generally be one rod driven apart. He goes on to say that it is the number of rods, outside the sphere of influence, that has the effect on reducing the Ze. Hence his diagrams, supported by a mathematical formula, showing that three rods, one driven rod length apart in a triangle formation would result in a better (reduced) Ze value than two rods several metres apart.

This is especially critical where access space is limited, as around here, by concrete gardens.

As for what the 'D' which happens in 11 days represents, I doubt that many of us believe that is means 'disappearance'. However, I really do need to regain some time for the rest of my life activities, so I'm going to try to stick to my promise to myself that, once the 12 months is up, 'D' may at least stand for 'diminished participation'!

I took it to be 'Decision'

 

[westie101]

We've always installed at one rod length (however long the rod is if multiple rods are being driven) connected by 70mm2 copper wire

 

[JohnW2]

However, does not your view of common sense correspond with mine, in feeling that what should be required is 'no overlap' of the resistance areas?

(A) Indeed there should be no overlap and that was what was being demonstrated in figure six .....
(B) .... Once the additional rod was outside the sphere of influence it could be placed in position - this did not need to be at least 2 rod driven lengths apart and could generally be one rod driven apart.

Can you help me understand how to reconcile (A) and (B) above? If the rods are of equal length and not at least two rods length apart, then there will be an overlap between their resistances areas (speheres of influence).

I took it to be 'Decision'

Oh, there'll be a decision. Indeed, the decision has already been made - to attempt to appreciably reduce the impact of this forum on my time. Having been through this a good few times before, I fear that it may prove to be like giving up smoking or heavy drinking - attempting to 'cut down' just doesn't work (one always 'slips back'), so one ultimately ends up having to make a choice between 'a complete break' or staying with the status quo (the latter being something I really can't afford). In other words, I have no more 'will power' than most other people! Anyway, we'll see

Kind Regards, John.

 

[riveralt]

However, does not your view of common sense correspond with mine, in feeling that what should be required is 'no overlap' of the resistance areas?

(A) Indeed there should be no overlap and that was what was being demonstrated in figure six .....
(B) .... Once the additional rod was outside the sphere of influence it could be placed in position - this did not need to be at least 2 rod driven lengths apart and could generally be one rod driven apart.

Can you help me understand how to reconcile (A) and (B) above? If the rods are of equal length and not at least two rods length apart, then there will be an overlap between their resistances areas (speheres of influence).

You are making the assumption that the sphere of influence radiates out equally in all directions. According to the author, and demonstrated by figure 6 in the document, this is not the case.
http://electrical.theiet.org/wiring-matters/2006.cfm

 

[JohnW2]

(A) Indeed there should be no overlap and that was what was being demonstrated in figure six .....
(B) .... Once the additional rod was outside the sphere of influence it could be placed in position - this did not need to be at least 2 rod driven lengths apart and could generally be one rod driven apart. Can you help me understand how to reconcile (A) and (B) above? If the rods are of equal length and not at least two rods length apart, then there will be an overlap between their resistances areas (spheres of influence)

You are making the assumption that the sphere of influence radiates out equally in all directions.

I am indeed - well, equal in all directions in the horizontal plane. Nothing else would make any sense since, if there were inequalities in the horizontal plane, how on earth would 'nature'; know which directions to favour?

According to the author, and demonstrated by figure 6 in the document, this is not the case.

Well, for a start (and cointrary to what you say), Figure 6 does appear to show the resistance area (sphere of influence) equal in all directions in the horizontal plane (but much longer in the vertical plane). However, the 'problem' with that Figure is that it shows the horizontal (equal in all directions) extent of the resistance area as being far less than the '1 rod length' suggested by the IEE report quoted by riveralt. In fact, the radii of the resistance areas in that Figure look as if they are not much more than one third of the driven depths of the rods - hence they have managed to draw a diagram showing resistance areas which fail (to a substantial extent) to overlap, despite the separation of rods being only one rod length.

I'm not suggesting that the Wiring Matters article is wrong in what it's saying (indeed, I assume it's correct, since it's presumably based on empirical data). However, if one believes (as you seem to have agreed) that resistance areas should not overlap, the implication is surely that (per Figure 6) those areas must be much smaller than suggested by the other IEE report, isn't it?

Kind Regards, John.