high earth loop impedance

[Notasparki]

Hi, looking for a bit of assistance if anyone can help. my brothers boiler was recently serviced by british gas. he was left a safety advice notice by there engineer stating "ELI above 200, please seek electrical support" he has a TT system, the house was fully rewired about 6 years ago. Please could anyone tell me what posisble causes could be. I am an maintenance electrician in a factory so i have good electrical experience but not in domestic properies. Any help would be much appreciated.

 

[chivers67]

What's the condition of the Earth spike/rod?

Have you got a meter you can test it with yourself?

Was a certificate provided with test results when it was rewired as this may show if it was "ok" back then.

 

[ricicle]

A TT system usually consists of Ze running into tens, or even a couple of hundred ohms. Although it is possible for Ze to be as high as 1666 Ω with a 30mA RCD, regulations state that anything over 200 Ω could be unreliable.
To get the Ze below that figure will require new, longer or even several earth rods.

 

[Notasparki]

the house was rewired before he moved in and the records were never passed on. the rod looks ok to me but i'm no expert. would i be ok running another earth in along the same path as the other and adding a second rod in the same pit?

 

[holmslaw]

..

 

[ricicle]

The rods normally need to be at least the rod length apart.

 

[riveralt]

the house was rewired before he moved in and the records were never passed on. the rod looks ok to me but i'm no expert. would i be ok running another earth in along the same path as the other and adding a second rod in the same pit?

Deeper is better is my experience - one rod screwed to the other.
If you intend to run a second pit then the distance from the first rod should be at least as far as the rod is deep.

 

[holmslaw]

..

 

[ricicle]

The rods normally need to be at least the rod length apart.

Its at least twice the length apart.

Yes, sorry, two rods means twice the length apart.

 

[riveralt]

The rods normally need to be at least the rod length apart.

Its at least twice the length apart.

Yes, sorry, two rods means twice the length apart.

Holmshaw you are incorrect - there is no additional value obtained in a second rod being more than twice the length of the rod.
Ricicle, you were correct in your initial statement but not in the latter.
The value of Ze obtained from utilising earth electrode rods is determined by several factors. These include the size of the rod both length and diameter, the type of ground that it resides in, the weather conditions, the time of the year and where appropriate the number of rods connected to the original earth electrode.

This is a paragraph from the IET report on the TT multiple earth electrode installations,

Some ground conditions may warrant a number of earth rods connected in parallel in order to achieve an acceptable value of RA. The benefit of multiple earth electrodes is that the overall resistance is then
virtually proportional to the reciprocal of the number of earth rods installed 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. A separation distance in excess of twice the driven depth offers little benefit.

In my experience, if ground conditions allow, deeper will be more cost effective than multiple rods. Recent example in sandy soil with 5/8" 4 foot rod - Ze = 247 Ohms.
When connected to additional rod of same size and driven deeper - Ze = 75 Ohms.
Interestingly when connected to the main protective bonding (gas/water) supplies Ze = 0.25 Ohms.

 

[ricicle]

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"

 

[efiste2]

Purely from a DNO point of view, when installing Primary and Secondary substations I find that in good ground, stacking the earth rods can achieve good results, but in poor ground we usually place earth rods some distance away from the sub (50 metres plus on primarys)and knock in rods every 10 metres or so along the bare earth cable length back to the substation....

 

[holmslaw]

..

 

[JohnW2]

...When connected to additional rod of same size and driven deeper - Ze = 75 Ohms. Interestingly when connected to the main protective bonding (gas/water) supplies Ze = 0.25 Ohms.

Interesting. Those figures are almost identical to those I've mentioned before in relation to my TT installation (with a single, old, earth rod of unknown length), in terms of the Ze measured on the rod alone.

As I've said before, in my case, I think the low Ze with main bonding connected is probably aided by the fact that the metal water supply pipe goes to a neighbouring property that has a TN-C-S supply, so their main bonding will be giving me a connection, via water supply pipes to their PME earth. Interestingly, even if I disconnect just the main bonding, the Ze is still only about 0.35Ω, presumably by virtue of incidental parallel paths from CPCs to the water supply pipe.

Kind Regards, John.

 

[JohnW2]

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).

Kind Regards, John.