<1.0 ohm - how many earth rods will I need

[mfarrow]

Hi

I need to instruct the installation of an earth farm requiring a Ze of less than 1&#937;. I've not done this before, and neither have the guys undertaking the work, so I'm just wondering how many 5/8" rods will generally be needed in this circumstance?

I know the answer is probably somewhere between 1 and infinity, but a rough guide based on experience would be helpful. Afterall, I don't want to under-order and then have to break the day to get some more.

The area is (apparently) carboniferous limestone and is roughly 100m from the coast, so I don't know how far we'll be able to knock them down.

Any help much appreciated.

 

[ebee]

I think you might just be asking the impossible.
For a start your supplier will only be able to guarantee within 20 ohms anyway which means in practice a figure of 10 ohms or so might be the best they could offer you.

Lets say on a good day you might get 60 ohms for one rod.
In theory two rods sufficiently apart might therefore give you 30 ohms (35 to 40 being more likely though) .

So, if you have between 60 and 100 rods you might, just might achieve 1 ohm on your earth, add that to the suppliers earth then add the little bit from the suppliers line conductor and you are still well above what you hope to achieve for a Ze

Why do you want this figure? No RCDs ?

Actually I think the suppliers stated is 21 ohm not 20 ohm anyway

 

[mfarrow]

Sorry I'm unfamilar with this 20 ohm figure. Is this the value at the DNO's star-point on their transformer? I would have thought this would be pretty good though on a TN-C-S system?

The supply feeds a UPS for a secure supply. The earth needs to be there in case the DNO is lost and the UPS continues to generate, although if the DNO can get away with 20 ohms at their point of generation then can this be said of the UPS?

 

[Adam_151]

Normally I aim for sub 20 ohms when installing a rod for generation. I think less than 1 ohm will not be very easy.

You will need a earth rod resistance tester to test the thing, although as this is a UPS rather than a genset in middle of a field, I suppose you could get an indictaion by using a loop tester and 'borrowing' the mains feed, of course you'll be measuring the irrelevant DNO side with this as well, but as you say, on TNCS it'll be low ish

Lowest Zdb I've seen on TT was 1.44 ohms, that was a row of 14 metal frames units, each with their own electrode, and all with a common bonded steel frame which was also set into the ground

 

[ericmark]

I was given the figure of 8 ohms and around the site it varied between 3 rods and 8 rods 5 being about standard. This will vary with different ground conditions but to get 1 ohms you are looking at an array of rods not a single.

Where the supply entered the site we sank four rods in a square and used bare copper cable between each rod which was after buried. However to measure would be a problem as the distance between the probes would be well over the 30 foot normally used so we just measured each rod then assumed it would be below 2 ohm but the total was not measured.

There was a massive resistor between the national grids earth and ours only ever seen it on the one site I assume it is to allow for the earths to be a different voltage without excessive current flowing. I have seen where a radio ham laid a very good earth where when there was a fault on the supply it burnt out the earth wire even when the power was turned off so one does have to be wary of very low values.

Except for radio transmitters I fail to understand why anyone would want such a low reading. The DNO I think have to have around 20 ohms on a TT supply so even if you got zero one would still need RCD's as the total loop impedance could raise to 20 ohms. Normally I look for around 60 ohms on a domestic as being low enough even if the ground should dry out to trip the RCD.

So who has given you the figure of 1 ohm and why? It must be some very large installation likely with it's own generators to need that sort of figure.

 

[westie101]

The figure of 20 ohms is used when DNOs have separate HV & LV earths at a transformer (the HV being 40 ohms)

If the earths are combined the figure must be 1 ohm or below.

Never assume it is one or the other!

This may be of assistence

The resistance to earth of a length of buried uninsulated cable sheath (or bare copper conductor) depends upon the soil resistivity. The length L required to achieve 10&#937; is given by: L = 30 x (R/100) where L is the length in metres, R is the soil resistivity in &#937;m (eg if R = 1000&#937;m, L = 300m).

Typical soil resistivity values are given in Table 1 below:
Table 1
Ground Type Resistivity (&#937;m)
Loams, garden soils etc 5 to 50
Clays 10 to 100
Chalk 30 to 100
Clay, sand & gravel mixture 40 to 250
Marsh, peat 150 to 300
Sand 250 to 500
Slates and slatey shales 300 to 3000
Rock 1000 to 10000

Table 1 may be used to assess the likely resistance to earth of existing buried cable sheaths or conductor. It may prove useful for assessing the likely lengths required of additional conductor laid to form electrodes. The 10&#937; value may be obtained from distributed electrodes and may include
rod electrodes.

 

[RF Lighting]

Best I've managed is about 1.5 ohms on a farm, but there was 20m² of buried steel concrete rebar mesh, and 4 no. seperate 1.2m rods to acheive this.

 

[JohnW2]

Best I've managed is about 1.5 ohms on a farm, but there was 20m² of buried steel concrete rebar mesh, and 4 no. seperate 1.2m rods to acheive this.

Any idea what the resistance was after a year or two of rusting of that rebar mesh (or did you find some SS mesh?)?

Kind Regards, John.

 

[RF Lighting]

No idea! The mesh has only been in about a year. The rods were put in to actually provide the main earth. The rebar in the concrete was 'accidentally' providing an additional source of earthing by being bonded.

 

[JohnW2]

No idea! The mesh has only been in about a year. The rods were put in to actually provide the main earth. The rebar in the concrete was 'accidentally' providing an additional source of earthing by being bonded.

Oh, I didn't realise it was acrually in concrete - I thought you menat taht you just buried the mesh! Mind you, just 4 1.2m rods would presumably get you nowhere near 1.5&#937;, so I guess the 'accidental' elements were mainly responsible for the low resistance!

Kind Regards, John

 

[RF Lighting]

Sorry I should have been clearer. The rebar mesh was installed to facilitate the bonding of a concrete floor. I can't remember the reading off the rods, ~35&#937; seems to ring a bell. It was certainly good enough for use as a TT earth, but no where near as good as when the rebar was connected.

 

[mfarrow]

Hi

Sorry for the late reply. The need for 1 ohm has been decided by the designer.

The UPS is a 20kW unit, single phase.