Todays Earth Rod Efforts

[uksparks]

Hi,

As I mentioned last night, I was going to be putting the earth rods in for this summer house this morning.

I put 8 rods in, this was one stack of two, and two stacks of three rods.

The readings from the two that were 3 rods deep were around 10 Ohms per location, and the two rod stack was approx 17 Ohms.

I tested the first rod at approx 10 Ohms, then connected the second rod to the first which was 17 Ohms which them reduced the total down to approx 6 Ohms, then connected the third rod which was 3 deep and that bought it down to 3.2 Ohms.

So the Ra that I achieved today was a respectable 3.2 Ohms

All rods were connected together with 16mm earth.

I am going to go back next week and bang in another couple of piles of two rods so another four in total because I want to get that sub 1 Ohm reading.

Anyway, what do you think, 3.2 is not bad is it!!


Reading from first rod: 10.6 Ohms (3x 4ft 5/8" Rods)


Reading from second rod: 17.2 Ohms (2x 4ft 5/8" Rods)


Reading from third rod: 9.6 Ohms (3x 4ft 5/8" Rods)


Reading from first and second rod combined: 6.3 Ohms


Reading from all three rods connected together: 3.2 Ohms

 

[IzzyS]

It's an impressive result with so few rods. I'm guessing it's mostly clay?

I didn't mean to offend by calling it an experiment - it's just that you're exceeding the minimum requirements of BS7671, something that hardly anyone seems to do these days. I may have mistakenly got the impression you were just trying to see how low you could go on this particular job, rather than it being a contractual requirement.

In my experience, there's little chance of achieving reliable "TN" values, at a sensible cost, using driven rods (assuming correct spacing and buried interconnecting cable), so you must still rely on the RCD anyway.

Can you post up the measured Zs - it would be interesting to know how your Ra relates to the the overall loop impedance?

 

[Deleted member 9648]

Interesting read, I wonder how many people would have been happy with a reading between 100-200 ohms...

Just out out of interest, how come you decided to tt the summer house?

+1 :yes:

 

[Richt16]

Hi,

As I mentioned last night, I was going to be putting the earth rods in for this summer house this morning.

I put 8 rods in, this was one stack of two, and two stacks of three rods.

The readings from the two that were 3 rods deep were around 10 Ohms per location, and the two rod stack was approx 17 Ohms.

I tested the first rod at approx 10 Ohms, then connected the second rod to the first which was 17 Ohms which them reduced the total down to approx 6 Ohms, then connected the third rod which was 3 deep and that bought it down to 3.2 Ohms.

So the Ra that I achieved today was a respectable 3.2 Ohms

All rods were connected together with 16mm earth.

I am going to go back next week and bang in another couple of piles of two rods so another four in total because I want to get that sub 1 Ohm reading.

Anyway, what do you think, 3.2 is not bad is it!!


Reading from first rod: 10.6 Ohms (3x 4ft 5/8" Rods)
View attachment 29132

Reading from second rod: 17.2 Ohms (2x 4ft 5/8" Rods)
View attachment 29134

Reading from third rod: 9.6 Ohms (3x 4ft 5/8" Rods)
View attachment 29133

Reading from first and second rod combined: 6.3 Ohms
View attachment 29135

Reading from all three rods connected together: 3.2 Ohms
View attachment 29131

Can I ask were they placed in a straight line and what distance apart thanks for sharing

 

[Robson689]

Can I ask were they placed in a straight line and what distance apart thanks for sharing

They should be separated by the depth of the rod, so 3.6m deep, 3.6m apart for example with 3no rods coupled together

 

[Richt16]

This is where I get confused so if I drive two in to the ground 2.4m and then another two at 2.4 should the space between be 2.4 or 4.8 between them as seen different answers thanks for your support

 

[nicebutdim]

This is a question that attracts many different answers.

General consensus seems to be minimum separation of 1x - 1.1x driven depth. If you can go twice that distance, there's certainly no harm in doing so.

 

[pc1966]

Basically you only get "independent" Ra values to parallel if the rods are far apart, as they get closer together each rod is in its neighbours region of influence so the reduction in Ra from multiple rods gets less. This article has various formulae to allow estimation of those values:

Measurements and Calculations Of Earth Electrode Systems (BS 7430) | EEP

System earthing involves the provision of a connection to the general mass of earth. This connection should have a resistance not greater than that required to operate safety mechanisms to isolate the electricity supply from a fault situation.

electrical-engineering-portal.com

If you have 2 rods and the separation is around the rod's length then you are getting about 80-90% of the improvement you would see if they were very far apart, and that is usually good enough.

Generally if you can get down deeper, below 1m or so, it is better as what is critical to low Ra values is moisture in the soil. The top layer often dries out during summer droughts or can freeze in winter, greatly reducing the effectivity of the top 0.5m or so.