How to install earth pit?

[bonebill]

I saw an earth pit in Screwfix (yes I know it's a shed).

Was wondering how to install such a thing.

Looking at the photo, it looks like there is a small outward flange at the bottom with what looks to be mounting holes in it.

So I am guessing that it wants to sit on something like a concrete footing (with a square hole in it - for the earth rod to go through), buried in the ground so that the top of of the footing is below the surface by the height of the earth pit assembly.

Does that sound right?

 

[Beanzy]

Dig square hole, no need for concrete. If very stony ground, move the pit to the least stony area. Run a bit of PVC conduit from destination point to earth pit. Use the conduit bending spring to make the necessary shape. drop a draw-string through attatched to a weight so you can pull your 16 or 25mm G/Y cable in. When drving the rod put the clamp on first, if it's one of those they sell on SF, as the top will mushroom out and prevent it being installed later. Back fill around the outside of the PVC box and all done. Good idea to protect the destination end of the conduit so water only enters the pit from the soil around not down the conduit.

Then do your earth loop measurements if testing the rod that way. What kit do you use for testing?

 

[Big_Spark]

Before installing the pit, perhaps you should read up on the requirements for testing Earth Rods and the precautions to note when doing so.

It is more than just "sticking a copper rod in the ground"

 

[bonebill]

Dig square hole, no need for concrete. If very stony ground, move the pit to the least stony area. Run a bit of PVC conduit from destination point to earth pit. Use the conduit bending spring to make the necessary shape. drop a draw-string through attatched to a weight so you can pull your 16 or 25mm G/Y cable in. When drving the rod put the clamp on first, if it's one of those they sell on SF, as the top will mushroom out and prevent it being installed later. Back fill around the outside of the PVC box and all done. Good idea to protect the destination end of the conduit so water only enters the pit from the soil around not down the conduit.

OK.

Then do your earth loop measurements if testing the rod that way. What kit do you use for testing?

Good question.

Before installing the pit, perhaps you should read up on the requirements for testing Earth Rods and the precautions to note when doing so.

It is more than just "sticking a copper rod in the ground"

OK it looks like I will have to look into it some more.

The point of the exercise is to establish a proper earth for a building with a TT supply which currently has an earth of unknown pedigree, so I'm not going to be satisfied unless it's measurably proper.

Bill

 

[mapj1]

For a TT supply I presume you will have a 100mA or 300mA slow tripping RCD immediately after the meter, or at least before the first earthed box...
or maybe even a 30mA normal RCD, if the circuit if occasional nuisance trips are acceptable.
What you need to ensure is that under any fault which does not trip the RCD, the touch voltage (between the earth terminal, which will be made slightly live by the fault current) and true earth (terra firma, and any pipes, fence posts stuck in it cannot exceed 50V, or for an 'agricultural installation 25V). The fault current is taken as
For a 30mA RCD, this implies an earth resistance of less than 1 kilohm for agriculture, and 2K for an all indoor installation, (if it was 25mA and 25V it would be 1K exactly) but one would try to get much lower than this to allow for soil drying out in summer etc. Of course for a 100ma then instead of 1K read 250 ohms, and for a 300mA trip read 100 ohms. Regardless of RCD type, I'd worry if it wasn't less than 100 ohms when new and damp, personally, and consider adding another rod, a rod length or two away connected in paralell, even though that might not strictly be needed to 'squeak a pass'.
There are 2 ways to test, one involving another earth electrode, and passing a fault current down the earth under test and looking at the rise in voltage, and the simpler one which is the basis of 'earth loop' type testers. The simpler method measures the phase to earth terminal voltage, and then (for a very short time !) connects a low value resistor (usually either 10 ohms or 100 ohms and wound like a fire element, to allow it to swea off the heat) between them, and monitors how much that voltage droops. The change in voltage divided by the current through the resistor is the impedance of the 'fault loop', i.e. the resitance of the live feed and the earth path together.
In a TT system, it is usually the earth rod resistance that dominates, as the live feed resistance will be sub ohm, otherwise the full load drop would be noticable.
While the test is underway, no-one should be near the earth electrode, as the 'step voltage' (the voltage between two nominal wet feet on the ground one pace apart ) could easily be dangerous.
Also for this test the live must be taken from the supply side of any RCD, as of course otherwise it will trip it.
actually, now I've typed all this guff I realise its all here... with nice diagrams.
http://www.tlc-direct.co.uk/Book/8.6.1.htm
oh well, happy reading.
regards M.

 

[bonebill]

For a TT supply I presume you will have a 100mA or 300mA slow tripping RCD immediately after the meter, or at least before the first earthed box...
or maybe even a 30mA normal RCD, if the circuit if occasional nuisance trips are acceptable.

The plan is to have a new, separate 100mA RCD before a new split-load CU (with 30mA RCD, so we have discrimination).

The existing arrangement is what appears to be an trip switch (ELCB?) before a distribution board (with fuses). In fact there is a Henley block feeding two trip switched (ELCBs?), each feeding a fused distribution board.

What you need to ensure is that under any fault which does not trip the RCD, the touch voltage (between the earth terminal, which will be made slightly live by the fault current) and true earth (terra firma, and any pipes, fence posts stuck in it cannot exceed 50V, or for an 'agricultural installation 25V). The fault current is taken as
For a 30mA RCD, this implies an earth resistance of less than 1 kilohm for agriculture, and 2K for an all indoor installation, (if it was 25mA and 25V it would be 1K exactly) but one would try to get much lower than this to allow for soil drying out in summer etc. Of course for a 100ma then instead of 1K read 250 ohms, and for a 300mA trip read 100 ohms.

OK that makes sense.

Regardless of RCD type, I'd worry if it wasn't less than 100 ohms when new and damp, personally, and consider adding another rod, a rod length or two away connected in paralell, even though that might not strictly be needed to 'squeak a pass'.

I didn't realise 100 ohms was achievable with a single rod.

There are 2 ways to test, one involving another earth electrode, and passing a fault current down the earth under test and looking at the rise in voltage, and the simpler one which is the basis of 'earth loop' type testers. The simpler method measures the phase to earth terminal voltage, and then (for a very short time !) connects a low value resistor (usually either 10 ohms or 100 ohms and wound like a fire element, to allow it to swea off the heat) between them, and monitors how much that voltage droops. The change in voltage divided by the current through the resistor is the impedance of the 'fault loop', i.e. the resitance of the live feed and the earth path together.
In a TT system, it is usually the earth rod resistance that dominates, as the live feed resistance will be sub ohm, otherwise the full load drop would be noticable.
While the test is underway, no-one should be near the earth electrode, as the 'step voltage' (the voltage between two nominal wet feet on the ground one pace apart ) could easily be dangerous.
Also for this test the live must be taken from the supply side of any RCD, as of course otherwise it will trip it.
actually, now I've typed all this guff I realise its all here... with nice diagrams.
http://www.tlc-direct.co.uk/Book/8.6.1.htm
oh well, happy reading.
regards M.

My feeling is that this is the kind of test I would do so infrequently that I would need to get a man in to do it to make sure it's done properly.

 

[bonebill]

The point of the exercise is to establish a proper earth for a building with a TT supply which currently has an earth of unknown pedigree...

Saw the next door neighbour's board today and his supply is PME, complete with a little notice saying so

I didn't look to see if his supply comes off the same pole.

Looks like I might need to put in a call to the REC...

Bill

 

[ebee]

It might still be an idea to put your earth rod in.
a loss of N on PME although rare does happen at times.
Your earth rod could just help mitigate some of that step voltage

 

[riveralt]

Don't you just love these very early twenty first century posts

 

[ban-all-sheds]

You'd think that by now ebee would have learned....

 

[ericmark]

I would agree although I think the problem is Google I have tried to find information in the past and been directed to some very early posts.

I was surprised at the name "Earth Pit" although looking on screwfix they do call it a "Plastic Inspection Earth Pit" I would have looked for earth rod cover. I would have considered an Earth Pit to be where one had dug out and put tape between the rods. I would have called the item an earth rod cover.

I was also surprised at the advice on fitting the collar on first as rarely have I been able to get the readings with a single rod and I have tested each rod as it went in and only fitted the termination once I had the reading. I used a petrol band jack or "Paddies Motorcycle" as we called it and it was not easy to drive between 3 and 7 x 1.2 meter rods into the ground.

However we were required to get a reading of 8 ohms or better with a house I expect one would accept a much higher reading.

But my concern is the voltage gradient under fault conditions the main reason for the cover is to ensure one can't get closer than 12 inches to where the gradient starts. Personally I would prefer to see concrete even if in slab form around the top of the cover so one one is temped to try and plant anything near it.

One hopes that the RCD will disconnect under fault conditions so the gradient is short lived. Items like lighting may not be RCD protected if one can get an 8 ohm reading and I have never really studied as to how far out the gradient starts.

 

[JohnW2]

You'd think that by now ebee would have learned....

We've all done it - even you I presume it usually results from clicking on one of the 'Similar Threads' at the bottom of the page and then forgetting that one has thereby stepped back in time!

Kind Regards, John.

 

[ban-all-sheds]

Actually, I'm as certain as I can be that I never have.

I have joined in replying to an ancient topic after someone else has resurrected it, not noticing the dates, but I don't think I've ever initiated the resurrection.

 

[JohnW2]

Actually, I'm as certain as I can be that I never have. I have joined in replying to an ancient topic after someone else has resurrected it, not noticing the dates, but I don't think I've ever initiated the resurrection.

That may be true. As you say, you've certainly joined into a ancient thread ressurected by someone else, without initially twigging the 'thread history' (and ending up having to post a ), but it may be true that you've never been the primary culprit.

Kind Regards, John.

 

[ebee]

Ok there was a few days gap between the former posts and mine.

I`ll give you that

Consider it a pause