Earthing rod question

Hi, I was thinking of taking a spur from the ring main in my house out to the garage where I plan to use a tumble dryer and a freezer. I would install an RCD protected double socket in the garage:

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thought I would provide a separate earth. We are on clay so I think I can get away with banging an earthing rod through the garage floor.

I read that I need a low resistance, say 50 ohms. But what is the circuit? Form where to where? I can connect one terminal of my meter to the earth rod but where does the other terminal go?

Reply to
John Smillie
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floor.

Good write up here:

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Reply to
Andrew Mawson

Providing a TT setup for a shed power supply could be overkill.

Are there any reasons why you cannot export the earth from the house? Have a look at

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Reply to
ARWadsworth

Does the existing circuit not already have RCD protection?

Do you want your freezer protected by a RCD circuit?

Why do you feel the need for a separate earth?

There are pros and cons of doing so, much depends on your circumstances. Details here:

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will need to know what sort of earthing your house uses. See here for a description of how to tell:

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I read that I need a low resistance, say 50 ohms. But what is the

The test procedures are described here:

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(nice and simple if you have an earth loop impedance meter, bit more complex if not. Sometimes buying a second hand one on ebay and flogging it after is a sensible approach)

Reply to
John Rumm

Thanks for both answers, which provided very comprehensive links.

Reply to
John Smillie

Except that you will not sell it:-)

The worst case is having to hide it behind a tin of old paint in the shed and just telling SWMBO that you have sold it.

Reply to
ARWadsworth

No well, *I* wouldn't, although some might...

(long since learnt, one of the fundamental rules of test gear of any sort - it usually pays for itself fairly quickly)

Mine seems content to allow any number of tool purchases! ;-))

Reply to
John Rumm

You connect a nice thick earth wire from the earth rod to the earth connector block by the CU. The meter isnt generally earthed.

As mentioned, I dont know whether its worth doing this, or simply using the main house earth.

You dont need a fancy tester to determine the rod earth's resistance, very basic equipment will do that. A car battery and a multimeter should suffice.

NT

Reply to
NT

And it doesn't need to be a 'nice thick wire' as the ground resistance is sufficiently high that even the resistance of standard 2,5mm earthing cable is insignificant.

Rob

Reply to
robgraham

For testing, it does not need to be anything special. For installation, it depends on if the earth wire is protected from corrosion, and if its buried or not.

See table here:

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Reply to
John Rumm

But there are minimum CSAs set depending on the route, so that the wire is suficien from a mechanical POV. If it's in the open unclipped or buried direct, it's something like 6mm2 (I don't have the book to hand, could be

4mm2, but definately bigger than 2.5)
Reply to
Tim Watts

To the 'general mass of earth' (which does not exist as a convenient physical terminal...

Re-repost:

In the absence of a fancy earth tester[1], the following simple DIY procedure that I've posted a few times before will give results of entirely adequate accuracy:

[Repost from 20/04/1998, Message-ID: ]

Anyone with a bit of electrical common sense, knowledge of Ohm's law, and a decent multimeter can measure earth electrode resistance quite easily. You need to isolate the electrode in question and then find a way of getting some current to flow into it. A safe way to do this is to use a double-wound mains transformer with a secondary voltage of around 24 (exact value not critical). Connect one end of the secondary via a suitable length of wire to the main earth terminal in the house and connect the other end to your earth electrode via an ammeter.

Energise the primary of the transformer, and the secondary current which flows will immediately give you a rough idea of the total resistance in the circuit, most of which will be attributable to your electrode. For a more accurate result, drive a second temporary earth electrode (a 2ft offcut of 15mm water pipe will do) into the ground at a distance of 10m or more from the one you're measuring. Then use the meter on volts to measure the voltage drop between the two electrodes. Dividing this figure by the electrode current measured earlier gives you the earth resistance. (Reactance in the circuit will be negligible.) Move the temporary reference electrode to a second position and repeat. Average the two values obtained, but if they are significantly different, try further positions for the reference electrode.

[1] Not to be confused with a loop tester for measuring earth fault loop impedance, which *is* an indispensable piece of kit.
Reply to
Andy Wade

how to make life hard. Just connect car battery between mains earth and the new rod via an ammeter. So much easier.

NT

Reply to
NT

according to the rules it does. There are reasons why.

TN

Reply to
NT

I'm obviously suffering from thickness this morning as I fail to see how either the ac or dc methods work.

Draw a box representing your house - draw another box inside representing the load items. Two wires go into the box and are connected by a coil or resistor; thus representing L,N and the load and are all isolated from the building. A third wire now goes from the outside of the load box to either the battery or transformer

2ndry, with the other lead of the power source going to ground.

No current can flow because the case of the load is floating so how does this work ?

I can only assume I'm missing something somewhere as the instructions are written with the utmost confidence, and seeming clarity !! :>)

Rob

Reply to
robgraham

I bow to my learned friend Mr Wade's knowledge and respect for Brother Meow, but would this be sufficient volts to drive meaningful amps through the ground especially if its bone dry like at the moment?..

Reply to
tony sayer

Uh? The current flow is between two things independently connected to earth:

(i) the means of earthing for the house installation, usually the TN earth provided by the electricity supplier (DNO). By statute this must have a resistance to the general mass of earth of less than 20 ohms. In practice it will be very much lower than that in most urban areas; and

(ii) the isolated earth electrode that you are testing.

If the house earthing is TT (with its own electrode and no metallic earth from the DNO) then the resistances may be comparable so you would have to use the reference electrode method to get valid results.

DC methods can be affected by incidental electrolytic PDs (different metals, salts in the soil, etc.) so AC testing is preferred. If you must use DC, reverse the battery and average the results.

Reply to
Andy Wade

That's ok for this particular application where there is a mains earth to make use of. However with a TT only install you won't have a "known good" earth connection to take advantage of.

Reply to
John Rumm

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Having re-read the Wiki to see what I had missed, and decided that it really isn't very clearly explained for a TT system (there is a little bit of an indication that the author was either bored at this point or knew so much that his wording isn't quite clear) - anyway I went off to see what Mr Google could find and came up with this far clearer explanation, which I would commend to anyone seeking to test their TT system.

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Reply to
robgraham

The resistance you get (Vbattery/i_flow) is the R of the 2 earth electrodes added together. In a lot of cases the main house earth will be of very low R, giving only a slight addition to the total R. If the main house eaeth is also a local rod, it will have significant resistance. This can be measured fairly easily, with enough accruacy for this purpose, but hopefully it wont be necessary.

NT

Reply to
NT

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