TT Earth electrode

[googlebot]

Hi all, I am in the process of replacing the earth rod on my TT system. I am using the standard earth rod enclosure from screwfix.

What I intend to do is leave around 7 inches of rod sticking up and then put a 6 inch deep layer of concrete about 6 inches wider than the box all around the base in order for me fix the bottom of the box to something that will ensure it will not move. I will then fix box down and use the last inch of the rod sticking up to connect to the earth cable. ( I am using 2 rods just to be sure). The earth wire will be run in conduit from start to finish.

My only worries are, will the last 6 inches of concrete affect the impedance much and should i find another way of sticking the box down. I just cant see the concrete being as conductive as soil.

All replies and help much appreciated.

John

[John Rumm]

The short answer is "yes". However by how much is uncertain - but it is likely to only be a marginal difference. It may or may not matter either.

The only way to know for sure is to test the rod with an earth loop impedance tester and see what you get. A TT system it is designed with the assumption that the impedance will be notably higher than with a TN system. Typically up to 200 ohms is considered acceptable (although depending on the RCD thresholds used you could sometimes get away with more). Obviously 50 ohms or lower would be better. If when installing a rod you don't get an acceptable reading, you can join on another and keep sinking it deeper. Repeat until you get a result you are happy with!

[googlebot]

OOOppps just read your post properley , I believe you are talking about an earth loop impedance tester and not a multimeter. I wondered why i sounded stupid. Looks like getting a sparky in. :-(

[googlebot]

Thanks for your reply John, sorry to sound stupid but where do i attach my multimeter to get the ohms reading. I have a vague idea that i jab one end into the soil and then the other end on top of the electrode but could be totally wrong. That sounds silly anyway, or is it me thats stupid? :-)

Regards

John

[Chip]

On Tue, 19 Jul 2005 22:35:14 GMT,it is alleged that "googlebot" spake thusly in uk.d-i-y:

Hmm, I have a few dangerous ideas, I think for fear of liability I will keep them to myself and state that it should take a sparky less than 10 minutes to get you an earth impedance figure, and thus not cost much:-)

[John Rumm]

Yup usually done with a bit of test kit designed for the purpose...

However there are two subtly different tests here:

One is an Earth Fault Loop Impedance test. This is done with a suitable tester and needs to be done at mains voltage and while passing a reasonbly substantial current to earth. It will give you a good measurement of the performance of the whole path to earth from the simulated fault location under (the equivilent of) fault conditions.

It is also possible to just measure the earth electrode resistance (i.e. rather than its performance under fault conditions). This measurment can be done with a multimeter and some other bits. Andy Wade has posted a method for measuring this before. Try here:

$0cd8b9e0$LocalHost@dog40%3E&rnum=1&hl=en#627d615b252e3aa8or > Looks like getting a sparky in. :-(

Unless you are near SE Essex, then you could borrow my ELI tester ;-)

[Ian White]

Since the OP was using a Screwfix terminal cover, it may be worth pointing out that the nominal 9mm x 1.2m rods that Screwfix sell cannot be joined.

More seriously, I'm wondering whether those Screwfix rods are well enough copper plated? I've been using a few as site markers for the past month or two, and the copper plating is so thin that they're already quite seriously spotted with rust. The spotting is equally bad both above and below ground, with additional scarring on the parts that have been driven into the ground. I feel highly dubious about the state they'd be in after a few years underground.

In contrast, the "proper job" 15mm x 1.2m threaded rods (from TLC) can be extended, and are much more heavily plated. I recently pulled one of these out after about 10 years in wet clay, and it washed up like new.

See

and the pdf document about earth rods.

[Andrew Gabriel]

If they're the same as the B&Q ones, first cold spell we had and the brass clamp bolt snapped. I couldn't find the head, it must have shot off like a bullet. Using a spare nut, I reclamped it. Next cold night same thing happened again, and the nut and snapped stud had gone about 6' from the earth rod. Then replaced the stud with a steel one, which has been OK ever since. Also, just gripping the rod with the brass clamp bolt was enough to strip the copper plating at that point.

[googlebot]

$0cd8b9e0$LocalHost@dog40%3E&rnum=1&hl=en#627d615b252e3aa8> or

Much appreciated John but alas I am in South Staffs. Thanks for the offer anyway.

It may be a new toy to buy. ;-)

Thanks all for your help.

John

[John Rumm]

Yes good point...

Give me a few years and I can probably tell you ;-)

I used a TLC one for my main earth rod on the house. Into nice permanently damp clay I was getting 11 ohms with just one rod, which seems quite reasonable (same as I measured for the significantly longer water and gas pipes on their own).

However when I measured my outbuilding rod it was giving approx 24 ohms. So, since I also had a Screwfix rod to hand, I slapped that in as an additional one a few yards away from the first. The resultant combined total of approx 12 ohms would suggest its resistance was similar to that of the first outbuilding rod. I shall repeat the measurements over time and see if the resistance starts to drift upwards!