Electrical supply to a shed

[John Rumm]

As it seems to be a common question in these parts, I thought I would try to organise some words on the subject.

See what you think:

comments and additions are appreciated as always.

(and check the sums, I was probably half asleep when I did those!)

[robgraham]

Having three external supplies I felt that I could reasonably take up your request for comments.

Could you check through for TLA's and list them somewhere - I don't know what CPC is for instance.

There is a hole (word chosen specifically!) in both this entry and in the DIY Wiki in general on the subject of installing a TT earth - at least I can't find anything more than a definition of TT. My house is TT and at the moment all supplies are attached to the house earth which particularly the greenhouse and garage should be locally TT'd. My understanding is that this is just a paralleling of earth points and they are not independent earths - if that is correct then your Wiki should say that, and maybe I've missed it, but is it possible that you could give some guidance on installing TT earths please. That would knock off another couple of round tuits.

I did some time ago pick up a procedure using a low voltage transformer for testing TT earths but have not tried it out. I can't remember whether it was posted here or if got it somewhere else. Is it of interest to you?

Rob

[John Rumm]

They should all be in here:

linking to the glossary from the text does not work quite the way I would like yet (it takes you to the top of the page rather than the actual entry). I need to add more links to the text as well.

CPC = Circuit Protective Conductor i.e. the earth wire in a cable.

Yes, good point. Might be worth a separate article...

The can be paralleled or independent IIUC (mine are the latter). The greater the separation distance, then the more logical it is to separate them since there may be differences in the local earth potential between sites.

It has been posted here I think. It would probably be worth including in a TT article.

My personal preference for testing them is using a loop tester... make sure all bonds etc are disconnected for the test, plug in, hit button, read result ;-)

[PeterK]

Is there anything in:

of inclusion or linking to? (

PeterK

[John Rumm]

think John S has already linked that.... (see the external links section at the bottom)

worthy of inclusion or linking to? (

Yup, that might be a good one for an article on earth rods etc.

[Andy Wade]

Re-repost:

In the absence of a fancy earth tester[1], the following simple DIY procedure that I've posted a couple of 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.

Indeed. Bear in mind though that this measures the impedance of the whole external earth-fault loop (Ze), not just the resistance of your own earth rod/electrode/system (Ra).

[John Rumm]

Yup, I will use that for a TT article. Got any other TT related tips to include?

True, but I have found with most electrodes the the contribution of the extraneous wiring is small when compared to that of the electrode, especially if you choose the socket you plug the tester into carefully. Also you are typically need to know the electrode resistance with less precision anyway - the nearest ohm being.