Just put an electricity supply in to a shed at the bottom of the garden
- 4mm armoured cable to a separate CU in the shed. We have TT earthing, so didn't use the cable earth and put in a 5/8" x 1m earth rod outside. Measured the earth loop impedance, and I have 320 ohms - far too high isn't it? Any suggestions?
Max 200 and ideally less than 100 ohms.
More earth rods and/or longer earth rods if you are able to drive them into the soil.
This time of the year its wet and more imperative to aim for 100 ohms for times when the ground is dry.
+1
I did a site supply this the summer. Just at the end of the 4 weeks without rain. Could we hell get the Ze below 220 ohms. Last week I added an extra 110V supply and it had pissed it down all week. Now down to 30 ohms and the apprentice lost a work boot in the mud.
Seriously though, I am not really following the logic of the earthing regimes. They are now so muddled that really any earth is better than no Earth, Brian
Is it permissible to bond the local earth to neutral in this situation?
Pardon my ignorance, but is there any reason you can't connect the house TT earth to the shed via the cable? By all means use the new stake in parallel. An RCD at on or another end of the cable is probably needed either way.
No. That would give you a TN-C system and that is not allowed in domestic settings.
I actually had a very nice TNC-S supply to work with. Exporting the earth was forbidden.
What you suggest sounds logical, but I was told not to use the house earth but disconnect it and put in a separate TT system. Not sure why, what do the experts think?
I was aware of the advice for that advice for the TN-C-S system if there was likely to be extraneous earthed conductors in or near the outbuilding. But I can't see why that should apply to a TT system. Though I would also like to hear from the experts on this.
Its higher than ideal certainly, although will still pass enough fault current to trip a 30mA RCD in most circumstances.
To get a lower Ze, you will need either/or longer earth rod / additional earth roods spaced away from the first one. Most rods can be coupled to additional ones to increase the length.
You would want to avoid exporting the the house earth in some cases...
With TN-C-S systems, while you can legitimately add an earth electrode of your own to it (it just becomes another of the "multiple" earths of a PME system), exporting the earth also means you have to extend the equipotential zone to the outbuilding. Depending on what it is and the nature of its construction that may not be practical; e.g. a greenhouse with easy access to an independent earth, or you may not have enough cross section of copper available in the sub main to have it function as a main bonding conductor.
With TN-S you won't want to join the exported earth to the TT system since there may be significant distance between your earth spike and that of the substation, and so there could exist a difference in the local potentials, giving rise to higher than expected protective conductor currents. (also in the event of a fault in the suppliers earth, you might find your spike and the sheath of your supply cable now becomes the main earth for the neighbourhood!)
With TT you could join them, although its still wise to aim for a system that can function within sensible parameters without the Ze reduction you get from the "other" bits of the system.
Update on this shed ELI problem. Following the suggestion here I did export the house earth as well as leaving the new earth rod in circuit. This reduced the reading to 109 ohms.
Still wondering why the shed TT earth was so high, I decided to fit an extra earth rod. The rod I fitted was 4 feet not one metre, so obviously by fitting another one it would go down 8 feet. In the end I couldn't get the last foot down so I sawed it off leaving 7 feet in the ground. The reading was now 212 ohms - with the house earth disconnected again. Better, but still not good enough - and I can't think why it remains so high. Anyway, I then reconnected the house earth and got a reading of 59 ohms. I think I'll leave it at that - especially as it's improved the ELI throughout the property!
Still baffled about the high reading though.
What kind of soil do you have? Stony, or sandy soils can be difficult to get a low reading in. As can anywhere that is very dry.
Clay is usually fairly easy. (our TT earth is around 7 ohms)
Due to low Summer rainfall, the subsoil is still very dry this year. Unless you are in a valley bottom, this may be your problem.
When I was in the electricity board decades ago, we used to bury a lump of cast iron in a bed of coke dust for substation earths.
Our soil is quite stony, and consequently well drained, so that's probably the reason.
You're probably right, and our soil is well drained. Overall, I'm happy with the situation now so, as I said, I'll leave it at that.
I wonder? Does the house earth benefit from other earth sources such as copper/lead water pipes, or did you test the house earth rod on it's own.
No, I didn't test the earth rod on its own. As I said in my first post I don't have the tester to do that. The house doesn't have gas and the water pipes - coming into the house at any rate - are plastic so I am assuming that there are no parallel paths to earth and that the readings I get at a socket are fairly accurate.
I take it you are testing with a plug in earth loop tester?
If so you can always temporarily disconnect any equipotential bonding wires at the main earthing terminal while doing the test if needs be for more accurate results.
If you wanted to test the rod in isolation, there is a method for doing so without any special test gear described here:
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