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.
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.
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
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.
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.
On Friday, 16 November 2018 17:48:51 UTC, Pat Pending wrote:
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.
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:
OOI, is there any reason I can't use the neutral incomer as an
alternative to "Supplier's main earthing terminal"?
If there is a small residual voltage on the neutral, swapping the
transformer phase and taking the average of two measurements would help
but should be a fraction of 24V.
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