The wiring of a 32A circuity shouldn't object to 23A fault current, how does it know it's a fault current and not a 5kW load? Even the CPC in
2.5 T&E should handle it for a considerable amount of time.The danger here is that the "earth" will have risen above real earth and exposed metal work connected to that "earth" will pose a shock hazard.
Now if you drop that earth impedance to 5 ohms, things are different.
That's not an earth fault as such, and as you say would look no difference from a normal 5kW load.
With a real 5kW load (i.e. between L & N) most of that power dissipation is in the load itself, only a very small amount is in the wiring system.
With that sort of current flowing to an earth stake, then there is the potential that a good amount of power is actually being dissipated around the stake itself. A situation that can result in local heating which drives off moisture and as a result makes the earth less effective still (so the fault current falls, and the touch voltage rises).
With 10 ohms, the equipment earth in the building is going to be sat at near enough full mains for the duration of the fault.
Not that much - a 32A MCB will supply 46A for a fair amount of time.
Also IME, getting down to 5 ohms with a single stake is difficult. I usually find that a single 1.2m stake into moist Essex clay will give around 10 - 12 ohms. If you parallel a couple of them up then you get down to 6 - 7 ish.
Good point, I was only thinking of PME (which is what most houses have by now, right?).
I would guess that there are more TN-S supplies than TN-C-S supplies in the UK.
Depends on how you interpret that. I would expect most new installs are TN-C-S (PME) in preference to TN-S. However I would expect there is still a greater number of TN-S installs actually in service.
Of course you & Adam are right --- I'd forgotten to make that distinction. What are typical resistances to earth on TN-S and TN-C-S installations?
The guideline figures that one uses in the absence of actual values, are
0.35 ohms for TN-C-S and 0.8 ohms for TN-SHowever in reality either can be significantly lower.
Thanks.
There isn't any kind of breakdown within "cooking appliances", is there? I'd be interested in that, since I've always been told that chip pans & smoking in bed constituted the biggest fire dangers.
On Sunday, 18 November 2012 13:24:27 UTC, John Rumm wrote: [snip]
That's a bit worrying. I'm getting 52 Ohms on the one I knocked in for the = shed yesterday! Should be plenty low enough to trip the RCD and RCBOs I sup= pose, but a bit disappointing nonetheless. I went to get an extra rod at lu= nchtime, but I'm having trouble finding anyone that stocks couplers...
Cheers,
Colin.
Much depends on your local soil conditions and type. In some cases getting under a few hundred ohms can be hard.
52 if the same all year round is ample with a RCD.
In addition to Johns reply "significantly lower" is a bit of an understatement - John knows that I have a TN-S supply with a Ze of 0.00 ohms:-)
As for a typical Ze then there is no definate answer - however my experience is that most built up areas have a Ze lower than 0.20 ohms regardless of the type of supply to the house. Smaller villages and houses out in the middle of nowhere are usually (but not always) the ones with the higher maximum readings.
"Ze" for "zero"! Have you had John calibrate your meter? ;-)
Of course, the electrons have to swim upstream.
Metrel calibrate my meter.
Thanks for the confirmation. I'll just check it now and then...
Cheers,
Colin
It should really have a LCD segment that lights up with "There is a substation in your garden, isn't there?"
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