We have a TT supply at our house, i.e. the overall protection is a
100mA Time Delayed RCD and we have local earth rods.Is there a "main earth bond" in such a system? If so where is it? I'm concerned only because if there is a "main earth bond" then should it be some proportion of the meter tail size (even on a TT system)?
TT doesn't need the large earth conductor sizes that TN-C-S (PME) requires, as the RCD ensures that any earth current flow is short lived and requires only 100mA to trip, so earth loop impedence is not particularly relevent (within reason).
However, it is a VERY good idea to install the TN-C-S recommended sizes, as it will enable you to upgrade to TN-C-S when it becomes available in your area.
Christian.
P.S. The On Site Guide has the required dimensions for TT (which vary depending on whether mechanical protection is available). But, as I said, if this is a new install, use the TN-C-S sizes.
Christian.
It's not a new install. I have the OSG too, I should have looked in there, thanks!
Why do you see PME as an improvement ? Where we are the PME earth is so far above real earth you daren't trust it and have to convert back.
You can design for improved circuit protection without either having the whole house on one or two RCDs, or having to use lots of RCBOs.
What sort of voltage difference are you talking about? The supplier is required to ground the PEN (Protective earth and neutral) conductor within some fairly short distance from your house (like 60m, but can't recall for sure). Any significant PEN to ground voltage would indicate a supply fault.
It is(was) about 45 volts, varying throughout the day. The earth is at least half a mile away, I can see it dangling down the side of the pole which feeds another farm first. Problem is when the last farm (of five) on the chain runs a heavy load the earth really moves.
A while back we had a small water leak from the bottom of our hot water tank which sits just above a stone floor. Shouldn't have been a problem to fix but one couldn't touch it without getting a small shock due to the wet floor and of course there is no 'off' switch for ground so we had to wait for the whole thing to drain into the ground before we could deal with the leak.
Went off PME big time then and installed my own earth rods.
There should be one where your service line joins the distributing main. The 45 volts would also point to a fault. However, you can't be losing 45 volts (and probably the same again from the phase conductor) as you'd be pretty screwed if you only had 150V supply. If it's just 45 Volts loss in the neutral and not the phase condutor, that could point to a bad connection in the PEN which is very bad news. If the 45 volts difference is being yanked towards a different phase than yours, then the your mains voltage would be -cos(120º) x 45 =
23 volts higher than it should be, which should also be noticable.You should report this as a fault to your distributor. There's not enough info to tell what's wrong here -- it would be interesting to know the three voltages L-N, L-real ground, N-real ground, but don't try measuring these unless you are compitent at working with mains voltages.
L-N is 251 volts. Drops slightly when the end farm is on load but not much. L-N at the first farm is over the 253 volt limit but they don't want to complain. L-N at the end farm can go as low as about 210 volts on load I gather but they have been told tough.
N-real ground is typically 45 volts. It varies from 40 volts upwards but I have never seen more than 50 volts. Do you think this still indicates a problem at the PEN which as I say is about 3/4 mile away ?
Problem is the Peak Park Authority won't allow any more cables in the Park and those we are using aren't really up to the task. Ideally the farms need splitting into two groups with a separate transformer for each, and I am sure United Utilities would do this if they were allowed. Remember the prime goal of the Peak Park Authority is to turn the whole national park into an unihabited tourist attraction.
Farms (particularly livestock) shouldn`t be provided with a PME earth under the supply regs. Its down to the individual farmers to provide their own earthing arrangements. There *may* be some latitude where only crops are grown.
I know of a case where a horse was killed by a fault on the wiring to a stable because the owners didn`t have a proper earth.
Interesting. But we definitely have a utility installed PME system from many years ago complete with pure green earth wire into the neutral connection. I'll check with the other farms tomorrow.
Does this apply to those buildings on the supply chain that are not farm buildings ? Ours is a farmhouse with separate supply to the barns.
You only reported 2 of the 3 measurements.
That's because I installed a local earth and didn't take that measurement before I did it. I'll try at my neighbour's place this week.
I think I was drunk when I first posted the reply :-}
I don`t think they`re allowed to provide an earth by any means to a farm, particularly with livestock - not just a PME :-}
ISTR any services from a stretch of overhead line that may serve a farm with livestock along its length aren`t supposed to be provided with an earth as it can change the line characteristics.
Some lines that have been refurbished may have an earth facility available, but shouldn`t be used if livestock etc etc.
Earthing never was my strong point :-}
As this line hasn't been refurbished in the last thirty to forty years as far as I can tell I'm now a bit concerned as to how well the other farmhouses are earthed, if at all. As mentioned elsewhere I'll go test them.
BTW, why does livestock make the difference ? Can't see how our sheep or next farm's cows are going to reach any of the supply lines.
The only thing I've ever seen is being careful about the siting of earth rods if livestock are around.
A hefty fault current through an earth rod can create a significant potential gradient radially in the locality. Couple that with nice wet earth and grass and Miss Lamb will be meeting Mr Mintsauce quite soon.
In which case I would have thought it was better that the earth came via the supply???
Timbo
Can`t find anything more specific than this at present, and its not particularly readable in my copied and pasted from page 20 of this pdf file :-} :
Oh ho-ho. There's a single thick wire running down the pole bolted to what I assume (hope) is a buried metal thingy.
(regulation 7(1));
Well if it breaks (as it does on a regular basis) it pulls the line wire inside it down as well so I suppose that's safe-ish.
I don`t know the full circumstances of the dead horse incident, suffice to say when electrocuted to death, they apparently get a h*rd-on.
I believe the vet was less than impressed too, receiving a shock while trying to attend to it (already dead though) :-}
You need to measure all 3 at the same time (i.e not two yesterday in one house and the third in a different house). The measurement is against a local earth anyway, not the supplier's.
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