There have been various posts in here over the last few weeks on earth rods and their applications.
A question to those of you who get to play with these things on a frequent basis: What would you consider to be a decent earth loop impedance to be achieved with an earth rod?
I did some measurements on my system prior to starting the upgrade on it, and found that my main house earth (TT system) provided by the gas main with no other connections, rods, or services bonded was giving an impedance of 11.6 ohms. I them repeated the measurement for my outbuildings which have their own dedicated earth rod and got a reading of 22.6 ohms. This latter reading, although well within the 200 ohms allowed in the regs, still seems a bit poor to me, any thoughts?
What was the reading with the earth rod connected. Tested my TTsystem and it was 51 ohms but if i tested today, ( wet and damp ) probably would be lower.
Sounds low enough to me but do the maths with fault current flowing at 22 ohms and you will realise why a TT system is RCD protected.
The house does not have an earth rod at all (yet). The only earth is provided by the (steel/iron) gas main. I was taking a reading so as to put a stake in the ground so to speak, before putting a (earth rod) stake in the gound! I will also add bonding the water main.
Anything under 200 ohms is good, unless you're using a 300 mA RCD. The regs per se require 50 V / Itrip (Itrip beig the nominal trip current of the RCD) so, with a 100 mA RCD, a value as high as 500 ohms would comply. For agricultural and horticultural installations the voltage rise limit is reduced to 25 V, making the max. earthing resistance 250 ohms. The On-Site Guide (OSG) recommends working to 200 ohms max. (a) to give a safety margin, (b) because in most soils an electrode reading over 200 ohms is probably broken, severely corrded or otherwise faulty, and (c) to permit simple measurement of Ze with a loop tester, most examples of which only read up to
200 ohms.
BS 7430 "code of practice for earthing" gives the resistance of a round rod or pipe electrode as
R = rho * ( ln(8*L/d) - 1 ) / (2*pi*L)
where R is the earthing resistance [ohms] rho is the soil resistivity [ohm.m] L is the electrode's length [m] d is its diameter [m] ln() is the natural log and pi has its usual meaning.
Armed with that you could sink a test elctrode and measure your soil resistivity. IME a single 4 ft. rod section will typically give 40 - 100 ohms, but two such sections (8 ft.) will give 15 - 60 ohms (implying that rho decreases with depth, which is not surprising). These are rough figures for 'normal' moist soils. In dry or rocky situations rho can be very much higher and it can be hard work to get a good low resistance. BS 7430 is the bible here and contains much practical advice on the subject.
I usually recommend people to sink at least two sections. The BS cautions that the contribution from the first 3 ft. or so should be ignored for design purposes due to the possibility of the ground freezing. For most of the UK that's perhaps a bit over cautious, but the principle is valid.
It is of course not permitted to use gas or water pipes for earthing [BS
7671 reg. 542-02-04]. (Yes, they should be bonded, but they must not be used as the sole means of earthing.) As well as the obvious possibility that they could be replaced by plastic, a new or re-worked gas meter installation with a metal service pipe could include an insulating insert on the input side of the meter. Transco don't like the idea of electrical fault currents flowing in their pipes for some reason...
Remember that when you measure the earthing resistance (or Zs) you should disconnect the main bonding to water, gas, etc., so that any fortuitous earthing provided by service pipes is not included in the measurement (see Section 10 of the OSG).
That seems an excellent value, although you haven't said what length of rod is involved. With a 100 mA RCD you'd only get 2.3 V rise before it trips. Stop worrying.
I was aware of that bit.... (appart from the horticultural installation bit - bit there is not much chance of any of that going on round here ;-)
That is useful to know - thanks...
Our soil is very heavy clay - and once you get to a depth of a foot or more seems to be permenently moist - so that may account fo the improvement.
Which was part of the reason for the planned upgrade...
Currently there is no bonding to anything (equipotential or otherwise)!
Oddly the earth connection is made to the incoming section of pipe on the gas main _before_ the meter.
That was easy in my case - do nothing ;-)
My main reason for testing the current setup was to see just how good or otherwise the performance with just the gas main was before I add a dedicated rod etc. I was anticipating that the gas main would provide a resonably decent earth bearing in mind its construction and location.
In fact the out buildings are on a 30mA RCD (unlike the rest of the house which it transpires only has a ELCB). The earth rod on the outbuilding predates our living here, so I have no idea how long it is. Having said that I have a 4' one that I plan to install for the house - so a mesurement from that should give some clue as to the likely length of the existing rod.
Was not worried as such - more a case of not having any empirical data to compare against to know where my observations came in the grand scheme of things. Judging by your comments, the readings I was getting sound quite good.
I need to make changes to the electrical system (new CU etc) for other reasons, so that gave cause to investigate in some detail what we had installed to start with (never having looked in that much detail before). Having discovered what is there is quite some way away from current best practice, this seemed like a good opotunity to get the whole lot sorted out.
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