Actually in the UK 100 ohms is not a 'pass/fail ' limit on a TT system - more a threshold for 'needs investigation'. The immediate fail would be if the resistance was high enough that the exposed voltage on the largest non-tripping fault is below the ELV limit - i.e just under 30mA, for a 30mA RCD, should not result in an electrode voltage rise of more than 50V.  So it could be  about 1500 ohms on a 30ma trip - though in practice  anything anywhere near  that would be very much 'needs urgent  investigation' as probably something has corroded off or been cut. It is however possible to have say 200 ohms in a hot dry summer, and after an inspection of conditions and wiring, to conclude there is no real fault, and it is safe to remain in service.


Also on a lone transformer feeding one or two farms, it is not that certain that the substation electrode is that great - a regs compliant site could be as high as 20 ohms so long as  the LV and HV earths are separated electrodes.

In such cases the results of  a multipoint electrode test and a Zs test diverge  as the substation/transformer electrode resistance is in the loop test, but not the mutipoint probe tests. (though the natural variation in repeated tests of the same electrode on various days and with different test electrode positions is probably comparable)

If the consumer electrode is actually much  lower resistance than the one at the transformer end, then it is not good, as the NE offset voltage  at the transformer (and for all other customers on the same LV branch) moves significantly during a low impedance fault.

In that case an  installation as a PNB connection may be  more satisfactory - being more like TN-S in terms of where the fault currents flow (even if some folk think of it as a variant of PME)

Soil resistivity mapping is available for the UK from the Ordnance Survey.


That is way way beyond the level of planning used by most electricians, they just look for somewhere no one will trip over the box on top of the rod.


If you look at YouTube videos of UK electricians installing an earth rod they are usually knocking a 4’ rod in with a 20 oz carpenters claw hammer around the back of a shed, you don’t seem to see them standing on a hop up driving it with a 2lb lump hammer or a small sledgehammer which you generally need to do with a 12’ rod.


I live on the second terrace River Severn alluvial flood plain, which is actually about the present day flood levels, I can see the river from my bedroom window, but the flood water only gets to the bottom of the road. We are over sand and gravel, which is still quarried just up from us though some of the pits are now flooded as nature reserves and for water sports. Over the sand and gravel is class one horticultural soil, so driving rods actually around where I live is not difficult, but not so far away the bedrock comes to the surface and driving rods is a nonstarter.


But around here if you have not got a low reading at a depth of 12’ it’s not going to make a huge difference going any further, I put a job up for assessment with a rod reading of over 400 ohms with the rod twelve feet into the ground and the assessor agreed that at that depth it’s a stable reading and unlikely to change.


You have to bear in mind that here in the UK we are putting a lot of faith in RCD protection rather than getting the earth resistance down in TT installations. Being “old school” I like a 100 mA S-Type RCD main switch upfront of the 30 mA RCDs, but with the change from Type AC to Type A RCDs that is not possible, so the upfront RCD needs to be a 300 mA RCD, which means a lower Ra is required, however 300 mA Type A RCDs are made for the European market so are not readily available rated at more than 63 amps and UK installation practice is to have a 100 amp main switch, so availability of suitable RCDs can be an issue.


So a lot of installers use a 100 amp main switch and 30 mA RCBOs , which is deemed acceptable. However some use a consumer unit with a 100 amp main switch and a split arrangement with two 30 mA RCDs supplied by internal tails , now bear in mind that we have to use consumer units with steel enclosures in domestic installations, so that is not deemed acceptable. It was something my NAPIT assessor quizzed me on this year as it is a recurring problem he is seeing.


So personally I see issues with the general standard of TT installations in the UK, many are still protected by voltage operated earth leakage breakers, despite them having been obsolete for over forty years, others have RCDs that do not trip, that have not been tested since the day they were installed and the are some very questionable rod installations.


Andy Betteridge

Just to show the other side of the coin too, this thread has just prompted me to do a quick Zs test in my TT'd detached garage - which has in the past given me a reading as low as 36Ω from a single 1200mm rod. Today it's still only 42Ω and that's after a very unusual dry spell for this part of the world - the best part of two months with negligible rainfall.


   - Andy.

Nice one Sparkingchip.

On the very rare occasions I`ve done a TT I use a minimum of two rods. I place them 2 rod lengths apart and test each one independantly. If I get around 60 ohms or less each and don`t expect much in the way of seasonal rise then I`m happy. I do have some extendable rods but have not needed to use them. I`ve found that two rods at 60 appear to give around 40 when both connected. As I said though very rareI use any

I am not saying a £2.99 earth rod won’t do the job, but there does seem to be a great reluctance amongst electricians to move up to the next level of installation practice.


Andy Betteridge

But and it’s a big BUT, there was the proposal that we would have to install installation earth electrodes for each individual installation with a PME connection, in case the PEN failed and all the extraneous metalwork and conductive parts connected to the installation is liven up to 240 volts.

This led to some debate of this forum, but didn’t make it into the regs.


However unlike MEN earthing we would not be able to assume that some of the neighbouring installations were still connected to benefit from their earthing arrangement, so as a stand-alone set up we would need to install a very low resistance rod or tape and apart from the cost, it is often unachievable due to there not being anywhere to install it, because there is little or no access to land around the property.


Andy Betteridge

I remember some New holiday cottages in the Mourne mountains close to where I live had real issues getting a DNO connection because the contractor could not get the electrodes below 200 ohms. Effectively built over granite and a few centimetres of peat, he had tried just about everything including long trenches with large csa bare copper wire and plate. Enlisted a device called chemrod which sorted the issue pronto. Needs maintenance mind you.

Lyle recently posted about an interesting TT earth electrode installation.


Trying to determine why someone did what they did can be intriguing at times.


 Andy B

Right over my head there Andy

If we could easily remove dangerous voltages under fault conditions by simply knocking a metal rod into the ground it would have been done for over a hundred years, but it ain’t that simple.


The question is, do you want to remove external influences on the earthing system within an electrical installation by relying on RCD protection to limit the effect of faults within the installation?


Andy Betteridge