Lol my mate tried to dig out some discarded re-bar with a mini digger the other week ; bit like David and Goliath- hilarious 
Termination of SWA on distant end of TT supply
- Thread starter shedbuilder
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Sounds a bit like my house's 'TT' system with a Ze of about 0.25Ω (with bonding in place), despite and earth rod around 70Ω-80Ω when measured in isolation!
Others may disagree, but I personally seriously doubt that the 1.3Ω is down to the rod alone, even if it within the 'zone of influence' of PME earths. Are you absolutely sure that nothing is 'bonded', deliberately or accidentally, to the TT system that could account for the measured low resistance earth? If there is bonding within the garage, did you disconnect it when you measured the 1.3Ω?
Again, others may disagree, but it sounds OK to me, provided that everything relevant in the garage is bonded to the garage's TT system (except when you're measuring the earth resitstance!). You already understand that the second RCD is a bit redundant, except as an isolator.
Kind Regards, John.
1,3 ohm is highly unlikely through a significant mass of earth. As you mention much more likely that there is some accidental/intentional wired path somewhere.
In my experience, most TT systems show a Ze of under 2 ohms with all bonding in place (of course that is not the official Ze) the official Ze tends to be much higher as you mention (but hopefully under 100 ohms)
I have seen a few TT Ze reading (with all bonding in place, not really Ze) of around 5 ohms. I had to conclude that the extraneous conductive elements (gas and water pipes) where probably very short runs and then transitioned to plastic in the street. Was forced to write 1,667 in the Maximum Zs expected by BS7671 on the EIC.
The best I've seen on a TT was 2 ohms.
It was in a dairy farm, and consists of 3no. 2.4m long rods at various different locations, and a 2m x 20m metallic grid buried in concrete.
It was in a dairy farm, and consists of 3no. 2.4m long rods at various different locations, and a 2m x 20m metallic grid buried in concrete.
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I had one 40 ohms then down to six ohms when the water pipe was connected , old lead water main run under a row of terraced houses close to an old pit head.
That's cheating
What's the lowest anyone has seen with a bog standard single domestic TT rod - 25Ω or so perhaps?
Kind Regards, John.
As previously reported, mine is probably 'borrowing' an adjacent property's TN earth, via bonding and the water supply pipe network - Ze 70Ω-80Ω rod alone, down to 0.25Ω with all bonding in place, and still 0.35Ω when all main bonding disconnected (hence only incidental parallel paths to pipework present).
It's a case of 'what I am allowed to rely upon' (e.g. for earth fault protection) and 'what I've got' being utterly different!
Kind Regards, John.
I'm a bit sceptical about earth rods being "significantly" influenced by ground currents (no matter what the source, sink is) I have practised the rules when measuring Ra with a specific earth rod test meter, making sure the test rods are the specified distance apart etc (I just use an loop impedance tester these days)
OK I can see that the impedance of an earth path can be reduced when there are significant "other" currents flowing in the same area of an earth rod. I have not thought about it too much but assuming it is to do with increased mean free path and abundance of charge carriers not greatly dissimilar to current in a semiconductor device but without the energy band gap of course. Anyone have any information on this?
"
OK I can see that the impedance of an earth path can be reduced when there are significant "other" currents flowing in the same area of an earth rod. I have not thought about it too much but assuming it is to do with increased mean free path and abundance of charge carriers not greatly dissimilar to current in a semiconductor device but without the energy band gap of course. Anyone have any information on this?
"
I think mine was more to do with the ground conditions of the abandoned pit , nearby 100 + ohms is the norm and the whole area is TT.
The gardens are constanly wet and the houses damp.
I'm only speculating about the reason for the very low measured loop impedance via my water supply pipes, but I can't really think of any other very credible explanation for such a low figure.
I live on the edge of a village in a rural area. Until a few years ago, the entire village had overhead supplies and TT systems. Then, a few years ago, following a few incidents involving 'firework displays' in roadside trees, nearly all of the village's supply was shifted underground - and, I presume to some TN system (I really must have a peep at a neighbours installation!). However, my house at the edge of the village and a few surrounding small cottages were left with their overhead supplies and TT installations. I'm not sure of the reason. It may be because they didn't like the look (or length) of the underground route to my house, or may perhaps be related to the fact that mine is (as far as I am aware) the only property in my part of the village with a 3-phase supply.
Anyway, the point of all that drivel is that it leaves me with a neighbour's house, probably about 50 metres from mine (and maybe nearer 100m 'as the underground water pipes fly), which very probably has a TN earth bonded to the water pipe - and my assumption is that I'm probably 'benefitting from' that! - but who know?!
Kind Regards, John.
About 30 for me. I then poured a glass of salt water on it and it went to about 25 for a day or two then back to 30. Mother earth always wins
The effect that pouring water on a rod and seeing a significant improvement, albeit a temporary improvement shows that the transition (metal to earth, clay etc) is quite imperfect does it not?
Mother earth always wins
Your'e right there ,since they shut all the pits up here and turned all the pumps off we have major flooding in the fields when it rains heavy, nowhere for the water to go.
Your'e right there ,since they shut all the pits up here and turned all the pumps off we have major flooding in the fields when it rains heavy, nowhere for the water to go.
Indeed. I've now found the figures I posted recently:
... and that's even without counting the CPC core.
Kind Regards, John.
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The TT earth problem has always been something which I have talked about since my friend lost the neutral on his TN-C-S supply and whole street was connected via his earth system in his radio shack.
But as to dangers if the combined neutral is lost we need only consider fire as it does not really matter if you have a TT supply or not if the neutral is lost the shock danger exists anyway and has nothing really to do with the TT supply.
In the case I refer to his shack was not TT and the 4mm cable was not man enough for load and melted. After whole house was turned to TT.
With the size of cables you are unlikely to have the same problem even if you did use the TN supply. And in real terms if the ground is so conductive then no real problems.
Oddly in fresh water there is a bigger problem then in salt water as since salt water is such a good electrolyte (not really a conductor) the voltage gradient will not electrocute swimmers. But it can with fresh water.
From the reading you give therefore I would say it does not really matter you are highly unlikely to ever have a problem.
But from an interest point of view I would consider some one falling down could at maximum span around 9 foot we will call it 3 meters. And within that distance we want a gradient of no more than 50 volt. The max voltage under fault conditions is 400 volt so the safe distance between items connected to TN supply and TT supply is 8 x 3 or 24 meters.
This is rather a long distance and in real terms likely only 230 volt and body around 2 meters so 4.5 x 2 or 9 meters. But even that is quite a distance and in a typical caravan site or marina getting that distance between the toilet block and caravan etc is really unlikely.
However a caravan has to have a two pole RCD so really we rely on the RCD for protection. Which returns us to your outbuilding. As long as an all pole RCD is used in real terms you have no worries what ever method you use.
So down to nitty gritty the only issue is the gland on the SWA. I would use cleats to ensure physical attachment and a plastic stuffing gland. You can leave the metal box.
I have tried to give reasoning behind what I say since I realise it seems a rather simple approach. But in real terms as long as the building is 6 meters away from any earthed metal on hoist building then that is really all that is required.
But as to dangers if the combined neutral is lost we need only consider fire as it does not really matter if you have a TT supply or not if the neutral is lost the shock danger exists anyway and has nothing really to do with the TT supply.
In the case I refer to his shack was not TT and the 4mm cable was not man enough for load and melted. After whole house was turned to TT.
With the size of cables you are unlikely to have the same problem even if you did use the TN supply. And in real terms if the ground is so conductive then no real problems.
Oddly in fresh water there is a bigger problem then in salt water as since salt water is such a good electrolyte (not really a conductor) the voltage gradient will not electrocute swimmers. But it can with fresh water.
From the reading you give therefore I would say it does not really matter you are highly unlikely to ever have a problem.
But from an interest point of view I would consider some one falling down could at maximum span around 9 foot we will call it 3 meters. And within that distance we want a gradient of no more than 50 volt. The max voltage under fault conditions is 400 volt so the safe distance between items connected to TN supply and TT supply is 8 x 3 or 24 meters.
This is rather a long distance and in real terms likely only 230 volt and body around 2 meters so 4.5 x 2 or 9 meters. But even that is quite a distance and in a typical caravan site or marina getting that distance between the toilet block and caravan etc is really unlikely.
However a caravan has to have a two pole RCD so really we rely on the RCD for protection. Which returns us to your outbuilding. As long as an all pole RCD is used in real terms you have no worries what ever method you use.
So down to nitty gritty the only issue is the gland on the SWA. I would use cleats to ensure physical attachment and a plastic stuffing gland. You can leave the metal box.
I have tried to give reasoning behind what I say since I realise it seems a rather simple approach. But in real terms as long as the building is 6 meters away from any earthed metal on hoist building then that is really all that is required.
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