Yet more on PME earth export
- Thread starter JohnW2
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May I probe you a little further ....
... If I did as you say, how would you suggest that I should supply the various lighting installations and sockets scattered around the garden (and, in case you're tempted to suggest it, many are a long way from the greenhouse, so not particularly practical to supply them from there).
Kind Regards, John
Wyou will have arranged with a known or recommended electrical contractor at a time and date to suit yourself so you will be there.
Even if you get a jobsworth it ain't legally binding!
I don't think you have necessarily interpreted "when I'm not around" in quite as decisive a sense as I meant! Hopefully, I'll 'be around' for a very long time, but ....!!
... and, no, not legally binding, but it could possibly cause hassle and distress (and maybe even unnecessary expense) to someone less able to defend the situation than me.
Kind Regards, John.
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Ah got you!
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The question is what is the equipotential zone? Where an estate has a square of houses with the gardens all surrounded by the houses then the whole of the garden area is likely considered as part of the zone.
In real terms we don't have an equipotential zone but we have a gradient and the aim is to make the gradient that shallow that the voltage differential the body can touch will not adversely effect it.
To take a garden shed and use a local earth is all well and good until you use an extension lead and export that local earth to another area.
Having a garden shed or other out building with a remote earth can result in the gradient becoming too steep.
So to use a remote earth rod is to me clearly not to be desired. The rod should be local to item earthed. The area covered may be increased to install earth rods in each corner of the garden with joining tape would likely ensure the whole garden was equipotential zone. This was done by my friend but not to stop getting a shock but to produce a good ground plane to ensure minimum EMC problems when transmitting.
With my son he also did not want and voltage gradient the first method was to use diodes in the earth connection to get a 1.2 volt barrier to stop damage to cathodic protection. Latter he stopped importing the earth all together using an isolation transformer.
To me the biggest problem with the earth system is not getting a shock but damage caused to metal in the ground. Try finding a steel earth rod. Why use a copper coated rod when the metal being bonded to is steel? Even worse where zinc lumps are welded to the steel. As to an aluminium frame like with a green house even worse. I would be looking for a large lump of aluminium as an earth clearly not copper to use copper would be crazy but people do. Why?
In real terms we don't have an equipotential zone but we have a gradient and the aim is to make the gradient that shallow that the voltage differential the body can touch will not adversely effect it.
To take a garden shed and use a local earth is all well and good until you use an extension lead and export that local earth to another area.
Having a garden shed or other out building with a remote earth can result in the gradient becoming too steep.
So to use a remote earth rod is to me clearly not to be desired. The rod should be local to item earthed. The area covered may be increased to install earth rods in each corner of the garden with joining tape would likely ensure the whole garden was equipotential zone. This was done by my friend but not to stop getting a shock but to produce a good ground plane to ensure minimum EMC problems when transmitting.
With my son he also did not want and voltage gradient the first method was to use diodes in the earth connection to get a 1.2 volt barrier to stop damage to cathodic protection. Latter he stopped importing the earth all together using an isolation transformer.
To me the biggest problem with the earth system is not getting a shock but damage caused to metal in the ground. Try finding a steel earth rod. Why use a copper coated rod when the metal being bonded to is steel? Even worse where zinc lumps are welded to the steel. As to an aluminium frame like with a green house even worse. I would be looking for a large lump of aluminium as an earth clearly not copper to use copper would be crazy but people do. Why?
We're really talking about something far more fundamental than voltage gradients in the ground since (as you partially point out yourself) those (usually pretty shallow) gradients can be bypassed by cables.
What you say may be of of some importance when one has high density housing and small gardens. In that situation, the gardens may be to some extent within the area of influence of nearby PME electrode(s). However, in many situations, particularly rural ones that may not significantly be the case. My greehouse is certainly a very long way from the nearest PME electrode and the ground there can, I would think, be regarded as being at roughly true earth potential.
Consider the situation we are contemplating, in which I take a long copper cable, connect it at one end to the PME earth of a property and run the other end down to the vicinity of an outbuilding. If (in the extremely rare scenario we are contemplating), the potential of that PME 'earth' rises to a high potential relative to true earth potential, then there will be a potentially dangerous pd between it and the ground in the vicinity of the outbuilding. Anyone touching it, or anything connected to it, whilst standing on that ground (as I've said, in the worst case with bare wet feet) is at risk of (potentially serious) shock, without the benefit of any RCD protection.
I'm not saying anything 'for' TTing the outbuildings (which, I agree, brings its own issues and problems) but, rather, am pointing out the theoretical hazard which causes some people to be 'against' exporting the PME earth. I agree that such a hazard exists qualitatively but, as I've said, I believe that the magnitude of the risk is far too small for it to be of concern to me personally
Silly though it might sound, I think the analogy I cited yesterday is valid. Consider a house with a PME supply and the required main bonding to the metal water pipework of the house. That pipework is in electrical continuity with taps and pipes touchable within a shower cubicle. If I then installed a stainless steel shower tray in the cubicle and (in a moment of madness?!) didn't bond it but, instead, connected it to something close to 'true earth' potential (e.g. an earth rod), I would expect you and others to say that I was being crazy, and acting in a potentially dangerous and certainly non-compliant manner. However, it's functionally hardly any different from the outbuilding situation - a TN-C-S-dertived earth connected to touchable metal parts in an area where someone (perhaps with wet bare feet) could be standing on something close to true earth potential.
Kind Regards, John
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