I'm in the process of setting up some "permanent" HF antennas here, it's a slow process but it is under way in a "sometime this spring/summer" time frame.
Now, one of the antennas will be a long wire fed via a 9:1 un-un transformer and shaped as an inverted-L, the intention being that the ground side of the transformer will be created by the mounting post.
I live in Bedfordshire, in a river valley, so there is a fair amount of clay below the top soil that prevents the sub-soil from drying out really badly although the top soil is already dry and cracked. I have just spent an hour or two driving a 6 foot galvanised steel scaffolding putlog into the ground and there is about 4-1/2 feet of it buried now.
What I'm wondering is, should I be thinking about driving in a couple of copper earth rods as well and bonding them to my steel post or does the collective intelligence of the newsgroup think that the post itself will provide a broadly similar ground connection?
I appreciate that this isn't a dead simple question to answer, and I also realise that I may end up needing to bury some earth wires and bond them back to the post but it would be nice if this step were unnecessary due to the inherent ground being "good enough".
Typically, a single 1m earth rod will give a resistance of 100-250 ohms - won't tie anything down to ground from a mains point of view and you need plenty of them to make a useful rf earth. I'd go with Spike's advice - I suspect wires near or on the surface work best, if soil is lossy you don't want to send rf through more of it than you need to. G4FGQ always argued that there was no need for wire to be in direct contact with the ground, capacitive coupling through insulation was sufficient.
And, if you've a "PME" mains supply, do take the necessary precautions. My preferred approach is bonding.
As for the RF side, I've gradually added more and more buried radials whenever I can- under patios, under the new drive, along fences, etc. All are bonded to a common point. When we have the back lawn re-turfed, it will have radials under it ;-)
An article in Rad Com June 1987, "The Killing Ground" by G3RZP explains the situation. Electrical regulations have changed since then but I believe the advice given there is still valid under the current regulations (17th edition) but this should be checked.
A some questions, for the sake of enlightenment....
THESE POINTS ARE FOR DISCUSSION ONLY
How many Radio Amateurs have been killed by faulty PMEs coupled to their earthing systems in the manner mentioned in the articles? Is it more or less, for example, than the number of Amateurs killed driving to rallies, climbing aerial masts and trees, being struck by lightning?
DON'T DO THIS, IT IS FOR DISCUSSION ONLY: Has anyone wearing normal clothes and shoes ever touched a mains live point without touching any other metalwork? What was the result? Is there any reason why the concept of every piece of metalwork in a house floating at 240VAC during a neutral fault is not a good idea?
If you think that the Electrical Regulations now have safety in mind, why has the number of deaths per annum from unsafe installations gone up since the introduction of Prat P, from 3 to 12?
Do the suggestions in the articles meet the current regulations? Has anyone had their domestic wiring that incorporates the measures mentioned signed off by a competent person under Prat P?
What do Radio Amateurs in other countries do about domestic mains wiring and RF earths?
Anyone can touch the live under the conditions you mention with no shock.
No problem with that if every piece of metalwork is indeed at the same potential. If you have PME *and* an extraneous conductor, possibly your radio earth or something else not bonded, then a hazard exists in the unlikely event of a neutral break as stated in the article.
Part P was a stupid Labour law brought in to add to their desire to control us in every way they could think of. You are quite right in that deaths have gone up as previously competent DIYers and non part P electricians are now more or less outlawed. Cowboy "electricians" are having a boon and don't give a toss whether their work is safe or not.
AFIK, they do subject to conductor sizing and routing.(minimum 10mm sq) and bonded to the earth at the consumer unit where the PME bonding is done.
If they have PME, then common sense suggests they would do the same.
If you want to confirm my viewpoint, then uk.d-i-y is the place to ask questions about PME or any other electrical question in fact. There are several very knowledgeable people on there who will explain the situation much better than I can. In fact to save you the trouble, I have added that group.
The basic description of the danger is still correct. However the suggested mitigations possibly miss the most obvious solution.
Indeed the number of folks electrocuted each year in the UK is very small (typically 20 or less - although since part P came in its (as predicted) been rising). However (if you believe the stats of the ESC) the number injured to the extent requiring hospital treatment is actually quite large.
The vast majority of these result from (mis)use of appliances rather than faults with fixed wiring. Apparently there are still 50% of households with no RCD protection at all.
The concept of an equipotential zone takes on a particular relevance with PME supplies since you have to allow for the fact that it might be the supplier's earth (via their Protective and Neutral (aka PEN) conductor) being the thing that introduces the hazardous voltage into your equipotential zone. Maintaining EQ zones inside buildings is not usually that difficult. However it does mean you have to export that equipotential zone to any outbuildings etc if you also plan to export the supplier's earth.
Exporting the EQ zone means that any protective conductor used to export power, must also be sized to be adequate as a main bonding conductor as well (i.e. typically 10mm^2 copper or copper equivalent).
Indeed. A bleeding obvious case of unintended consequences that seemed obvious to all except the legislators.
The big problem with a PME earth is when you try to take it some place where there is easy access to an alternative "real" earth. Outside being an obvious one. (same problem with power to greenhouses etc).
An alternative use for the D in DIY I suppose?
The obvious solution to this collection of related problems would seem to be to make sure your entire rig etc is powered from a TT supply and follow normal practice for such supplies.
A typical setup might be to take a submain to your equipment room. The submain can carry and be protected by the house PME earth. However at the equipment room, the connection would be made off in an insulated enclosure so that the PME earth is disconnected from anything downstream of this point. Then using an appropriate CU with 30mA RCD protection and an earth provided from a local earth rod power all your equipment. Make sure that the TT supply feeds all circuits used in the room - so there is no accessibility to the PME earth. If there is additional extraneous conductive paths to local earth, then include them in your main equipotential bonding with your TT earth.
This solution would also eliminate the problem of the supplier's earth being a source of additional noise into your systems.
For further info:
Background on earthing types in general including PME:
Concrete conducts. Some other things conduct. Outdoor ground conducts.
Stand on a concrete floor, touch it, and let me know. It ain't so.
Sure. But the point of PME is that its earthed at multiple points, making the risk of a live neutral/earth conductor very remote. PME supplies are far safer than TT generally.
That's what happens when people don't care about getting the facts right, and pass laws without a proper thorough consultation process. Whether Labour even cared about the safety implications is unclear, it seems to have been very much about revenue and control, and the negative safety implications seem moderately obvious.
this case, I feel I was justified. You have given us amateurs some very good advice and guidance in this safety-critical area that is not obvious to some of us.
Some will be to stubborn to take the good advice but that is their problem.
Its interesting from our point of view as well since its a novel variation on the "I want to take power to my garage/shed/greenhouse" question that we more commonly get.
Such is always the case. While the earth on a PME supply carries an elevated risk due to its potential failure mode, its also important to keep that risk in context. The reality is that the electricity supplier will go to some lengths to ensure that disconnection of the PEN conductor in isolation is unlikely to occur, and even if it is broken along the way, it is also connected to earth at frequent intervals along the route to your house. So in absolute terms the chances of something bad happening are pretty slim.
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