Dr. David Kirkby wrote:
> I know someone who managed to get free power by making use of the small
> voltage between earth and neutral. He knew of someone who claimed to
> heat his greenhouse from this, but taking much current will soon trip
> any earth leakage circuit breaker.
Must be on an older 3 wire system then. Most new domestic supplies in
the UK are two wire, with PME mains distribution systems.
So, although you have a neutral and an earth connection marked up in the
distribution board, the neutral and earth block in the cut out (the box
on the end of the cable before the meter) are actually the same block of
metal.
Which is why the safety people get paranoid about guarding against any
break in the neutral/earth connection to the property and why all the
metalwork in the house (pipes, fixed units like metal baths, sinks etc.)
should be all bonded together so that there can't be a difference in
voltage between one metal fixing and another in the case of the earth
getting disconnected *and then* the live getting connected to some
metalwork due to a fault. Although, it does seem to me that some of the
senarios of the neutral earth getting disconnected and then one metal
item being 'live' and another 'earthed' and then someone stretching
their hand from one to the other all at the same time, seem to stretch
credibility at times... A bit like the rule about having a larger earth
cable connected inside the distribution board than the one in the cable
connected to the cutout, but again I digress. :-)
In theory, it can also be a problem if you have your own earth
connection to rods in the garden as, if the netral/earth to the house
gets disconnected then any fault current will go through the
neutral/earths connected to your radio gear and down to earth via the
connection to the earth rods. An old friend of mine from MEB (Midlands
Electricity Board) days did a write up on it which many people have
read, but few seem to have done anything about. Me included... But,
then again, I don't use rods to the general mass of earth for RF earths,
or any other earth come to that. In fact, I don't even use radials on
the ground, because I don't have a vertical for HF at present. Plus,
rods in the groud actually make fairly poor RF earth connections anyway.
Great for lightening protection, if you have enough and they are well
connected to the ground, OK for 50/60Hz AC earthing, as long as you take
account of possible problems on PME systems, but not a great deal of use
at RF. But that's another off topic conversation.
Dave (G0DJA)
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