In some countries, connecting the neutral to an outside earth could be
extremely dangerous, leading to fires or even electrocution. Where a Protective
Multiple Earth system is used, a ruptured neutral could pass many amps down any
earth lead, or lead the coax outer to float up to full mains voltage. This is
why it is not permitted to bring any conductor within the unipotential zone
unless it is bonded to the system earth, and earth leads need to be able to
carry large fault currents.
Where PME is not used and the neutral is earthed somewhere else, you can still
get quite a large potential between neutral and earth. When I lived on a
housing estate in Swindon where the whole estate was fed from a substation
where the neutral was earthed, on a Sunday morning with washing machines,
electric cookers, immersion heaters etc all blasting away, I could get 25 volts
at 500mA - free - between a decent ground and the mains neutral. No RCD (GFI in
the US) there - if there was one fitted, earthing the neutral to a decent
external earth would be very safe - you would have no electricity supply
because the RCD would trip!! At this QTH, at the end of the line feeding other
houses and a half mile of wire from here to the transformer, the neutral is
usually around 1 volt above earth potential.
So any system earthing the neutral is not a good idea and may even be illegal
in some places.
If one used TLT instead of conventional transformers, the blocking capacitors
would need to be Class X, and for 160m, at least 10nF. But Class X capacitors
are not designed for handling large amounts of RF current (ca. 3 amps at 500
watts) , and are not as low inductance as one would want for 10 metre
operation. Cascaded RF transformers is, to my mind, asking for trouble. From
memory, an isolating transformer to meet the regulations needs to withstand
2kV, although I could be wrong on that.
As Leigh says, the other regulatory requirements offer a problem too. Although
in Europe at least, kits for amateur radio and home brew equipment are exempt
from the requirements of the Radio and Telecommunications Terminal Equipment
and EMC Directives, it is questionable as to whether such exemption applies to
other directives. Interestingly, kits for non-amateur purposes, such as model
train controllers, aren't mentioned anywhere......
73
Peter G3RZP
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