Hi John:
Also, it might be worth reminding folks that single-pole fusing is OK for
defined-polarity mains supply systems (e.g. the UK, and some N.American
systems) if it is in the Line/Live/”Hot” conductor, but not in
undefined-polarity systems as found on the European Continent and many other
regions/countries – those need double-pole fusing or else there is the risk
that the unfused lines will have to carry the full mains-supply prospective
fault current in the event of a similar fault condition with a
reversed-polarity connection.
Well… I don’t agree. The mains conductors must have at least basic insulation
to PE. The PE, a supplementary safeguard, must withstand the worst-case mains
fault current. So, if the mains-PE basic insulation should fail, then the PE
(supplementary safeguard) carries the fault current until the overcurrent
device in the installation operates. That’s why we test the PE at high
current. And, that’s the principle of two safeguards, basic and supplementary.
No need for double-fusing.
The mains wires and the PE wire can be appropriately sized on the load side of
an equipment fuse. (As a general rule, the mains wires and the PE wire must be
the same size because, in the event of a fault, the PE will need to carry the
mains current.)
Best regards,
Rich
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