Hi John:
> >You may come across some distributors in Norway asking for DI or RI
> >between PRI-EARTH, even for Class 1 power supplies.
>
> That's because Norway uses the 'IT' power distribution system, where
> neither mains conductor is earthed directly, but one is earthed through
> a quite high impedance to prevent common-mode charge build-up.
I fail to understand how, in a Class I product on an IT
power distribution system, PRIMARY-EARTH double or
reinforced insulation provides protection against
electric shock while, on a TN power distribution system,
basic insulation is sufficient to provide protection
against electric shock.
If the product is earthed, then in the event of a fault
of basic insulation, the voltage on accessible parts
will remain close to earth potential. This applies
equally to IT and TN power distribution systems.
My understanding of the Norwegian unwritten requirement
was because many of the socket outlets, while of the
earthing type, do not have the earth terminal wired to
earth.
Under this condition, double or reinforced insulation
makes sense.
Best regards,
Rich
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