Hi Greg:
> > >Why does "True SELV" require basic insulation
> > >between SELV and earth?
> > >
> > >What is the hazardous voltage source, and what
> > >is the current path through the body if that
> > >basic insulation should fail?
> > >
> > >
>
> I believe (and I could be wrong) that it's a philosophy of keeping SELV
> intact under single fault conditions. If there's a single fault bridging
> mains and earth, then an SELV circuit is exposed to mains voltage unless
> there is basic insulation protecting it. Since it is estamited that close
> to
> 50% of the homes in the US have improperly grounded outlets, the earth
> connection itself is not taken into consideration as a reliable means of
> protection. In other words, for the purposes of the standard, an open
> earth
> connection is not considered a fault.
Thanks for the explanation.
One implication of this explanation is that
an open earth connection is not a fault
condition, but a normal condition. (Failure
of basic insulation is the fault condition.)
This would suggest that it would be better to
double/reinforce insulate the mains against
earth, in which case SELV could be earthed.
(This also solves the problem of accessible
earthed parts becoming live when used on a
system with no earthing connection.)
But, unfortunately, the requirement remains
that "true SELV" cannot be connected to earth,
and must have basic insulation between it and
earth.
Best regards,
Rich
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