Hi John:
> SELV can protect under single fault conditions. BUT, as I tried to
> explain, under some conditions, it can allow a single fault *to persist
> undetected*, until eventually a second, unrelated fault occurs which
> then results in a serious hazard.
This is a problem of the double-insulation scheme: one
cannot know when the first insulation has failed. So,
your argument not only applies to SELV but also to
ungrounded accessible metal parts and any other double-
insulation scheme.
If we pursue your argument, then we should outlaw double
insulation as an acceptable scheme of protection against
electric shock, independent of SELV.
And, we should add a "new" criterion that failure of any
safeguard should be obvious to the operator *without*
presenting a hazard to the operator. An interesting
design problem.
> With PELV, this does not happen: the grounding ensures that the
> protective device operates.
This scheme requires that the path between the ungrounded
PELV pole and the grounded PELV pole be capable of carrying
the fault current until the protective device operates. In
other words, the ungrounded PELV pole must carry 25 amps for
1 minute (or appropriate criteria). In turn, this means the
fault current, 25 amps, must flow from the ungrounded PELV
pole through the PELV source to the grounded PELV pole. In
my experience, there are few PELV circuits that can meet this
criterion. In the PELV circuits I have worked with, the 25
A would cause the PELV source to open before the operation
of a protective device, and the mains voltage would appear on
the PELV ungrounded pole.
> Well, perhaps I have made it clearer now. My beef with SELV is the ban
> on grounding, whereas PELV which is grounded AND double/reinforced
> insulated is clearly safer for systems extended in space.
Agreed. In the products I deal with, this is our construction.
However, we do not test the capability of the ungrounded PELV
pole to carry fault current.
Best regards,
Rich
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