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.
>
> No, you are 'extending the argument' until it looks unjustified and then
> using that as a hook for your critique.
Your thesis is that:
Ungrounded SELV can "under some conditions," "allow a single
fault *to persist undetected*, until eventually a second,
unrelated fault occurs which then results in a serious hazard."
You are applying this principle only for SELV. The solution is
to ground the SELV.
I did indeed extend the argument, not to make it look unjustified
but to understand why the principle applies only to SELV and not
to similar constructions. My remarks certainly were not clear on
this point.
> I said quite clearly that failure of double or reinforced insulation is
> acceptable because failure of it has an acceptably low probability.
Now I am extending the argument in the other direction. If
the failure of double or reinforced insulation has an acceptably
low probability of failure, then if SELV is double- or
reinforced-insulated from a higher voltage, why must it be
grounded?
I presume the answer lies in the proviso of "under some
conditions." I guess I missed those conditions in one of
your messages.
> >> 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).
>
> Yes, 'appropriate criteria'. 25A for 1 min is an extreme criterion.
> Let's go to the other extreme, 150 mA for 100 ms (protection by RCD).
Yes, "appropriate criteria." The worst-case would be 25
amps. In most cases, the SELV circuit is on the load
side of the primary circuit protection and thus would
only be subject to twice the rated current of the
equipment overcurrent device.
(In the USA one cannot rely on a building installation
RCD/GFCI for protection from a single fault in a product.
The RCD/GFCI can be used for protection of a double fault
in a product.)
> Well, you might consider *designing* it to carry a realistic fault
> current, then there is probably no overwhelming need to test.
Hmm. Design, but no test.
This would certainly present a challenge to write such
a concept for SELV circuit-carrying capacity into a safety
standard.
I presume that the fault current path for the ungrounded
SELV pole must be routed through the SELV source to ground
as that path is the lowest impedance path to ground. It
seems to me that the fault current would have to pass
through the transformer winding. In some cases, the fault
current may have to pass backwards through the rectifier
to ground -- which is an impossibility.
Now I am wondering about the practicality of this
proposal for any part of the SELV circuit except the
transformer winding.
Best regards,
Rich
ps: I recall a high-power power supply where the 5-volt
SELV winding was a single turn of a robust copper
sheet. One end of the copper sheet was grounded.
Any insulation fault within the transformer would
have been grounded through the copper sheet.
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