Point well taken. You're right, safety is relative.
I think that your message (below) points to the fact that products that are
double line fused should have labeling to say that they are double line
fused (as the clause in EN 60950 that you mention points out) It also
points to the fact that service personnel should never make assumptions and
either work with all hot and neutral lines disconnected or assume that the
circuit is hazardous.
However, I don't think that it is a condemnation of double line fusing. The
situation that you described can happen with single fuses as well under
different circumstances. Consider a single fused product which has had its
"hot" line fuse opened. A maintenance person could assume that the
equipment will not be hazardous since its "hot" line is opened. However,
if the product in question is in certain countries (countries that don't
have Earth referenced "neutrals") this service person could be making a
fatal assumption since the "nuetral" line could still be providing lethal
voltages to the product.
Chris
> -----Original Message-----
> From: Peter Tarver [SMTP:ptar...@nortelnetworks.com]
> Sent: Tuesday, October 10, 2000 10:26 AM
> To: emc-pstc
> Subject: RE: Overcurrent Protection: One or Both Sides?
>
> Chris Maxwell asks: "Can anybody shoot a hole in this theory with a single
> fault condition?"
>
> This subject has been debated before, on this list and elsewhere.
>
> Safety is always relative. In the scenarios you painted, Chris, the phase
> conductor always has an intact fuse after the neutral fuse opens. This
> can represent a risk of electric shock, by casual or intentional contact
> with the live circuit, to anyone attempting to replace the fuse that
> mistakenly believes the phase fuse is opened. Hence the service person
> warning requirement in 2.7.6 of IEC60950.
>
> Regards,
>
> Peter L. Tarver, PE
> ptar...@nortelnetworks.com
>
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