-----Original Message-----
From: owner-emc-p...@majordomo.ieee.org
[mailto:owner-emc-p...@majordomo.ieee.org]On Behalf Of Rich Nute
Sent: Tuesday, September 03, 2002 7:31 PM
To: soundsu...@aol.com
Cc: Product Safety Technical Committee
Subject: Re: electric strength test
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.)
No I think not -
1) Standards are not designed on the premise that they will not be obeyed.
2) Please remember that Earth Leakage current is limited so that it will not
be a hazardous source in the event of failure of Safety Earth Ground.
3) Remember also that SELV may reach an upper limit of 120Volts (for a
limited time) under single fault conditions - therefore the insulation
between SELV and ground must be adequate to prevent breakdown and
(potential) sustained fault currents.
This would suggest that it would be better to
double/reinforce insulate the mains against
earth, in which case SELV could be earthed.
Insulation is good and can (if one is careful not to compromise its
integrity - e.g. a detached wire/connector, reduced insulation) simplify
the equipment construction.
Please refer to Conformity August Show Issue p 38.
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