Hi Doug:
> Does this section mean to say that the voltage on "internal SELV" may in
> fact go higher than the 71V Peak and 120 V DC for a short time? Naturally,
The standard says:
"In a single circuit... other parts of the same circuit
do not comply with all of the requirements for SELV
circuits and are therefore not permitted to be operator-
accessible."
The catch is the phrase "do not comply with all of the
requirements." The standard gives no guidance on which
requirements are "optional" and which are not. So this
will be an interpretation on the part of a certification
house. So, we cannot predict whether the voltage can go
higher or for a longer period that specified.
Clearly, if the potential is not accessible, then it would
seem reasonable that the construction should be acceptable.
But, certification houses are not necessarily reasonable.
I think I would take a different tack. I would not call
that part of the circuit SELV. Instead, I would identify
whatever it is that "separates" the "internal SELV" from
the "accessible SELV" as an isolating component or circuit.
This gets you out from having to identify the circuit as
SELV, and therefore no requirments would apply. Since
that part of the circuit is not accessible, it would not
have an effect on your design, and would eliminate the
un-predictable situation.
> Is someone able to give me a clear definition of TNV and what the acronym
> means? The standard seems a little ambiguous in my mind.
The requirements in IEC 60950 are for TNV circuits, not
just for TNV equipment or products. This is why the 3rd
Edition has the requirements integrated into the standard
rather than its own section as it was in the 2nd Edition.
TNV is Telephone Network Voltage.
TNV exceeds ELV, and is therefore a hazardous voltage and
circuit, but is not dangerous to the same extent as mains.
A TNV circuit should be treated the same way a mains circuit
is treated: Isolated from everything.
Accessible parts, including SELV, must be isolated from
TNV circuits. However, the isolation need only be basic
insulation.
The TNV circuit must be isolated from the mains to the
same extent as for SELV or any accessible part.
Sorry, but I can't answer the questions about EN 61010-1
and EN 50178.
Best regards,
Rich
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