Dear Boštjan:
a) If internal distances input to output
comply with functional insulation only, fault
condition tests need to be performed on functional
insulation (including transformer) in order to be
sure that output remains SELV. In addition,
transformer needs to pass electric strength test
for basic insulation
If the results of the fault test are that the
output remains SELV, then basic insulation is not
required. The test proves that the failure of the
functional insulation does not result in a
non-SELV output.
If the results of the fault test are that the
output exceeds SELV, then the insulation (between
the 88 volts peak circuit and output) must be
comprised of BOTH basic and supplementary
insulation. IEC 60950-1 does not contemplate a
SELV-to-SELV dc-dc converter that generates
greater than SELV in the converter circuits and
impressed on the isolation transformer and
isolation insulation. That said, the dielectric
strength test voltage (for 88 volts peak primary
to the secondary) would be taken from Table 5,
Part 2. Vb, for double or reinforced insulation,
would be 1257 volts rms!
a) If internal distances input to output
comply with basic insulation, there is no need for
fault condition tests.
No. The insulation from the 88 volts peak
circuits to the secondary (output) must have
double or reinforced insulation. Clearances are
specified in Table 2M, and would be 1.4 mm for
reinforced insulation.
Creepage distances are related to the r.m.s.
voltage of the primary, 2.10.2.2. The r.m.s. of
88 volts peak is 62.2 or less, depending on
waveshape. Creepage distances are specfied in
Table 2N and would be 0.1 or 0.16 depending on
material. But, a creepage (physically) cannot be
less than a clearance, so the clearance
requirement of 1.4 mm applies.
Note that some authorities would require isolation
of the input from the 88 volts peak circuits. A
fault test will prove that this cannot happen and
that only functional insulation is required. (The
law of conservation of energy.)
a) Since measured working voltage 88Vpk is
above TNV-2 limit it is considered as hazardous
secondary voltage and we need reinforced
insulation in order to avoid fault condition
tests.
Yes. But, if fault tests prove that the output
remains SELV, then there is no requirement for
reinforced insulation.
Good luck, and best regards,
Rich
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