Hi Richard:
You suggest that Y capacitors are "fail-safe," and self-healing.
Its not quite that simple.
Y capacitors can fail short, but not permanently.
Y capacitors are not required to be self-healing.
There are four Y capacitors (from IEC 384-14:1993):
Y1 Double or reinforced insulation. Rated up to 250 V.
Y2 Basic or supplementary insulation. Rated from 150 V to 250 V.
Y3 Basic or supplementary insulation. Rated from 150 V to 250 V.
Y4 Basic or supplmenetary insulation. Rated up to 150 V.
(The Y3 cap does not have an impulse voltage prior to endurance testing.)
Table II specifies that Y capacitors shall not have any PERMANENT
short-circuit failures as a result of the IEC 384-14 tests.
IEC 384-14 does not specify that the capacitors must be "self-
healing."
Turning to a publication by Evox-Rifa, here is what they say in
"Capacitors for RFI suppression of the ac line: basic facts,"
fourth edition:
"There are three types of RFI capacitors commonly used on the AC
line. These are impregnated metallized paper, metallized film,
and ceramic. The first two are "self-healing" or "clearing" while
ceramic is not."
