What does the overvoltage rating really mean?
It could be a real reliability-impact based rating
or it could just be a UL, arc / creepage sandbagged
number.
If there is a max duty associated with the 500V / 1 min
spec then you could criticize the waveform against
that. I imagine that the "effective duty cycle" (of V>350V)
is mighty low. Certainly there is a real duty cycle limit
(like, 500V for 1 min, 0 for 10 sec, 500V for 1 min... would
likely violate the test paradigm, if any). Whether Vishay
will provide that, who knows? Could be something in
the application notes for pulsed power applications.
The wording of the detail spec makes me think this
is not specifically about voltage across the resistor,
but voltage from the body to "ambient" (i.e. any close-
in conductor). You might want to dig for more clarity
about what it's really expressing:
Permissible voltage against ambient (insulation):
If 500V (350V) is an end-end, terminal voltage value
then you could use 2 series devices and split the
stress.
Might also check the power dissipation which gets
a call-out as another reliability limit, possibly more
of a bind than voltage depending on value.