Brian,
I find portions of our discussion has been transcribed
to the entire IEC TC 108 group. Thus I feel I should respond
to your TC 108 comments
A problem NOT ADDRESSED by TC108 is the increased energy AFTER a surge, or
during a SFC, due to the 120% rating requirement. Perhaps this was a
principal intent of UL1449 3d ed - verify that the high E and I do not make
the MOV puke it guts and start a fire.
I don't think TC 108 is deficient in missing out AC
powering when surging. I'm not in love with some other stuff
they do, but that's another discussion.
My reasoning is, if the surge damages the MOV, then all
the applied AC does is provide a power source to cause a
component power loss and temperature rise that might lead to
thermal runaway and component destruction.
Without the application of AC power, there are
measurement techniques available to tell if the component
has been damaged. At the component level, measuring the MOV
nominal voltage at 1 mA DC is a common approach. If this
voltage changes by more than a given amount then the MOV has
degraded due to surge.
I've spent a deal of time studying UL 1449 recently
and, as I understand it, in the most stressful surge test UL
doesn't apply AC power (like TC 108).
Type 2 SPDs have a Nominal discharge current test (As
the MOV isn't a GDT with a gaseous filling, you can't have a
discharge current in it, so the correct term is the more
mundane nominal surge current).
Preferred current values are 3 kA, 5 kA, 10 kA and
20 kA. These are currents in the SPD, not short-circuit
ones, produced by an 8/20 current generator. No AC power is
applied.
Type 3 SPDs have an operating duty cycle test with AC
power applied. The surge generator is a 1.2/50-8/20
combination wave generator set for 6 kV open-circuit and 3
kA short-circuit. Once you included the coupling/decoupling
network and the component, it is only a quasi-8/20 current
of a smaller amplitude than the short-circuit value in the MOV .
UL measure pre and post test limiting voltage - this is
a very crude validation assessment method. A lot has gone
wrong to cause a significant change in limiting voltage. On
the bright side, because UL throws a lot of extra tests in,
there is a leakage current test, which performs a similar
appraisal as the component 1 mA nominal voltage test.
Regards
Mick
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