Jetstream67
I cannot speak for the particular cell pack in the ELT but having dismantled and rebuilt quite a number used outside aviation it is
standard to include a fuse or a polyfuse (effectively a self resetting fuse) in a pack. These often turn up as links between the actual
cells of multi cell batteries but on single cell designs they are at the cell ends of the leads or on an attached protection circuit board.
Whoever did that design would have been expected to test their design (i.e. apply both increasing and random levels of excess
discharge current and finally an immediate short circuit across the battery terminals or wires) to ensure the battery shut down
and stayed shut down safely in each case.
I assume that a polyfuse would effectively restrict the current during the "short circuit" to that necessary to keep it at ~125C.
If so, the heat-flow from the polyfuse might become significant. Two extreme examples are:
1) If thermally clamped to the battery, the polyfuse would presumably heat the battery to ~125C at the point of thermal contact.
2) If thermally clamped to the ELT case (via the shorting wire), the polyfuse might need to pass a significant current to maintain
its ~125C temperature.
I assume that (1) is designed out, as it's effects would occur in all short-circuit testing situations. However (2) would only arise if
the short-circuit event also provided a "new" thermal pathway, so might not appear in "normal" testing.
Do you have any thoughts on the polyfuse's likely degree of thermal isolation in this incident; both from the battery and from the ELT case?