Speed of sound,
Codswallop, total and utter codswallop.
So given the fact that none of these ELTs have been known to have started a fire onboard before, you don't think it is worth examining other ELTs which have undergone similar environmental changes to the 'incident unit', ie sat installed but unused for over three months?
Reuters:
The Ethiopian Airlines plane sat outside in the hot African sun for months, raising questions about whether that could have affected the battery in the locator beacon, said the source.