According to a biography of Franklin I read recently, Ben didn't actually
conduct the experiment himself. He was well aware of the dangers, having
not too long before narrowly escaped death by inadvertently discharging a
series of Leyden jars (which at the time were normally charged during
electrical storms) through his body (while trying to electrocute his
thanksgiving turkey). The author asserts that the long delay between the
actual execution of the experiment and its publication indicates that Ben
was wrestling with a guilt trip--that he had had the experiment carried out
by his house slave, rather than himself, on the consideration that the
slave was expendible while neither he nor his son (also present) were. Had
Ben known that his son would end up on the Tory side during the revolution
he might not have been so reluctant to let him do the experiment.
The author admits he has no documentary evidence that this was the fact,
and he is speculating, based on the knowledge that Ben knew the dangers and
the long delay before publishing what he knew was certainly an important
result.
So, if our author is correct, it wasn't old Ben who was lucky to have
survived, but his slave. I believe that Ben was a little more careful in
his experimental design than were the subsequent experimenters who died. I
don't remember the details of his setup, but my recollection is that,
although risky, it was not as dangerous as other versions. Perhaps it
wasn't all luck after all.