Outdoor GFCI with HD extension cord feeding Mosquito Magnet bug light (total load about 20W) fell to ground and was covered by water during storm. Current proceeded to travel through side of cord socket/ receptacle and melted plastic body. Eventually panel breaker tripped, but GFCI never did.
Thoughts???
Best Answer
GFCIs are not magic everything-fixers
It's like asking an ambulance crew "why didn't you arrest those graffiti artists?" "That's not what we do".
GFCIs do one specific task that is related to human safety, and they do it well. They detect current leakage between the intended hot-neutral loop and anywhere else. However they do nothing about any fault that stays inside the hot-neutral loop.
Hence their name, "Ground Fault Circuit Interruptor", which is ironic since they don't interact with ground at all. It's presumed that the "anywhere else" must be ground.
Those other bailiwicks
It sounds like you had more of a "poor connection/arcing/overheat" problem. That is more in the bailiwick of an AFCI. (Arc Fault Circuit interruptor). Those work by listening to the power line for that familiar "crinkle-crunch" sound of hooking up speakers with the amplifier turned on, or a shoddy headphone jack, etc.
You note also that the GFCI did not trip and the breaker did. That's another thing GFCIs aren't. Some people think if they put a 15A GFCI on a 20A circuit, they can extend from there with 15A wire and the GFCI will protect it. The opposite is true, 15A GFCIs are specifically rated for 20A pass-through. (because of a NEC technicality).