It likely started as high resistance
and as the plastic melted it possibly
As the springs in the male plug weaken over time
, in the 12V+ tip and ground wings, the plug can develop a sloppy fit and work loose. When it becomes loose in the socket we create a point of high resistance
which can begin to heat the plug and outlet to the point of fire all at well below a fuses trip point. I have seen this happen with a 15A rated fuse and the fuse never tripped because the owner was not exceeding 15A but the outlet was scorching hot and melting due to high resistance
If you follow a few simple rules for 12V DC sockets they can
be used in a safe manner.
ALWAYS USE OVER CURRENT PROTECTION
Max fuse size 7.5A (preferably not more than 5A)
DO NOT plug in pumps, motors or search lights
Choose high quality sockets rated
for at least 15A (but don't run them at that)
Contact points need to be kept clean (A Dremel works, 12V Power off
DO NOT plug in small 12V to 120V inverters.
If the male plug becomes loose fitting stop using it and replace it.
Use only for cell phone
, tablet or laptop charging
with 12V loads less than 5A - 7A
If you follow this you will likely never have a 12V outlet issue again. High resistance can start fires well below the fuse trip point!
Absolutely and this is a major goof
. Every wire on-board your vessel, not just a 12V socket, needs to be over-current protected.