I want to know what do you think?
What was probable cause?
Before that, for last 2-ish years I must replaced 5 LED bulbs.
No other symptoms.
Right before failure, one or two LED bulbs started blinking.
I heard "scorching" sound.
And smell of burned electric/electronic (isolation).
Then I disconnected all circuits and opened all surrounding boxes...
Over-current breakers or RCD/GFCI didn't tripped.
Extra photos:
(ignore temporary jumper - fixed now)
Loose screw on the neutral bar. Maybe it loosened over time, maybe it was never tightened correctly to begin with. Probably both. Regardless, when the gap got big enough and the excess current on that circuit went back through the neutral, it overheated to the point of melting. Good thing you shut that circuit down, you very likely prevented a fire!
Now the big question is, why didn’t the breaker trip on its own?
A loose connection creates heat, but doesn't draw additional current. Assuming nothing changed on the load, the breaker wouldn't see excess current and would have no reason to trip. 16A is plenty enough current to get a bad junction toasty hot without tripping the breaker. Breakers protect from shorts, but can't protect all faults.
Not enough angry pixies to trip a breaker I think.
16 A, so at least 3.5 kW power needed → block with loose connection will burn, not melt.
Summary
Avoid Legrand brand at all cost:
.
About consumer unit:
.
I had a small UPS on top of the unit - shit...