I've been trying to find the cause of power supply failures, and noticed that an NTC is smoking if it runs at full power. Obviously the NTC needs changing. But is it possible that the smoke in the chassis could then allow an arc to form between 400 V traces or components when it wouldn't otherwise? Either carbon particles on the PCB or ions in the smoke.
Particles from cigarette smoke or smoke after a fire can land on electronics and can cause them to short circuit.
from a dubious source
(Or maybe it's possible, but components are designed with materials that won't have this effect?)
Interesting problem. Smoke can conduct electricity. From the research here (first Google link btw.), "A surprising result was that the conductivity between parallel plates remained high although the optical density in the smoke chamber dropped drastically, indicating that there was very little smoke in the air."
The research also states that high voltages promote the growth of 'soot bridges', further strengthening the assumption that yes, smoke can cause arcs between high-voltage traces.