In ,
George Herold wrote in part:

SNIP previously quoted material

Thanks John, I must admit it's hard to understand how a 3 watt 100k
ohm resistor across the 240 V lines would blow even if everything down
stream of it was shorted. (As your calculation shows.) A new circuit
board is in the mail, but this won't solve the mystery. The board has
been redesigned and is no longer analog but digital. Once I put the
new board in I'll look at the busted one and see if I can figure
anything out.

I have seen resistors fail in ways that cause their resistance to
decrease until they get toasted and then go open circuit.

My experience has been that this happens more with carbon composition
resistors. Those tend to be cylinders with untapered ends. Many times,
the composition is exposed. The composition may absorb moisture and
experience a decrease in resistance, especially if it is operated only
intermittently. This is more likely to be a problem where there is
exposure to moisture or high humidity or salty coastal air.

And carbon's resistivity has a negative temperature coefficient.

Preferably the resistor degrades rapidly and then goes open circuit
quickly by the time it allows anything else to get overloaded to a
damaging extent.

A possible other explanation is that a severe line voltage surge caused
something in the resistor to carbonize. The resistor becomes more
conductive and hotter. The carbonization may then go into a runaway
situation, leading to sizzle-sizzle and then to *pow*, *poof* or *phut*.

- Don Klipstein )