I wonder if any buildings in the US have burned down simply because of a F34T12 lamp installed in a full power F40T12 fixture?

It probably happened at least once. The thermal protector on older Class P ballasts can stick shut, particularly if the luminaire was poorly maintained and the thermal protector allowed to cycle a few times. Then once the ballast starts to overheat you have hot tar start dripping out as seen here.

For that reason, I personally like to add inline fusing to all older fluorescent luminaires I have in use, particularly with older PCB containing ballasts. Inline fusing is a option available for most fluorescent lights from the factory as some places require them.

Was this caused by someone installing a F34T12 lamp in a full power F40T12 fixture?

Was this caused by someone installing a F34T12 lamp in a full power F40T12 fixture?

This is the way how they are rated to be used, so I doubt it is the cause.

However the slightly higher ballast loading could become the "final nail in the coffin" in triggering the destruction of the ballast which then starts the fire. But there the cause is that ballast being cr@ppy in the first place.


But generally failing magnetic ballasts and bad lamp contacts, together with the dust accumulated on the top of the fixture, are notorious to cause fires, with any lamp type or size.

  It mainly the older Tar filled ballast with out thermal protection
  Lot of that is older preheat ballast and stuck starter. 
 But with 34 watt lamps it just a time factor how long 34 watt lamps have to run on such old ballast like these as  have higher current loading on the ballast as heat can break down
 Insulation.  But good for short run time to test out old Unknown
 Preheat ballast.

Dor123 knows of a building in Israel that caught fire in the 1960s by a overheating magnetic preheat ballast with a stuck starter.

in several of my church buildings, I've seen a few tar burner ballasts failures that I attributed to having had 34 watt tubes run in ballasts that were primarily rated for 40 watt.  The ballasts got HOT.  And I do mean hot.  The ballast covers were charred on the inside, and upon removing the fixture itself the ceiling was also heat damaged.  we got lucky in one case where the wire insulation melted to the point that a short circuit was caused causing the breaker to trip.  The smell took forever to get rid of.  After that, I disposed of all of the 34 watt tubes we had left in the buildings that I had not yet had a chance to retrofit from T12 to T8.  During the retrofits, I did encounter a few fixtures where tar had leaked and things had gotten very hot but not to the point of failure yet.  Most of these were 1970s vintage Universal and Magnatek magnetic ballasts.  And while some of the ballasts did state that they were capable of running F34T12 tubes, they were primarily rated for F40T12.  Additionally there were a few in areas where the minimum operating temperature was not met (60-65 degrees F ambient) in some cases during the winter months. 

Dor123 knows of a building in Israel that caught fire in the 1960s by a overheating magnetic preheat ballast with a stuck starter.

I've seen few lamps with fried ballasts due to stuck starter for long time.Luckily lamp was outside and fuse tripped.