PbF failure of 20mm fuse ?

[N_Cook]

How to determine whether tinpest layer round an end of the filament or a
physical break?, just durious as to failure mode. 200mm F type. Breaking
the glass is likely to disrupt the failure situation. Filament seems to
be in proper place , diagonal along the visible part of the tube and no
observable movement on normal handling. No circuit failure to have
caused long term aging stress failure of the filament at one end. I've
not tried 500V/1000V insulation test (could perhaps marginally "weld"
the failure) but DVM-R >30M

 

[N_Cook]

Set fuse in an open frame fuse holder. Ground into the glass until it
cracked. Could insert a needle in there and o/c in both directions.
One end broken , whether by me , unknown, other end fixed in. Crush the
glass in preset mole-grips and then I decided to prize out the caps from
the holder with a blade and the one with the still captive filament went
flying across the room