For those of us not familiar, 'splain, please?
A conventional laminated core has corners and stuff. Some parts run at
lower flux density than others, so are sort of buffers against hard
saturation. Toroids have nice uniform cores, so can be designed to
have all of the core material run near saturation. That's one reason
they are so small and light. The geometry favors low copper
resistance, too.
So switch off a piece of gear that uses a toroidal line transformer.
If you're unlucky, the switchoff will happen at maximum flux density
in one direction, and leave some residual magnetization. Now, more bad
luck, turn it on at the ac zero crossing in the same direction. All
the core saturates and a huge primary current flows. This cheerfully
takes out mdl or even slo-blow fuses, and sometimes power switches.
We've measured 1000 amp peaks on modest-sized transformers, and you
could hear the wiring jump inside the wall.
CE requirements don't allow over-rating fuses a lot, so that can be
really nasty. The super-slow TT fuses help, but are sometimes hard to
get.
John