"Paul E. Schoen" wrote:

It would also be helpful to know how the toroid was mounted, and if the
failure occurred where pressure was applied. Toroids are usually mounted
either flat with washers and rubber gaskets with a single screw through the
hole, or vertically in an "Omega" bracket, with some rubber cushioning
material around the periphery. But in either case there may be additional
pressure on a "high spot" where two windings may be crossed, and softening
of the insulation from overheating may also contribute to a short.

Not to mention the possibility of overtorqueing on top of that. What goes on in
some places is unreal. I've even seen steel TO-3 flanges dimpled by the
mounting bolts pulling into the clearance hole !

Graham