Having a polyfuse upwind of a crowbar circuit is almost a sure invitation to see your downwind circuits blow out in an over voltage situation. The reaction time of the typical polyfuse if quite long. Any serious crowbar application designed to blow the upstream fuse would do well to plan for a Fast Blow type of device.

In addition the SCR in the crowbar should be sized and biased in a way that it can easily place an effective short circuit across the power supply output. This needs to handle not the rated output of the power supply but instead a 150% or more of the maximum of current that the supply can deliver including outside its regulated range.

Michael Karas