Before we went to "industrial switchers" it happened quite often that we burned $1000+ of electronics because the control chip shorted in our "home made switcher" (= 24V on the Vcc line). Due to resistance against using an "industrial switcher" (at that time no economical such had been found) the solution was to add a crowbar circuit consisting of a MC3423 and a SCR. This circuit short the output and blow the fuse if the output voltage exceed 6V.

I suggest that anyone making a "home made switcher" include a crowbar, if the MC3423 or an equivalent can not be found in "your area" a somewhat less safe, but still better than nothing, circuit can be made with a SCR and a zener.

Erik