What is the best way to protect circuits from overvoltage. I recently had a case where a 5vdc switching power supply failed and went up to 9vdc. This fried the circuit boards that it supplied. MOVs might be a bit slow , I think. Any ideas, circuits etc will be appreciated.

 

Actually MOVs are extremely fast, but they have wide tolerances. The best solution is to correct the power supply design so it is less likely to fail in that way. If the power supply problem can't be fixed, the consider an SCR crowbar and a fuse.

http://en.wikibooks.org/wiki/Practical_Electronics/Crowbar_circuit

 

http://cds.linear.com/docs/Article/OVP LTC1696 257045.pdf

Have a look at that, it shows how opto-couplers can be used to protect against over voltage.