The OverVoltageProtection (OVP) is one of the mandatory things I planned to include in my PSU from the very beginning. Also many PSUs on the market are equipped with OVP. The old C128 PSU has OVP. OVP should constantly monitor the output voltage and cut it in case it goes over the maximum safe limit. But I have always felt uncomfortable with the fact that the OVP checks if voltage regulation fails, but who checks that OVP is healthy? You could have your PSU running for years with a faulty OVP without noticing it until you needed it, but then it's too late to know! So I added a somehow hidden button for testing the OVP. The button closes a resistor between 12V and 5V line, causing a voltage spike on the 5V line which is then cut by the OVP, provided it works. The resistor in between is needed to limit the fault current. This test only works when the PSU is not loaded, otherwise the current through the resistor is too low to cause a voltage spike sufficient to be detected by OVP. Actually I think there are not many PSUs out on the market having an OVP test button, but I think it's a smart thing to have.
The test button is hidden so you don't accidentally activate it: you need a toothpick to push it. Looking at the 5V line with the scope, you can see below what happens when the button is pressed.
