If a linear power supply provides overvoltage protection, where is the voltage monitored?

The primary goal of an overvoltage protection circuit (often called a 'crowbar' circuit) is to protect your expensive transceiver from sudden, dangerous increases in voltage. In a linear power supply, this usually happens if the voltage regulator fails and shorts out, which would send the full, unregulated DC voltage directly into your equipment. To effectively protect the connected radio, the monitoring circuit must be placed at the very end of the power supply's internal chain. By monitoring the voltage at the output of the regulator, the protection circuit can instantly detect a failure and short the output to ground, blowing the fuse before the high voltage can reach the radio. Monitoring earlier in the circuit, such as at the transformer or rectifier, would not detect a failure of the regulator itself.