Overvoltage + Overcurrent protection
For this. It would be a crowbar or an e-fuse. The crowbar is cheaper in general. But the e-fuse is much faster and non destructive

If your requirements are only 50V and 500mA it could make sense to just fit a buck converter on the input with a low drop bypass mode. When he circuit starts up. If the voltage is too low. Then the buck converter is in bypass. If the voltage rises. It leaves the bypass mode. And the buck converter fires up and keeps your circuit in spec.