All well designed power supplies will incorporate overcurrent / overpower protection in case the output load / overall output power exceeds the manufacturers specification. As well as protecting the power supply this also serves to ensure that the maximum current delivered to any given output rail is limited in the event of a fault within the host equipment. 

Overcurrent protection is generally provided by monitoring the output current of each output via a current sense circuit (usually sense txr) and shutting down the main PWM converter if the maximum threshold is exceeded.

Overpower protection typically measures the overall current (an hence relative to power) drawn from the mains via the PFC or PWM circuits and again shuts down the unit if the maximum power rating of the psu is exceeded.

In both instances we check the operation of these modes of protection (where employed) via electronic loads to ensure correct functionality to manufacturers specifications.