This is an interesting solution, from http://www.blacet.com/tech.html, to provide protection against reverse connection of power supply as well as overcurrent protection caused by any kind of short circuit:
Power input protection and filtering is highly recommended for all modules! See below.
PS1 and PS2 are polyfuse resettable fuses available in various mA ratings. A typical module with a 50 mA current draw would use 100mA versions.
A power supply reversal causes the diodes to conduct, creating a short circuit. That condition or a short on the PCB causes the polyfuses to heat up and go into current limiting.
Now I tend to simply use series diodes on power rails instead. Polyfuse might take some time to react, and then can cause some stress on your PSU. Besides, their “ON” resistance is not negligible.
Keep in mind that diodes have a dropout voltage when in conducting mode. Anyway, a good design should not use the power rails as voltage reference.