PTC fuses and circuit protection

I am connecting to an home HVAC unit. I want to tie into the same AC transformer as the thermostat and so will have voltage regulation on my board. I will have a relay out, so I do no expect any output issues.

however, I have a lot of room on the PCB and un-used pins. so I want to measure some additional devices that are unknown at this time.

in reading on inputs, the idea of connecting a PTC to the input to protect it seems like a great idea.

are there any guidelines on using these with the Arduino ? suggestions ?
even sources to get them ?

A PTC can open a circuit but may not return to its original "closed" resistance value. This could be a problem for circuits that are sensitive to voltage drop. As far as reliability goes, you can get much better than an old-fashioned fuse: it only has one mode of failure, and that is to open.