In the past OpenSprinkler has always come with a 750mA PTC resettable fuse. I’ve received a couple of report that in the summer when the temperature is extremely high, the resistance on the PTC fuse would increase to a point that it blocks the current supplied to the controller. So as you said, it poses a potential problem. Another issue is that these PTC fuses have a slow response time (typically a few seconds). So if there is a shorting on the solenoid, typically the transformer voltage would drop instantly before the fuse even catches on. So in practice I don’t know how useful it is. In the most recent kit I’ve removed the PTC fuse. Probably a better solution is to use a fast-action fuse. Unfortunately there is not enough space on the board to fit such a big fuse.