There are two types, some units come with internal/built in ones, you won't see those at a campground. I use an external for the time being. Cheap insurance. There was a power issue at one of the state parks I was at, my EMS shut off the power, the guy next to me had to call for a mobile repair service to come out and replace his fuse block and something else so he could get his jacks up to go home. Cost him over $1,000. I am guessing there was a power spike of some sort on our loop. I recommend the Progressive Industries models.