Okay thanks, that is what I wanted to know. If a surge pops the MOVs, the unit will no longer work.
I have one of the less expensive units that I use to check for proper wiring. It is rated for 1080 Joules. When I use it I often wondered how I would know if it was still working.
If I was only worried about polarity I could check that at my outside 120 volt outlet with a $10.00 gizmo from Home Depot.
I will say the little Camco unit is the simplest to use, but the power box door will not close completely.