There are certain conditions where voltage spikes can occur. I know this to be true, but I don't remember why so I'm hoping an expert will speak up as to the possible causes. A TVS protects against these voltage spikes. As per one of the other threads the effects of these can be cumulative, so there's a good possibility this is what happened with your Porter. The circuit breakers are there to protect your transformers and your wiring, not your electronics.
Pete
