Yosemite Ridge Resort
Four of us convoyed from the Bay Area to Groveland, CA. We arrived at the Yosemite Ridge RV Resort about 4 p.m. on Friday. One Roadster was plugged into the hotel next door, while the remaining three plugged into NEMA 14-50 outlets in the RV park. These have 50 amp breakers; our mobile charging connectors limit the current in this case to 40 amps. It was a cold day and the RV park was not full so we didn't expect any problems with overloading the system and we began charging almost immediately at 40 amps each. The fifth Roadster arrived about an hour later and started charging at 32 amps. All went well for about 4 hours. By about 8 p.m. I thought my car would be nearly fully charged so I went over to check on it. As I was peeking thru the window to read the VDS I heard a loud "Bang!" and a piece of insulation fell down onto my hood! As it turned out, I was parked directly underneath the power pole that serves most of the park. The bit of insulation was from one of the old style fuses that protect the high voltage side of the transformer; they had blown. The breakers on our 14-50 outlets and the breaker on the park's main service panel were OK. PG&E was called, and they arrived about an hour later and determined that the transformer itself was bad. A couple of hours after that (at 3 a.m.) they had a new one (twice as big) installed and all was back up and running again. We finished charging that night (and the following night) uneventfully.
This is not supposed to happen. We were well below the 50 amps allowed for each circuit and the total of 200 amps for the main service panel. There wasn't a lot of other load since it was a cold day and no one was running A/C (though some lights and electric heaters were probably coming on). Everything seemed to be operating within its rated limits.
My theory of what happened is this: the transformer was old (probably roughly 50 years old, as the RV park itself was built about 50 years ago). Perhaps it was about to die anyway; insulation doesn't last forever. Perhaps it was low on oil. We don't know what its power rating was but it may well have been underrated for the 200 amp service panel on the (reasonable at the time) assumption that the load would never really get that high. Perhaps the service panel had been upgraded without a compensating upgrade to the transformer. But for whatever reason, with our load of nearly 150 amps continuous for several hours, it just got too hot. The internal insulation failed, shorted out and blew the fuses. Transformers fail all the time and PG&E is used to this. It was no accident that they were able to replace it in just a couple of hours.
We were concerned that we were responsible and that the RV park owner would be upset with us. He was being a nice guy letting us charge for no additional fee and clearly we were the proximate cause of the failure. But after thinking about it a little, everyone agreed that whatever ultimately caused the failure was clearly PG&E's fault - even the PG&E guys. After all, it was their fuses that blew. The RV park guy was cool.
All's well that ends well.