According to the electrician, the cable failed due to old age in the marine environment.
I had previously noticed the cable got warm when under load. On this particularly hot day when the cable failed, we had arrived back into dock about 30 minutes before, after being out at anchor for two days or so, and so the boat was charging batteries, plus we had three air conditioners on. We had all hatches closed, including the companionway.
Our neighbour was cleaning his boat when he saw our cable in flames. He shouted "fire," "fire," and ran for an extinguisher. We heard his cries and rushed into the cockpit. I saw the end of the episode:
Then just as I was thinking of running to the dock breaker, the fire fizzled on its own just as the neighbour arrived with a large fire extinguisher in his hand. By this time, all power to the boat was already off.
I wanted the electrician to replace all the power cable including to the 230V box in the engine room, and he looked into it, but reported to me that it looked very difficult or impossible. He said that besides, the section of the cable enclosed inside the boat, being immobile, dry and shaded, wasn't likely to fail.
In any event, hull No. 350 had an unused pigtail right next to the failed cable, by the propane locker: apparently, a whole second cable, perhaps to be used in 110V installations? As this unused cable was found to have continuity/end at the 230V box anyway, the new cable connects into the formerly unused cable, and the end of the originally used cable was left in a pigtail in its stead.
SM2K N. 350
At anchor, Sardinia