After further inspection, I discovered that the factory installed 105C wires in the heaters were failing. I'll see if I can find 130C wire. However, the customer happened to be home when the fireworks occurred, and the breakers DID NOT TRIP - she had to stop the sparking and "carryin'-on" by turning off the thermostat.
In fact, her boyfriend "couldn't find the breaker that the heater was on"... Even when ALL breakers were off, the heater still had power. The breakers has failed to the point that they wouldn't turn off, let alone trip.
So... mystery solved...
Lesson learned... 22 year old 105C rated wire mounted inside baseboard heaters may fail to keep their temp rating.
I have the FPE panel and I intend to do some experiments with it this weekend to see just if and how many breakers have failed.
