This bandswitch was removed from a 3CX1000A7 HF/MF amplifier. The anode supply is 4000V @1A.

In my opinion, the arcing was not caused by high SWR, operator error, a surge in the electric-mains voltage, dirt, lint ,or unauthorized insects. The most severely burned contacts are at the 10m end of the bandswitch. I believe that VHF energy was probably responsible for the arcing.