OK, my posting yesterday stated, that the repaired hardening furnace is up and running again. But then it went dark in my shop.
At 950°C and 9PM the RCD switched off power. Complete darkness in the shop, but at least I didn't fall over things when I went to the fuses.
Checking resistance between PE and any of the heating elements, I read above200MOhm (don't have an isolation-tester). But only when the oven is cold. At about 400°C I measured 20kOhm **but** even -20kOhm. I interpret that as thermocouple-voltage. Could it deliver enough current to trip the RCD (20mA I think)? I could lay an extra cable to the furnace, bypassing the RCD, but I would like to avoid that by all means. I also wouldn't like to disconnect PE in the kiln (as it would be really idiotic).