I have furnace up-and-running, well up-and-heating at least. I have taken it to 800C so far…aluminium melts at c650C. No problems (well one problem, see below, nearly none.)

Lid-on and ready to go, with a nice ‘Ford Engine Blue’ ceramic paint coat (optional!)

Lid off. Ceramic shield in there. You can see the temperature probe.

The Earthing/ RCD Problem

I ran the mains earth wire from the power lead to the metal enclosure. But when I powered up the first time it tripped the RCD on our house switchboard.  This magical little thing, now a legal requirement I believe, in New Zealand at least, saved our dog’s life a number of times when she was a puppy: she liked to chew wires, then go to sleep/ piss/ play with them.

From what I understand, if the current coming into the RSD does not equal the current going out, it means something is ‘leaking’ current into the ground…which is bad…cos it might be via your heart. The RSD detects this. If the ‘in’ and ‘out’ are out by a few milliamps, it trips.

After a few trips with the furnace, I nearly threw the thing in the bin and gave up. I could not work out where the current was ‘leaking’: my connections were all solid and insulated, the PID worked by itself, the coil worked by itself… until I stuck my multimeter probes on a refractory brick. My refractory bricks have a non-zero electrical resistance, so the heating coil is electrically connected, in my setup, through the brick and the steel frame to my earth connection. If I disconnect the earth connection to the enclosure, no tripping.

Hum?

Pretty sure my refractory bricks are non-conductive so why on earth my ones conduct a minuscule bit I don’t know.  Perhaps there is still enough residual moisture in them?