Hardening furnace: Not yet over :-(

Dec 11, 2006 3 Replies
N

Hi



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 above



200MOhm (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).


Suggestions?



Nick


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A

Nick,

Two things:

firstly the lights should not be on an RCD protected circuit - potential safety issue in a workshop if you are plunged into darkness while things are still rotating to a halt

secondly, many insulators degrade markedly when hot.

The second point was brought home forcefully to me when a teenager. I found that I was able to wrap wires round the ends of a 1/4" glass rod separated by maybe 2 inches, put 240v mains on the wires, and heat the rod with a bunsen burner. When it got to dull red enough current flowed to keep it glowing ! Needless to say parents weren't aware of this experiment som 45 years ago !

AWEM

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N

Good point! I wasn't aware of that. But now it is too late to change it.

This is obvious. :-) I've decided to rewire one single wall-outlet and have a 300mA RDC there. Seems to be the safest option.

Nick

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N

Keep an eye open for building refurbs near you. They often throw out the battery-powered safety lights that come on when the power trips.

They're easy to spot, the light's on in the skip...

One or two of those dotted around are a major help, and easy to wire in.

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