Hardening furnace: Not yet over :-(

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Nick Mueller - 11 Dec 2006 14:18 GMT

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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Andrew Mawson - 12 Dec 2006 16:34 GMT

> Hi
>
[quoted text clipped - 16 lines]
>
> Nick

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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Nick Mueller - 12 Dec 2006 18:55 GMT

> 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

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

> secondly, many insulators degrade markedly when hot.

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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Nigel Eaton - 13 Dec 2006 22:15 GMT

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

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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Nigel

When the only tools you have are a Bridgeport, a CNC Taig Mill, a Colchester
and assorted other stuff, every problem looks like a steam engine.

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Model Forum / General / Models / December 2006

Hardening furnace: Not yet over :-(