Making a Safety Power Interruptor for an Electric Furnace

In my thread on making an electric heat treat oven I posed a question about setting up a way to mechanically interrupt the power to the furnace should an over-temperature condition occur. On that furnace I am using a Bartlett Kiln Controller board to program ramp and soak temperatures of the 4Kw furnace. Included on that very capable and convenient board is a 12v pin that sends out a signal (turns on) should an over-temperature criteria be met. My desire is that over-temp would completely cut power to the furnace and would require a manual throwing of a switch to restore power.

I posed this question here and on another board that I frequent. I got two good suggestions from the other board:

1) Use a manual magnetic contactor like those often used on motors---> powering a normally closed 12v relay(opens if the over-temp pin turns positive)--->powering the mains to the heater coil via a mechanical (or could be Solid State) relay controlled conventionally by the board. So, when the over-temp pin goes positive the power to the heater coil is interrupted and the main magnetic contactor opens until it is mechanically closed.

2) Use a shunt triggered circuit breaker. They are a unit combining a magnetic contactor with a coil that pops the contactor open should the coil be powered---like the over-temp pin going to 12v. Those circuit breakers tend to be expensive new. But are available at about 50 bucks or so on eBay used and usable.

I am thinking of going with option 1. I like the idea whichever option I choose an over-temp condition will kill the power and require physical intervention to restore power. I'll feel more comfortable running the furnace while I am otherwise occupied.

If there are other options, I'd like to hear them.

Denis

I decided to make a new thread as this problem has wider applicability than heat treat ovens.

 

How about implementing option one like this? Power the furnace through a double pole contactor with a 120vac coil, configured to be self latching, hit the push button switch and the contactor energizes and latches on, the 12v signal powers a double throw or normally closed relay which unlatches the contactor when energizered. The 12v over temp signal would kill the power to the furnace and the furnace would stay off until you to hit the push button switch again.

 

Yes, I think that is exactly the setup "1" I proposed above. Good idea! ;-)

Denis

 

BTW, I just checked by phone with Bartlett and the output of the Safety Pin and Outputs 1, 2, and 3 are all 12v DC with 500 mA capacity.

Denis

 

I thought that's you were going for but it wasn't completely obvious from your description.

 

You know the Kiln Sitter on all old pottery kilns does exactly what you want and has no electrical signal to fail. You just need a high enough cone you don't trip it on normal operation. I use my kiln sitter as a backup in the kiln I use for heat treat.

It's not very sophisticated, though. But you can protect from 1,100F to 2,300F depending on your choice of cones.

 

Using a LATCHING NC relay might just do the trick. I think I had generally not considered the latching option. I'll look at that some more.

If I had such a sitter sitting around, I'd use it as you suggest. I really am attracted to mechanical failsafes. Sophisticated depends on your definition. Simple and reliable are the ultimate in failsafes.

Denis

 

I don't think there are any thermal fuses in the desired temperature range available commercially but I wonder if there's some particular metal alloy like brass that will melt at an upper fail safe temperature and cut power to the heaters. It could be made into a fusible link inside the oven chamber. With enough weight it would pull open as soon as it reaches the alloy's softening point.

 

That is my impression too, that there are no fuses in the desirable internal heat heat range. And wiring in a chunk of brass so that it does not cause an unreliable electrical connection I would find challenging. But, over a few runs I will observe the external temp rise. It is ultimately the external temp that poses the threat of fire in the event of a runaway.. I imagine the external temp will not ordinarily go over a few hundred degrees. For that temp range a fuse could be set up. I am not sure I will add an external fuse, but it would likely be feasible.

Denis

 

Assuming you're able to find some suitable wire whose melting point is slightly higher than the maximum desired internal temperature of your oven, make one of your blu-ram standoffs with two small through holes, run a your wire in one hole and out the other leaving a small loop inside the furnace then make the connections to the wire outside the furnace.

 

Kiln sitters are practically free. Just look for an old broken pottery kiln. The sitter seems to never go bad.

That being said, cone collapse and sitter actuation is based on time and temperature. You would not get an accurate kill temperature when heating to below a cone rating. For instance, a cone 01 will trip the sitter at 2,079F when heated at 108F/hr but the same cone won't trip until 2,109F when heated at 270F per hr. If you are trying to soak at 1,900F for 12 hr you may find a cone 01 will trip before the 12 hr is up. I would use a cone rated several hundred degrees above my intended operating temperature.

Kiln sitters are very reliable shutoff devices and use a weighted bar which swings down and kills the circuit by tripping a mechanically latched switch.

 

That's still too much money imnsho.

 

And to think I cut my sitter off and tossed it in the trash. lmao.

 

Yeah, it's not for everyone.

The only time I ever got in trouble with a sitter was years ago. She was 17...

 

LOL!

 

Another principle that could be used is curie temperature, the temp at which a ferromagnetic material stops responding to a magnet. Cobalt, for instance would fall away from a magnet at 2060°F. The hardest part would be keeping the magnet cool enough.

 

Boilers, oil home heating furnaces sometimes have a 'break' wire on the fuel valve, given it's an OK spec it might be worth a look, that could lead you to mech/elec solution. Just a thought.

 

I looked a bit for more info, Peedee but found zip. Any specifics?

Denis

 

I'm struggling to find an example, it used to be a soft wire link on a weighted shaft, nowdays it seems a capillary valve is used instead. That's the trouble with getting old you get out-dated!!! I'll ask a boiler service friend for more info.