Heating Element Safety

[bigun]

I want to control a heating element with a fairly hefty SSR.  I understand basic electronic safety very well, but I have not worked with heating elements very much.

I have a 1,650 Watt heating element I want to control, however I don't know how long to safely leave it on before it gets damaged or dangerous.  Any help is appreciated.

[wasedadoc]

You have not provided anywhere near enough information for anyone to give you a reliable answer to your question.

[ebastler]

That would depend entirely on the surroundings of that heating element. What is the use case? Where does the heat go? Is there a fan to distribute it? What materials are used around the heating element and what temperature can they withstand?

In any case you should provide protection against excessive temperatures, i.e. not just rely on limiting the time you keep heating. (Probably under software control, which can't be considered fail-safe...) Define your required/acceptable operating temperature, add a safety margin, choose all materials appropriately such that they can withstand that temperature, provide a cut-off thermostat switch in series with the heater.

Or, if possible, don't homebrew this part at all, but buy a quality off-the-shelf solution. Getting this right, in a way where you can safely let it run unattended (or even give it to others to use), is not easy.

[bigun]

You have not provided anywhere near enough information for anyone to give you a reliable answer to your question.

Hence why I'm here.

I don't know what information is relevant or not.  I have a model number (ESGHER1017), I don't have a spec sheet.  It's mounted in an enclosed space, being used to heat a meat smoker.

[bigun]

That would depend entirely on the surroundings of that heating element. What is the use case? Where does the heat go? Is there a fan to distribute it? What materials are used around the heating element and what temperature can they withstand?

Thank you.  I'll fill in what I can.  It's used in a meat smoker.  Inside of a enclosed metal cabinet.  No fans unless I install some.  Not entirely sure what it's made of, the brochure says steel coated aluminum.  So far I only plan to take it to 350F/176C at max, most of the time will be at 250F/151C.

In any case you should provide protection against excessive temperatures, i.e. not just rely on limiting the time you keep heating. (Probably under software control, which can't be considered fail-safe...) Define your required/acceptable operating temperature, add a safety margin, choose all materials appropriately such that they can withstand that temperature, provide a cut-off thermostat switch in series with the heater.

It will be under software control, that's the plan.  But when I think about turning on the element, first thing I thought of, "What happens if my temp probe malfunctions?  How long can I safely leave this thing on?".  A thermal cutoff is a great idea.  The smoker is one of these with a dead controller with no option to buy a new one off the shelf.  So I'm making my own.

Or, if possible, don't homebrew this part at all, but buy a quality off-the-shelf solution. Getting this right, in a way where you can safely let it run unattended (or even give it to others to use), is not easy.

Can't, see above.  The other option is to toss and buy a new one, which I don't really want to do.  I have an old Raspi model A I want to make use of, time to spare, and a love for smoked meats.  It's happening.

*edit*

Where would be a good place to mount the fuse?  The element is near the bottom of the cabinet.  Near the element or at the top?

[tunk]

190-200C thermal fuse in series with the element? Close the the sensor or element.

[bigun]

190-200C thermal fuse in series with the element? Close the the sensor or element.

I think close to the sensor makes the most sense.  Close to the element is sure to pop it with a near guarantee.

[ebastler]

OK, so the materials all around the heater were designed to get hot; that's reassuring. Keep an eye on the wiring you use to connect the heater, sensor and thermal cutoff switch: Silicone insulation is probably what you want, maybe with an additional silicone sleeve for extra protection (and tidy wiring).

In general, you want to design everything "single fault tolerant" -- i.e. if one component fails, the unit should still be safe (although not necessarily in working order). The thernal cutoff protects against a failure of the sensor or the controller; grounding the external metal parts should protect against electrical shock if the insulation gets damaged, etc.

If you are particularly concerned about a specific component, you could provide redundancy for that one, e.g. by connecting two temperature sensors and letting the Raspberry shut things off if the readings deviate too much. But I dont think that's needed here, with the thermal cutoff already taking care of sensor failures. Note that using only two redundant sensors read via software, without a hardware-based thermal cutoff, would not provide single fault tolerance -- a single controller failure could make them both inoperable.

[Ian.M]

Google reckons its a Calrod style heating element as used in many electric ovens and grills.  Odds are, in free air it will survive nominal voltage 100% duty cycle for many hours or even days without issue.  However its mounting bracket and element shell *MUST* be reliably grounded so if it does fail, it trips the breaker rather than taking anything its attached to live.

I'd probably go for a manual reset thermal trip monitoring the temperature in the upper part of the smoker cabinet, and a thermal fuse on the back of the element's mounting bracket - fuse temperature to be determined with the aid of a thermocouple probe attached to the bracket after several hours operation with the new controller holding max. operating temperature.  The thermal fuse is for protection in the case anything blocks normal air circulation within the cabinet cutting the power before the wiring insulation fails, and the thermal trip is to handle all other runaways.

[BrokenYugo]

I'm confused, this thing was designed to be an electric smoker already, it should have thermal safety switches already fitted and suitable for use, so just use those? They may not be visible, but that's where power wiring disappearing into the insulation on these sorts of appliances generally goes. Wiring in this sort of stuff is generally just a big loop of switches, one of which is the thermostat, and the heater, if every switch is closed, you get heat, any one opens and you don't.

Replace the switch in the old thermostat with your SSR, do the wiring safely and neatly, and that's the hazardous side done if I'm understanding right.

[bigun]

I'm confused, this thing was designed to be an electric smoker already, it should have thermal safety switches already fitted and suitable for use, so just use those? They may not be visible, but that's where power wiring disappearing into the insulation on these sorts of appliances generally goes. Wiring in this sort of stuff is generally just a big loop of switches, one of which is the thermostat, and the heater, if every switch is closed, you get heat, any one opens and you don't.

Replace the switch in the old thermostat with your SSR, do the wiring safely and neatly, and that's the hazardous side done if I'm understanding right.

That's what I'm wondering, I haven't dug into the guts of this thing just yet, wondering if I may find a thermal cutoff already.  If it doesn't, it'll have one before I start using it.