Adding a 24V safety relay for the heaters, any thoughts?
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@deckingman Maybe its better if you ask the manufacturer what the appropriate design criteria is; the US is or is looking into using nitrogen for their capital punishment - which I doubt they'd do if it didn't have some adverse health effects.
Maybe they have added a scent or colour to the gas, idk. But inert gasses are dangerous, even a common CO2 extinguisher can take care of both you and the fire if you use it in a small basement without some thought to your own safety.
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@DaBit said in Adding a 24V safety relay for the heaters, any thoughts?:
given the limited power density of the heater
I have 750W, 1000W and 2500W silicone heaters here on some aluminium beds ... never tested how far they can go but they get up to 100C in seconds, I would never allow them to run without meltable fuse for security. Running low power beds that would settle at max power around 100-150C is safe but I don't run those and more and more printers are made/upgraded with high power beds.
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@bearer I appreciate your concern but extensive searches on my part don't return any causes for me to be concerned on my part. These readily available clean agent extinguishers replace Halon which is now banned in the UK for health reasons. In the worse case scenario, I'm reasonably confident that my extinguisher will put out any fire, without itself damaging the Duet boards as other system might do. At the same time, because the cylinder size is matched to the enclosure volume, I do not believe that it poses any significant health risk should it discharge inappropriately. Now I really do think that we ought not to hijack @zapta's thread any more than we have done already.
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@deckingman, I find it interesting. Alternative ways to achieve same goal.
Edit: I would think that with electrical fire it's important to disconnect power otherwise it will keep igniting. Is it so?
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@zapta Quite possibly. That's why I have a smoke alarm too. That would probably go off before any fire really got hold enough to trigger the automatic extinguisher. Realistically, my printer probably has more layers of protection against fire than any of the appliances in my house - including our built in electric ovens. Prior to moving it to it's new home which is fitted with an automatic fire extinguisher, it resided in a spare bedroom without any fire extinguisher but I still slept peacefully at night.
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@zapta said in Adding a 24V safety relay for the heaters, any thoughts?:
I would think that with electrical fire it's important to disconnect power otherwise it will keep igniting. Is it so?
that would probably vary quite a lot, but I'd guess more often than not, "no" as fuses would trip, flammable materials get burned up and if the fault required some logic state to start odds are that would be dead in the water.
however getting the power cut would certainly be a good thing
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@bearer most off the shelf fire alarms have a way to tie into other fire alarms so if one is triggered they all beep. Trying to find decent documentation on that is difficult though.
There are probably off the shelf systems for industrial environments that kill power on fire detection.
With UPS, the cut obviously has to be on the protected side of the batteries!
All that said the probability of a fire starting in a well designed system with thermal fuses, well protected wiring, redundant thermistors, heater interlock, etc is minimal and a fire proof cabinate design isn't without its challenges and risks strength to resist sudden increase in pressure and where and how that pressure is vented for the first two.
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Anybody got a Whirlpool, Hotpoint, Indesit or Credit tumble dryer? Come on, own up - there are 5.3 million of you out there with these defective appliances. Here in the UK there are on average 60 fires a week caused by faulty appliances - mostly tumble dryers and washing machines. And of course, the recent Grenfell disaster was caused by a defective fridge/freezer. The moral of this story? Don't set the washing machine, dishwasher or tumble drier going, then go off to work or go shopping. (A bit more difficult to do with a fridge freezer though.............)
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@deckingman risk assessment and safety on household appliances is poor in comparison to industral equipment standards. No harm in trying to do better. That said thermal fuses and resetting switches are common place even so.
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@Danal said in Adding a 24V safety relay for the heaters, any thoughts?:
Could you expand on that? It would seem a sprinkler requires a water supply, and it is hard to see how that is cheaper.
I think that is more applicable to my situation. Machines in the garage, and there is a water supply available.
For the price of one AFO fireball one can buy a lot of pipe and mounting clamps, and the sprinkler heads themselves are not very expensive either.In my situation it might be a good idea. The 3D printer is not the only machine.
@deckingman said in Adding a 24V safety relay for the heaters, any thoughts?:
Better still, use a clean agent fire extinguisher as I mentioned above - much kinder to the electronics than water (or foam).
I will look into that.
I am also wondering: how effective are one-shot devices when the power is not killed? Water would sooner or later trip the ground fault interrupter, and it keeps flowing (which might be a 'small' disadvantage also)@arhi said in Adding a 24V safety relay for the heaters, any thoughts?:
I have 750W, 1000W and 2500W silicone heaters here on some aluminium beds ... never tested how far they can go but they get up to 100C in seconds, I would never allow them to run without meltable fuse for security. Running low power beds that would settle at max power around 100-150C is safe but I don't run those and more and more printers are made/upgraded with high power beds.
Mine is a 600W heater on a 300x300x10mm aluminium plate. They can become way hotter than needed, but it lacks the power needed to reach autoignition-hot. I think the remaining risk after taking into account the overheating prevention of the Duet in combination with the regular SSR and relay is acceptably small. Can it be better? Sure.
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@DaBit said in Adding a 24V safety relay for the heaters, any thoughts?:
Machines in the garage, and there is a water supply available.
Got it! That does sound like a good idea, as a final safety net.
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@DaBit said in Adding a 24V safety relay for the heaters, any thoughts?:
I think the remaining risk after taking into account the overheating prevention of the Duet in combination with the regular SSR and relay is acceptably small. Can it be better? Sure.
...and there's where the art in the design of these failsafe and protective systems come in.
It'd be enlightening to the what the real probability of failures is and how it compares to risk of injury from mundane everyday tasks where intrusive safety measures are accepted without thought.
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TL;DR I performed a small experiment and PS_ON doesn't seem to be a good choice for everything-ok signal.
Experiment:
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Connected a small 24V bulb between PS_ON and +24V.
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Added the command M80 to config.g to turn PS_ON.
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While the printer is turned on and idle, shorted the bed power mosfet with a wire to have the bed permanently on.
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The bed temperature rose to 70c within ~6minutes (observed with DWC) but PS_ON stayed on.
I would expect the duet to complain about bed temp rising with no input and to turn off PS_ON. Any thoughts?
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@zapta said in Adding a 24V safety relay for the heaters, any thoughts?:
3. While the printer is turned on and idle, shorted the bed power mosfet with a wire to have the bed permanently on.
4. The bed temperature rose to 70c within ~6minutes (observed with DWC) but PS_ON stayed on.I don't see anything "alarming" there. It is quite normal for beds of certain power densities to be "full on" (no PWM) during their initial heatup phase. 70C is well within the temps required for certain materials.
What would trigger an error?
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@Danal said in Adding a 24V safety relay for the heaters, any thoughts?:
What would trigger an error?
that the bed temp rises without rrf turning the bed on presumaby?
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And how would this be distinguished from the bed being 70C on its way down from 90C while "off"?
In theory, one could specify a set of "trends" that would not false trigger on things like ambient room going from 20 to 25C because someone opened a door in the summer, or the bed taking a long time to cool (some do). This would require a lot of "prior state" or "state over time" tracking.
In practice, I'd be willing to bet that every firmware out there looks for "rising too fast" (note, not "rising too fast and the output isn't on") and "exceeded configurable peak".
Therefore, no, I wouldn't expect those 4 steps to trigger any alarms. Keep going until the currently configured max temp is exceeded and see what happens.
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I perform more experiment and the PS_ON doesn't badge, even when the duet firmware detects an abnormal conditions and disable the PWM bed and nozzle heaters.
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Heating the nozzle and then forcing overcooling by holding the heat block with parallel pliers. Firmware detection detects (good) but PS_ON stays on.
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In idle, shorting the nozzle heater mosfet, I let the temp rise to 240C but no firmware detection.
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With nozzle temp sets to 230c, shorted the mosfet. Temp risen, firmware detects but no change on PS_ON.
Ut seems that
a) the duet doesn't detect temp runaway when a heater is not turned on and
b) even errors that the duet detects do not turn off PS_ON.Edit: all tests were done on Duet2 Wifi 2.04
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@zapta said in Adding a 24V safety relay for the heaters, any thoughts?:
b) even errors that the duet detects do not turn off PS_ON.
And that’s good, because I would loose all of my cooling fans, too. PS_ON is not meant to signal an error, but to control the 12V or 24V power rail on systems with two separate PSUs.
It’s wrong to focus all hopes on a side effect (which, in this case, doesn’t occur).
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@infiniteloop said in Adding a 24V safety relay for the heaters, any thoughts?:
I would loose all of my cooling fans
interesting observation. while i wouldn't count on ps_on for safety I might opt for more 5v fans.
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@zapta said in Adding a 24V safety relay for the heaters, any thoughts?:
In idle, shorting the nozzle heater mosfet, I let the temp rise to 240C but no firmware detection.
2. In idle, shorting the nozzle heater mosfet, I let the temp rise to 240C but no firmware detection.
I regularly print at that extruder temp. I would not expect an alarm.
Delta / Kossel printer fanatic
