Adding a 24V safety relay for the heaters, any thoughts?

A Former User

@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

DocTrucker

@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.

deckingman

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.............)

DocTrucker

@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.

DaBit

@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.

Danal

@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.

DocTrucker

@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.

zapta

TL;DR I performed a small experiment and PS_ON doesn't seem to be a good choice for everything-ok signal.

Experiment:

Connected a small 24V bulb between PS_ON and +24V.
Added the command M80 to config.g to turn PS_ON.
While the printer is turned on and idle, shorted the bed power mosfet with a wire to have the bed permanently on.
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?

Danal

@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?

A Former User

@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?

Danal

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.

zapta

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.

Heating the nozzle and then forcing overcooling by holding the heat block with parallel pliers. Firmware detection detects (good) but PS_ON stays on.
In idle, shorting the nozzle heater mosfet, I let the temp rise to 240C but no firmware detection.
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

infiniteloop

@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).

A Former User

@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.

Danal

@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.

A Former User

@Danal said in Adding a 24V safety relay for the heaters, any thoughts?:

I regularly print at that extruder temp. I would not expect an alarm.

i'm not a fan of the nanny state tbh, but i don't think that there is a reason to not flag erroneous operation when possible, even if it doesn't pose a fire risk. detect rising temperature when the firmware should expect falling or stable temp is quite possible, and could at the very least avoid someone getting a burn, especially small inquisitive hands f.ex.

same with the bed at 70C; its technically wrong behavior and possible to detect so why not? (even though I don't see ps_on as a reliable means to remedy it)

DocTrucker

As I flagged up before the PS_ON control is disabled when the machine is Idle. It has been on the wishlist for some time to get similar controls for PS_ON while idle as to printing.

After all, if you are doing domething that is likely to trip the alarm with maintainance or tuning a temporary override and don't leave the machine unattended should suffice.

Perhsps @dc42 or @T3P3Tony can update us on the progress of this firmware wishlist item.

zapta

@Danal said in Adding a 24V safety relay for the heaters, any thoughts?:

I regularly print at that extruder temp. I would not expect an alarm.

The alarm is not not be about the temp but about the inconsistency between the input from duet (no warming) and the actual temp (do not cool down and keeps warming, consistent with a short mosfet).

zapta

@DocTrucker said in Adding a 24V safety relay for the heaters, any thoughts?:

It has been on the wishlist for some time to get similar controls for PS_ON while idle as to printing.

Note that PS_ON did not trip even during printing when over heating or cooling triggered the firmware PID error message.

To be fail, I don't think that Duet represented the PS_ON as an automatic OK signal so it does what it is advertised to do, a digital output that can be turned on/off with geodes to control a power supply.

It would be nice though to have an OK output from the duet. Doesn't need to be fancy charge pump and such, even a simple digital output that trips when an anomaly is detected will go a long way.

DocTrucker

@zapta it will/should drop as I understand it if you get a heater fault state during a build. It maybe that there is a delay of a period of time before the PS_ON signal is dropped.

I regularly get heater fault if I overcool the heater block (yes with a silicone sock in place) with a bad part cooling duct.

A three posision switch on the switched-ground side of a heater could test alarms. One side normal (normal control by duet), central break (heater disabled, mimics temp sense failure) in a single sensor system), other direct to ground (mimics a shorted MOSFET). My machine safey prints PLA anywhere between 180 and 230C so it would be interesting to test this for myself too...