@dc42 I have checked this and there is not short between the heater and the duet. I have also tested each individual component separately, and they all still work. Excepting the thermocouple daughterboard, we are just paying it safe and buying a new one. So still not sure exactly what happened here.
I am rebuilding with all of the tips in mind shown here. Plastic washers between the screw and board, and the board is mounted to plastic frame now. I have removed the peltier cooler from being controlled by the board altogether and am using a separate control board for this. I have docked it all in a computer case with adequate airflow and a fan to avoid dust. Hopefully all these combined will keep me good to go going forward!

Thank you to you and @NeoDue for all your help identifying points of improvement going forward. I appreciate it.

@dc42 I have checked this and there is not short between the heater and the duet. I have also tested each individual component separately, and they all still work. Excepting the thermocouple daughterboard, we are just paying it safe and buying a new one. So still not sure exactly what happened here.
I am rebuilding with all of the tips in mind shown here. Plastic washers between the screw and board, and the board is mounted to plastic frame now. I have removed the peltier cooler from being controlled by the board altogether and am using a separate control board for this. I have docked it all in a computer case with adequate airflow and a fan to avoid dust. Hopefully all these combined will keep me good to go going forward!

Thank you to you and @NeoDue for all your help identifying points of improvement going forward. I appreciate it.

@dc42 wow thank you so much, I will do that! I got caught up with some other things on Wednesday and am now out of town. I'm returning next week and will pick back up on testing. I've already got the new duet3, just want to make sure all components are safe before hooking it all back up.

@droftarts when we originally started using the daughterboard a year ago and ran into these problems, I actually tried other daughterboards on hand and got the same result. I also tried other duet boards with these daughterboards, and same results. So going forward, even if this heater still works, we're actually planning to change the sensor to a thermistor in order to bypass the daughterboard altogether. I may try these steps anyways though, just to see what it's current condition is... Thanks for the advice!

@dc42 We did not actually extend the thermocouple wiring, so it seems like that isn't the culprit.

I will check for what you advise when I get back to the workspace later today and let you know.

We started using these boards more or less than 2 years ago, from filastruder a distributor in the US.

@dc42 Thank you for your replies. The thermocouples are insulated from the hot end.

I have checked continuity for these and have not found any continuity where it should not be. I've tried every combination possible, thermocouple to heater, thermocouple to insulation, thermocouple to heater wires, thermocouple to heater wire insulation, and then the same combinations through the heater power wiring.

I'm now working on powering the heater through an older heat controller so as to not fry another Duet if it is the culprit. Just want to see if it still works as intended. If it still works, I'm sort of at a loss. I've also seen that length should be taken in account for thermocouples. The thermocouple wiring is about 19.75 inches. Is this an acceptable length?

FYI we are using the v1.1 daughterboard with a k type thermocouple.

@NeoDue Well as far as I know there shouldn't be any resistance at all between these two pins. It should read 0L, as it does for all other pins when connected to the ground. I also have a new duet3 board that hasn't been used yet, and it does not have resistance when touching the same pins: 3.3V and ground.

When I search the forum it looks like I'm not the only one who has had problems with the thermocouple daughterboards. Maybe I can get some good information from those threads.

@NeoDue it looks like I have a short where the thermocouple daughterboard connects. When I connect the multimeter (on ohm setting) to the +3.3V pin on the back of the duet 3 board and any other ground, the multimeter shows resistance. ~2.8-3.8. However when I do the same to the daughterboard itself there is no resistance.

I now see it would have been relevant to state the daughterboard has always given me trouble. Particularly when I had two thermocouples connected to it, it would power on with the Fault light showing on one of the two. I would simply disconnect and reconnect, power everything back on, and that would fix it. Sometimes I'd have to repeat many times. I tried multiple of these daughterboards and multiple duets, and still it happened. So I resigned myself to just check for the fault at each power up of the system. That seems like it was a mistake. Does this sound like it would have been the cause of the short?

I will continue to do more testing in the meantime! The continuity was the fastest/easiest to start with. Wires are next.

@NeoDue okay I'm going to try these things! Thanks again

@NeoDue Okay, so it could possibly be the screws, or possibly the lack of plastic spacer between the screws. I'll try to implement these changes along with the plastic mount. But otherwise the answer isn't obvious..

Circling back to the vout_0, I found older pictures of it which do not show damage to this wiring. Is it possible that this equipment (a cooler) failed and caused the short? Is there a recommended way to test each piece of equipment individually for fault? Without frying another board? We have multimeters available for use.

Thanks again for your help!

@NeoDue You are right, it was actually mounted to the metal. I am unsure if it is aluminum but will be able to check with the person who built this setup tomorrow. I have found multiple damaged chips (in addition to the ones found before) and am assuming this board is now unusable. I will be able to obtain a new one. Can you please help me to ensure it is safe going forward?

I have included pictures of how it was mounted. I am wondering if putting the board above the aluminum would have made a safe difference, even if it's still mounted to the aluminum? I also have a spare small mounting bracket printed that I found extra from someone else's setup. It does not have the fan, like yours does. I've included potential mounting set ups for this, are any of them appropriate? I think I should be able to implement your setup fairly easily. But what materials are appropriate for that to sit on top of? Is the wood okay, and are there any I should avoid completely?

I am concerned for the other Duet3's being used in my work space too. I have also included the photos of the set up for those. They do not have fans directly cooling the boards. They also seem to be mounted to metal (with plastic in between). Is this a safe set up for long term use?

We are using 24V power supply (also pictured).

I so appreciate your help. I am utilizing the system but am unfamiliar with how it actually works. I'm very much out my depth here and you've been a great help in identifying the problem. I sort of short circuited too, and was in a panic when I first posted. You have helped tremendously. Thank you.
Duet3 mounting pictures

@NeoDue @elmoret the metal plates are attached to the wooden base, the board is attached to the wooden base. The board and the metal plates don't have contact. There are also plastic spacers between the wood and the board. I can take photos and post them later today if that's helpful? You can see the edges of the spacers in that photo at the bottom of the board.

As for the v_out0, yes this looks like damage but that photo was from two weeks ago. I'm not sure if it had this discoloration when it was installed, but it's attached to a cooler and has been running for months without obvious issues. Could it have slowly been damaged? Is there anywhere else which you'd recommend to check for damage?

This system has been running for about year with changes/upgrades here and there. But no major changes to this hardware recently. I had unplugged a heater connected to the out_8 to change the housing, then plugged it back in. When I first switched the system on, the chip sparked and flamed within a couple seconds. The vout didn't have apparent damage at that time. There wasn't an arc or anything.

Thank you for your help