Halt and catch fire?  

RE: Halt and catch fire?

What was determined to be the root cause?

Posted by: @wink7887

It was an actual fire, a flame actually formed when I approached it.  Good timing on my part and a lot of luck

Cheers

RE:

I am now trying to figure out that very question.  Here are a couple more pictures to who ever may be able to shed light on this, but I disassembled the cover, wire brushed the plastic off, seems as the terminals where and are snug, though the wire to the left looks like it is freed/melted due to the pinching/having to force the cover plate on??  Does anyone know of a remix of that particular part, this was a kit, and I followed the directions to the Tee, just trying to give all the info.  Thanks in advance for any and all suggestions!

RE:

Here is my best guess as to what caused this and what happened.

The left terminal appears to have had a high-resistance (relative, I'm talking milli-ohm to low single-digit ohm range here) connection.  This could have been caused by a simple lack of tightness or dirt on the mating surfaces or oxidation or a combination of those.  The connection from the wire to the lug appears to be good, but if it was not, that could be a contributing factor.

IIRC the 24 volt heatbed has a resistance of about four ohms, and any other resistance in the circuit of any significance will be a power-stealing point of failure.

When the bed was heating, the temperature was slow to respond and the Einsy called for more and more current to compensate and to heat the bed.  This caused the heat at the relatively-high resistance point to build up to the point that it melted the plastic and began to oxidize whatever metal is there.  Since you say there was a flame, it either reached the ignition temperature of the plastic or the released gases.

My recollection is that the Einsy can push maybe 5-6 amps max to the heat bed, so, with say an 800 milliohm resistance at that point, that would be about 28 watts of power, and with the relatively lower mass in that section, it's going to heat significantly.

That's my take on the root cause, and over the years of my career I've seen similar meltdowns (caused by resistance in the circuit), a good example of which would be milliohm resistance in the filament circuits of high-powered transmitting tubes.

RE:

 I installed a smoke detector in each enclosure after my first experience with this.  

Posted by: @wink7887

It was an actual fire, a flame actually formed when I approached it.  Good timing on my part and a lot of luck

RE: Halt and catch fire?

I do plan on now either building an inclosure or purchasing one, and yes with a fire safety system.   Any recommendations on a build, for and enclosure, and what recommendations for what parts to use and where.  I and a DIY type been pondering the thought, what better time then now.  @JSW I do appreciate the time you took to give that analyst of your guess, sounds great, but from my education on that field, well I honestly didn't understand any of it, bit I do thank you, besides a smoke detector, what elsevcan be done to help prevent.  Thanks again to everyone for there responses and help in this matter

RE: Halt and catch fire?

the wire to the left looks like it is freed/melted

your last picture magnified: The washer on the right is assembled correct.
On the left it is wrong, you placed it between the PCB and the cable lug !

And one more: The left side does not look like a original Prusa Cable, the lug looks different to the right side.

If you built this cable on the left side by your own, there is also the chance that you did the crimp connection wrong. That can cause corrosion and increased resistance  over time, too.

RE: Halt and catch fire?

Posted by: @wink7887

I do plan on now either building an inclosure or purchasing one, and yes with a fire safety system.   Any recommendations on a build, for and enclosure, and what recommendations for what parts to use and where.  I and a DIY type been pondering the thought, what better time then now.  @JSW I do appreciate the time you took to give that analyst of your guess, sounds great, but from my education on that field, well I honestly didn't understand any of it, bit I do thank you, besides a smoke detector, what elsevcan be done to help prevent.  Thanks again to everyone for there responses and help in this matter

I built mine with lack tables.

RE: Halt and catch fire?

Agree with @stefan-3 re cabling.

See steps 2 through 12 here:

https://help.prusa3d.com/guide/7-heatbed-psu-assembly_172872

This is what the cables & connections should look like  -

Cheers

RE: Halt and catch fire?

Posted by: @stefan-3

the wire to the left looks like it is freed/melted

I also remember when assembling mine that the plastic holder was a poor fit unless I assembled the wire+crimps downward to be inline with the pcb board. This reduced the height of the wire/crimp by putting the bulk of the wire in line with the board and not above. I also used 1000 grit paper to remove oxide from the  pcb traces, the connectors and the washers to enhance the connection. The photos here does seem to suggest the washer was in the wrong position.

RE: Halt and catch fire?

Sorry for the long no response but been busy with life.  Anyway not sure exactly how it was caused but since then have replaced the heat bed assembly with new, just printed all my parts for a box, with temp control, heat and cool, with a smoke alarm that shuts off the printer, along with a camera and octoprint with a raspberry pi 4b+ I've had laying around.  So thank you to all for the help/suggestions and I'm sure I'll be banging my head some more with getting this enclosure built lol.  All in the name of fun!! I absolutely love the 3d printing, wish I had people close to me hear in the states to guide me as I'm really not the book guy.  Thank you all!  And here is the start of it now.

RE: Halt and catch fire?

I run my printer in a workshop/garage that isn't attached so for long jobs, I want to be able to shutdown and alarm if there is an issue.  I am looking at using a non-contact thermometer sensor and maybe in combination with a thermistor to detect over temps with an Arduino.  I did look at the fire suppression attachment for the enclosure.  Being in a garage, a standard smoke detector could be an issue.

Due to other comments and issues about vibration on the wires from the heat bed, and not having any 3mm nylon filament to provide some vibration relief similar to the print head setup, I did a mod using a zip tie.