Duet 8.5 Groundwire overheating

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dmo
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Duet 8.5 Groundwire overheating

Postby dmo » Sat Feb 03, 2018 12:21 pm

What's going on? I'm using 12awg power wires. Everything was tight and striped properly. None of the other wires were even warm, only the main ground wire to the Duet. The heated bed wires were fine. I've been using it over a week with no issues. I was 2 hours into the print when I smelled something burning. I could also use a source for a new connector and the part #.
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Re: Duet 8.5 Groundwire overheating

Postby dc42 » Sat Feb 03, 2018 4:59 pm

Looks like the screw terminal because loose. This happens when you put high current stranded core wires into a terminal block without crimping ferrules on the wires first (that's why ferrules are supplied with Duet WiFi and Duet Ethernet boards). You can usually get away with it if you re-tighten the screws frequently in the first few hours and days of use.

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Re: Duet 8.5 Groundwire overheating

Postby lordbinky » Sat Feb 03, 2018 10:52 pm

If you're using stranded wire, I'd highly recommend using ferrules (a type of crimped connector) with screw terminals. I know it takes yet another tool, but the reasons we crimp / tin / plate our connections are inescapable and screw terminals are really intended to be used with them. Once you strip the insulation off the copper it starts to oxidize now that it's in contact with the air. A proper crimp cold welds the wire and connector and further oxidation (and the associated increase in resistance and heat) isn't a concern for a long time. You get the same advantage to a smaller degree with solid wire since the contact surface between the wire and the terminal should slow the oxidation for a reasonable time frame. Tinned stranded wire also works for this but you still don't get the firm connection as using the ferrule and the shifting of the strands loosening the connection is still present.

p.s. I'm still lurking, but i thought a safety related issue was worth the struggle to make a post. I have my 1 mo. old baby boy taking up all my spare time right now (it's actually less than my wife in her third trimester though hahaha).

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Re: Duet 8.5 Groundwire overheating

Postby dmo » Tue Feb 06, 2018 1:29 pm

Thanks, that's a very good explanation. I'm wondering if using 12 awg wire made it worse. I'm very thorough with tightening when installing but didn't think to go back and retighten. Now that I know about ferrules I will certainly use them. I noticed SeeMeCNC doesn't use ferrules on the Artemis and I had read not to tin the wires. What's a good rule of thumb on how tight to screw those connectors?

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Re: Duet 8.5 Groundwire overheating

Postby geneb » Tue Feb 06, 2018 3:17 pm

Going back and re-tightening is always a good idea. Ferrules certainly can't hurt.

The only thing that would really need a ferrule in the Artemis would be the high-draw heated bed, and since it's no longer actually powered by the Artemis, it's not needed.

Note that I haven't done the re-tighten cycle on my Artemis and I've been printing fairly constantly since I got it in early January. Since the hot end doesn't draw nearly the power that the old 12V heated bed did, I doubt I'll ever get around to adding ferrules to the wiring.

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Re: Duet 8.5 Groundwire overheating

Postby lordbinky » Sat Feb 10, 2018 7:08 pm

Just to throw out a little big extra info, (geneb gives true advice again :lol: )I should have noted, that I meant pre-tinned wire specifically. Those who can tin the wires in the proper fashion theirself also know they can disregard the recommendation and not speak of it 8-)

Re-tightening is good enough within a few weeks of use. (practicality wise, as long as the copper is not brown, which you can restore in a few minutes or just clip it off and strip back fresh wire). This is largely the thermal expansion of the wires shifting each other around into a less strained/stressed arrangement, after that it's pretty likely to be good for a few years* until vibrations/movement work the connections loose, or the connection is messed with.

*Not to suggest waiting a few years until the issue shows up, just check every few months for warm fuzzies. Based on reasoning that in practical terms 4 years with even tight connections is enough for bare stranded wire (ie not crimps) to have oxidized enough to getting toward the hot-spot issues.

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Re: Duet 8.5 Groundwire overheating

Postby dc42 » Sun Feb 11, 2018 4:45 am

The reason we tell users not to tin the wires themselves is that if you do, it is absolutely guaranteed that the solder will creep and the joint will work loose. Anyone who does tin the wires themselves will need to re-tighten the screws frequently until they stop working loose.


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