Burnt power terminal

After much work, I finally my prusa i3 running beautifully. Some print quality issues, over hang drooping, adhesion issues, y'know, but working just fine. I was running a test with several objects on one plate, a bottle opener when one of the infill layers jerked off to one side, then the printer stopped entirely. There was a smell of burning plastic so i cut the power immediately. Lo and behold, the power terminal was burned out. See attachment. Just the outermost wire, though. The rest are fine. Did I overtax it or something? Or is this the result of bad parts? I ordered a replacement terminal that should get here in a few days, but I don't want to put it on until I find out what happened. I was running the printer with PLA, extruder at 190C and bed at 60C, well within the usual limits.

I had nearly the same problem on my printer, but found the scorching in the power terminals before it completely destroyed them. I tracked down the source of the cause to be too thin of wires on my power terminals, and it looks like you may be having that problem, too. I switched from thinner (around 18-20 AWG) wires to 16 AWG wires, andhave had no problems since. I think that is what caused your 'meltdown', and some thicker wires should help to keep things cool around the power terminal, and hopefully that should fix it.

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I agree - those wires look too thin. Also, obviously the weak part of the wire was the connection. Make sure those screws are down TIGHT.

The problem might be the connector itself. Friction fit connectors always fail eventually on 3D printers. The constant vibration causes the contact resistance to increase over time. I only use screw terminals.

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is this a 24v or 12v system?

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It's on 12V, and I thought my wires were a high enough rating, but I'll double check and upgrade. I thought maybe it could be the board since I've recently had to replace one of the polyfuses as well, and I suspect cost-cutting components. Thanks for the suggestions! I'll post again with results once my new parts are in.

Make sure you have thicker gauge wiring than you think you need. Also having multi strand wiring is important especially if the wiring has to move periodically. Another way to help protect your board is to move the power hungry heated bed onto a relay. Either a solid state or auto relay with diode will work great. This is a fairly common issue that has been seen. Always take time periodically to check wiring for this kind of issue as it could be a potential fire hazard. Another thing that could cause this is not enough wires out of the stranded wiring are making contact. This will cause the wiring to heat up significantly. You can buy connectors that are made for these kinds of terminals. These connectors will ensure that all strands are properly connected.

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This looks like one of the fingers in the plug connector became loose and caused heating and subsequent failure. The wire damage is from conducted heat.

Personally, I am not a fan of these connectors for carrying significant current because of these issues. I prefer screw terminal barrier strips to avoid this issue.

Another often overlooked issue with these Phoenix type connector is not stripping the wire far enough back when inserting it into the connector or stuffing the wire into the connector so far that it partially clamps down on the insulation. This has the effect of reducing the surface area of exposed copper inside the wire clamp and increasng the resistance. What is shown in your picture is thermal runaway. More resistance at a given current equals more heat which leads to oxidation over time whch leads to more resistance and the cycle continues until failure occurs. Im not saying upgrading your wire is a bad idea, or that this type of connector is optimum, i just wanted to offer some experience and insight into this type of failure. If it were just the result of undersized wire then melting or softening of the insulation would be seen elswhere.
I hope you find this post useful.