Safety Reminder Periodically check that your heater wire screw terminals are tight. My colleague’s printer caught on fire, right at the terminal strip.
After hearing his tale of woe, I’m also putting a smoke detector and fire extinguisher in the room with my printer
This happens more often than most of us realize. I found out by the time my bed stopped heating. By then the connector was allready black and cremated.
This usually happens because of bad contact between the wires and the terminals. Try to “tin” the end of your wires, and flatten them with pliers. This should make the surface contact optimal.
Anyway… soldering the wires directly to the pcb would be best.
Ran into that problem recently too: http://www.pappp.net/?p=1296
From my reading at the time, be really careful about tinning wires for insertion into screw terminals, it is fine to tin ahead of the contact area, but if you tin the area that is gripped by the contact it is likely to create a problem because of varied coefficients of thermal expansion. There is serious literature on the topic, but also a whole routine about it in the reprap wiki:
http://reprap.org/wiki/Wire_termination_for_screw_terminals
The “right” way is crimped terminals (Ferrules or screw rings or whatnot).
There’s one school of thought, that you shouldn’t tin wires used in screw terminals. The theory is that solder can cold flow over time loosening the connection and that bare copper is best.
@Paul_Eberhart , that’s very very true. This is the whole reason why the housing changeover from aluminum wiring to copper wiring was so fraught with burnt down houses. The changes in thermal expansion would cause spark gaps and fire.
Generally, make sure your connectors are rated for the current you push through them. 5.08mm screw terminals are rated for 10A, which is already exceeded by a single bed (12.5A). 2.54mm crimp plugs max out at 2A per pin, as do pin headers. Barrels plugs are related at 3A, but cheap ones already get hot enough that they deform at that current.
@Robert_Wozniak it’s even part of the VDE (basically the German part of the IEEE) rules not to tin clamped wires. Tin causes more fire long-term than it prevents short-term.
Im planning to change to off-board 4mm bullet connectors for anything carring a significant amount of current. Or more specifically HXT plugs, for polarity protection. I wonder what would make a good single removable connector for my carriage plans with enough pins for two steppers, two hot ends, four fans, two thermistors, a servo, an end stop and some LEDs. Perhaps a 24 pin motherboard connector with a few common 12v and 5v pins.
I know 2 or three people this has happened too. +1 on the fire extinguisher and smoke detector just make sure its an electrical extinguisher or you can make things worse.
You mean a powder extinguisher?
There are a few types, but for example you don’t want to pour conductive liquids on an electrical fire. There are different kinds usually they say right on them what they are for. Well I think it is the law, so always.
CO2 and powder are the two recommended types for anything involving electricity. Obviously, CO2 makes much less of a mess, but can suffocate you in extreme cases.
I just bought a small powder based extinguisher last week for € 7 EURO. Good to know I bought the right one.
Powder extinguishers can definitely cause you to suffocate as well
Yes but at least then you can “see” what you’re suffocating in. 
That’s very true 