Terminal block on Duet melted

Just had to unplug my Ormerod today due to smelly smoke from the Duet enclosure. The machine had just finished an 2 hour print and I had just set it up for another and was about to leave the area.

Turns out the top-most of the two wires from the PSU was melting, well, the printed enclosure and the terminal melted. The wire was too hot to touch.

I did not use the ferrules in the kit because my crimp tool made the ferrule and wire too big for the terminal block. I have a hard time imagine that just wires would give a higher resistance or smaller surface area than it would using the ferrules, so I don't think this was the reason. However I' curious is anyone else has had issues with temperature rise in the terminal block?

I didn't use the ferrules, but I did twist and tin the wires before putting them in the terminal block. No problems so far.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Twisting the strands for easy entry into the terminals cage, where it will be spread by the force of the cage to give maximum contact area. A ferrule will be easier to mount, look good and be remounted if needed. It dosnt reduce contact resistance.

If the duet survived I'll have to get a new terminal block with high amp rating and sufficient squarage for a heavier cable. Must then also replace terminal block on the psu board side. Then remove the original boxes and combine them both in one enclosure. Should probably add a fuse somewhere for safety.

This was probably due to the connector clamping force being too low. It is a timely reminder to check that these terminal screws are tight as they can relax!

It could also have been the soldered terminal pin on the PCB being slightly high resistance, but I would have expected the PCB to fail before the wire started to melt.

You didn't mention solder on the power wires, so presume these were not, but for others reading this post you should never solder stranded wire that is going to be inserted in a screw terminal, firstly the wire will break at the end of the solder if flexed, but also the solder will deform with age and become loose in the connector and over heat.

Hope your board is OK. Don't forget fuses will add to the volts drop.

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Ormerod #007 (shaken but not stirred!)

Oh yes, a slightly loose terminal screw or strands missing / unclamped in the block can introduce enough resistance over a small volume to heat the wire significantly at those currents. The hot wire starts to oxidise over time, which eventually increases its contact resistance further and it suddenly gets red hot. I've seen it happen many times! Also I second Treth's comment - twist stranded wire tightly before putting into a terminal block making sure you get all the strands in if it is carrying significant current, and do not tin the end. Make sure it goes in all the way so the screw is not gripping it right at the tip. Retighten the terminals occasionally.

Dave
(#106)

What I did from the word go was to take a piece of main's rigid electric cable (2.5mm2 or 1.5mm2 can't remember), stripped the grey insulation to get the earth and live wires out and used them to connect both boards. It is solid core, no strands, and the wires are not flexible.
No need to twist, less risk of it not being tight and because it is non flexible it doesn't sag and pull out of the terminals.

Quote
arnaud31
What I did from the word go was to take a piece of main's rigid electric cable (2.5mm2 or 1.5mm2 can't remember), stripped the grey insulation to get the earth and live wires out and used them to connect both boards. It is solid core, no strands, and the wires are not flexible.
No need to twist, less risk of it not being tight and because it is non flexible it doesn't sag and pull out of the terminals.

This may be OK, but I'm not sure. Thick rigid copper usually goes in a more robust terminal block with larger screws and pointed ones that are strong enough to compress and shape the copper to make a good connection. There has been problems in the past with the so called chocolate block connectors (seem to be better produced now) where the smaller flat ended screws do not apply enough pressure to distort the rigid copper and grip it securely.
So you may be correct, but I would not go down that route from my experience with the type of terminal block supplied.

Incidentally the screw relaxing the pressure on copper and the solid copper 'relaxing' and deforming over time is one of the main reason all mains electrical installations should be regularly inspected (10 years?) and why some companies will never use a junction box (hidden and not accessible for checking) on mains wiring. So we may not have this long term service issue, but it does show the importance of the clamping force.

Except for the difficulty of fitting, I prefer the crimped multi-strand solution as supplied.

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Ormerod #007 (shaken but not stirred!)

The Duet terminal block is rated 10 A. To me that sounds too small....