Electronics > Projects, Designs, and Technical Stuff
wire termination for screw terminals
duak:
If there's no way to use a crimped termination on a stranded wire that is held down with a simple screw I've found a few things that help:
1.) Leaving a bit of insulation on the end of the stripped section can help for some sizes
2.) if the screw is right handed, ie. the normal one that tightens clockwise, twist the wire strands counter-clockwise before forming it into a hook.
3.) use a washer underneath the head of the screw - the bigger, the better. I occasionally find products like Solid State Relays that have a square washer with folded wings that help hold the strands in place. I wish these were available more widely.
tkamiya:
--- Quote from: electrolust on June 04, 2020, 09:00:45 pm ---https://reprap.org/wiki/Wire_termination_for_screw_terminals
The terminals in question do have a flat pressure pad, and are tin plated. There is a mains voltage terminal and a control terminal. Both of these are meant to be used with solid wire.
It's clear from inspection that solid wire does not contact the terminal pads very well. Round vs flat, so you get limited contact. There's limited torque you can apply, so you aren't biting into the wire much either -- only enough to prevent pullout. This is a high quality product, so that must be within the design spec.
--- End quote ---
Does the flat pressure pad snugly fit the slot and have a downward bend at all 4 corners? If this is the case, the terminal may be designed to accept two wires, one on each side. You'll have to consult with manuals to determine if this is the case. In those cases, you don't need to make a U hook. Just straight wire will do. That little downward bent will help with fraying of stranded wire, too.
As to solid wire not making good connection, it's been this way for many years. Even 100 amp circuit breaker is made this way. I think you are over thinking this. I will not suggest flattening the wire either. You'll just make a weak point for wire to break. There are manufacturers of terminals that writes in manual AGAINST using too much torque, as it will squish the wire and possibly damage the terminal.
As to compression wire end terminals, please note, insulated type is for stranded wires only. You will not get a good crimp on solid wires. (even if you use expensive manufacturer supplied tools) Quality of crimp, if you choose to use them with stranded wires, depends on use of proper tool. Too much or too little pressure can cause eventual failure.
If I were to do this, I would use stranded wire larger than spec calls for, and crimp an insulated Y terminal with approved tool. Another way I might do, assuming it is not subject to vibration, is use a solid wire, make an U shape and torque appropriately. Then bundle the wire for extra measure.
FYI: I'm a licensed electrician (but license is from Japan, so it's not valid in USA)
tooki:
--- Quote from: Vovk_Z on June 05, 2020, 05:01:05 pm ---
--- Quote from: tooki on June 05, 2020, 04:31:12 pm ---It’s more likely vibration causing stranded wire to loosen.
--- End quote ---
- No, stranded wire loosen without any vibration. I've seen it many times.
--- End quote ---
Yes, sorry, I forgot to mention the other thing that causes loosening: thermal cycling.
Lomax:
I would never put bare stranded wire into a rotating clamping mechanism. And as pointed out by others tinning the wire may lead to problems in the future, especially in a high current/high temperature application. Uninsulated open barrel ring or fork terminals are the gold standard, but I find ferrules also work perfectly well in screw terminals. Considering that the cost & time for adding them is insignificant I can't see why one wouldn't.
wizard69:
--- Quote from: Vovk_Z on June 05, 2020, 05:01:05 pm ---
--- Quote from: tooki on June 05, 2020, 04:31:12 pm ---It’s more likely vibration causing stranded wire to loosen.
--- End quote ---
- No, stranded wire loosen without any vibration. I've seen it many times.
--- End quote ---
This very true and frankly I suspect this is why some are saying tinned wires are subject to loosening over time. The reality is if you work in a plant with a decent Electrical / Electronic PM program, you will be running around once a year torquing screws down until the stabilize. Vibration can of course be an issue but working in a plant where there is basically no machine vibration I can safely say that vibration is only part of the problem. A a result I've seen various issues and can safely say what is right depends upon what is being used at the time.
For example set screw type connections work really well with wire that has had a ferrule put on it. This especially if you have to terminate a very small wire into a larger terminal. Ferrule allow the wire to act like it is solid.
On the flip side ferrules really seem to make things worse on barrier type terminal strips. That is strips that have wire clamping washers under the clamp screw. Here it is better to have ring or fork terminals. It seems like these types of terminals work much better when the load is balanced under the screw. They also work with bare stranded wire fairly well as long as the diameter of the wire is not excessive. If you try to clamp large diameter solid, ferruled or even stranded wire, under one side of the screw you will have trouble. In such a case I've even seen the clamp screw end up bent and unable to secure the wire reliably.
One other thing tinned wire is not wire with globes of solder on it. Tinned wire is effectively plated with solder. I've never seen tinned wire causing a problem by itself. It however doesn't really offer any advantages either. If you put globes of solder on the wire then you will certainly have issues. In any event any place where tinning might offer a benefit, wire ferrules are easier and provide additional advantages.
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