I guess you're right. I think twisting prevents screws working loose from when the copper expands and contracts from heating/cooling down though (I wonder how many blackened cooker/immersion heater switches in the world have suffered from this)
Twisting conductors or not?
- Thread starter tew1
- Start date
I guess you're right. I think twisting prevents screws working loose from when the copper expands and contracts from heating/cooling down though (I wonder how many blackened cooker/immersion heater switches in the world have suffered from this)
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there's not often more than one cable supplying a cooker / imm heater though!
The 3D geometry would be quite difficult to fight with, but I doubt that's actually true - at least, if there are only two conductors. The 'before twisting' length of the bare conductors would have to be greater (for a given 'after twisting' length) and the actual areas of contact between conductors 'per unit length of contact' would probably be similar - so I think you'd be more likely to have more, rather than less, contact area with twisting.
However, despite that, and despite the fact that I was brought up to do it (several moons ago
), one should not twist. There's a small chance that it can do damage to conductors when it's first done, a possibility that it will reduce contact area with the terminal (or impair tightening), and a serious chance that one will do damage to the conductor if the twist is undone and redone, in the name of testing or maintenance!Kind Regards, John
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B
Big Tone
Thanks John.
As I say, it's not something I know either way and I'd love to put it to a scientific test on whether it increases or decreases the contact resistance between wires and a connection terminal.
The other important factor, as said, is the degradation or integrity of the wires by the very act of twisting them. Also, from my electronics background, I have been known to solder them together.
This gives me the confidence that they cannot displace, while putting them in place or when tightening the screw, but then neither Lead nor Tin are not as good conductors as copper of course.
It's an interesting topic for me. (But I need my bed right now).
As I say, it's not something I know either way and I'd love to put it to a scientific test on whether it increases or decreases the contact resistance between wires and a connection terminal.
The other important factor, as said, is the degradation or integrity of the wires by the very act of twisting them. Also, from my electronics background, I have been known to solder them together.
This gives me the confidence that they cannot displace, while putting them in place or when tightening the screw, but then neither Lead nor Tin are not as good conductors as copper of course.
It's an interesting topic for me. (But I need my bed right now).
Soldering wires that are going to be screwed is seriously bad
Solder is soft and will creep under pressure of the screw over time.
It's a particular problem when appliances came without plugs and people fitted plugs to the tinned ends of flexes.
I know that in the US, it's common to find some aluminum wiring (60's/70's) combined with copper. In the US, connector blocks with screws as used in the UK don't exist, and they use twist-on wire nuts (like in cars or the ceramic ones that used to be used in the UK). Fires often occur where aluminum and copper conductors are connected together alongside eachother in these nuts without twisting or "pigtailing" them first. The aluminium expands/contracts more than the copper, working the bolts loose, but if the wires are "pigtailed" (twisted) together first, then the bolt twisted on, this problem doesn't happen, I'm not sure why. (They also require specialist wire nuts which prevent the aluminum from oxidizing). They also forbit "double tapping" as they call it (screwing two individual conductors into the same terminal) in breaker panels (standard practice in the UK, I'm not sure why that is)
I suspect that it may well require equipment with more resolution than most of us have to detect a difference!
In my youth, I was tempted, too (but I don't think I ever did it with 'mains wiring'). However, as has been pointed out, it's actually a definite no-no, for the reason given.
Kind Regards, John
B
Big Tone
Argh, keeping me up
Solder, as you say, is soft so possibl/probably just fills the gaps between copper wire and terminal. If you tighten the screw properly surely the solder is displaced and the only forces and pressure are between the two?
Gotta say, with respect, I don't honestly know but based on empiricism I have re-visited jobs I have done using solder. They looked okay...
Really gotta Zzzzz now...
Years ago back in college, the tutor explained exactly what you've just said. I can't remember the exact reasoning, but it sounded good!
He also tried to discourage us from twisting the ends of stranded singles before terminating, however nobody bothered as when you tighten it up it just made the strands splay out everywhere giving a pretty crap termination. Although on paper its apparently better not to...
There's an awful lot which doesn't seem right to you isn't there.
Ever stopped to wonder why that might be?
That's what a ferrule is for on fine stranded wire.
Solder, as you say, is soft so possibl/probably just fills the gaps between copper wire and terminal. If you tighten the screw properly surely the solder is displaced and the only forces and pressure are between the two?
Gotta say, with respect, I don't honestly know but based on empiricism I have re-visited jobs I have done using solder. They looked okay...
Really gotta Zzzzz now...
The solder is pretty firmly bonded to the copper.
Get some ferrules.
There is a difference between twisting two wires together and wrapping them round each other.
I feel wrapping the CPCs together before they go into the terminal provides a fall back area of contact should ( when ) the screw in the terminal becomes loose. Thus the CPC continuity is maintained.
Loose screws in Live and/or Neutral tend to be noticed by the heat generated or the loss of power further down the line. A dis-connected CPC may not be discovered until after an event ( possibly a fatal event ) when it is too late.
Provided the wire ends are prepared ( maybe doubled back ) to suit the terminal and thus ensure good pressure onto the copper then wrapping has no real benefiit in the Live and Neutral terminals.
My pet hate is sockets where the terminals for Live Neutral and CPC are all the same yet in the cable the CPC conductor is smaller than the Live and Neutral conductors.
In one make of low cost socket I came across the L and N made good (?) contact under the screw but the smaller CPC could squeeze between the side of the screw and the wall of the circular bore of the terminal resulting in a very poor, zero pressure on the wire, connection. I have seen the same in a consumer unit.
Soldering plain copper wires before putting them into a screw terminal is not good, As has been said the solder under pressure will deform and the pressure on the wires will reduce over time. Solder doesn't bond molecularly with the copper and can be peeled off. There is some molecular bond between the tin and copper in tinned copper wire which makes solder joint on tinned copper wire stronger ( mechanically ) than on plain copper wire.
I feel wrapping the CPCs together before they go into the terminal provides a fall back area of contact should ( when ) the screw in the terminal becomes loose. Thus the CPC continuity is maintained.
Loose screws in Live and/or Neutral tend to be noticed by the heat generated or the loss of power further down the line. A dis-connected CPC may not be discovered until after an event ( possibly a fatal event ) when it is too late.
Provided the wire ends are prepared ( maybe doubled back ) to suit the terminal and thus ensure good pressure onto the copper then wrapping has no real benefiit in the Live and Neutral terminals.
My pet hate is sockets where the terminals for Live Neutral and CPC are all the same yet in the cable the CPC conductor is smaller than the Live and Neutral conductors.
In one make of low cost socket I came across the L and N made good (?) contact under the screw but the smaller CPC could squeeze between the side of the screw and the wall of the circular bore of the terminal resulting in a very poor, zero pressure on the wire, connection. I have seen the same in a consumer unit.
Soldering plain copper wires before putting them into a screw terminal is not good, As has been said the solder under pressure will deform and the pressure on the wires will reduce over time. Solder doesn't bond molecularly with the copper and can be peeled off. There is some molecular bond between the tin and copper in tinned copper wire which makes solder joint on tinned copper wire stronger ( mechanically ) than on plain copper wire.
B
Big Tone
One thing I’ve always noticed is despite twisting or not there is something, some action going on, which over a very long time loosens the screw. It could be that in every case the electrician has not tightened them up enough, but I somehow doubt that.
I concluded that it is the power surge and contact resistance which eventually takes its toll after 1000s of on/off’s. Again, I don’t know and I’ve never had a definitive answer on this phenomenon.
Interestingly though, it’s not something which I have ever noticed in terminals where they have two screws. Maybe one acts as a grip and protects the second from microscopic movements...?
I concluded that it is the power surge and contact resistance which eventually takes its toll after 1000s of on/off’s. Again, I don’t know and I’ve never had a definitive answer on this phenomenon.
Interestingly though, it’s not something which I have ever noticed in terminals where they have two screws. Maybe one acts as a grip and protects the second from microscopic movements...?
Copper under pressure deforms albeit very slowly and over a long period of time it moves away the point of pressure thus the pressure from the screw is reduced. Copper that has been heated and cooled slowly, ( as in a burnt connection ) is very much softer and deforms much quicker.
The two screw terminals may be better as the copper between the screws has no where to go to escape the pressure from the screws and so the contact pressure remains high.
The two screw terminals may be better as the copper between the screws has no where to go to escape the pressure from the screws and so the contact pressure remains high.
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