I was always taught that the tighter you make a termination the better.

I have split neutral bars like zippers from tightening. I have stripped out 600KCMIL split bolts from tightening with 2- 24" pipe wrenches.

I have always done this in the belief that I was doing a good job.

But now I use a torque wrench and it always seems loose to me.

I thought the resistance falls as the connections get tighter.

And if not, what happens when I use a hydraulic crimper, that is certainly very tight.

Can anyone explain what happens when you over torque?


[This message has been edited by iwire (edited 01-26-2003).]

over torquing can be just as bad as under torquing. When you over torque you can mash and severe conductors, thus creating a poor connection and possibly a high resistance fault or arching. That is why there are torque specs.

Iwire, threaded fasteners have a natural elasticity that helps them stay tight. Overtightening stretches the fasteners past their elastic limits. Vibration and thermal expansion/contraction will make the conection loose because the fastener can no longer return to it's original length. Think of a spring. If you stretch it longer than designed, it will distort and no longer be stretchy.

stamcon,
I know exactly what you are talking about with the elasticity of fasteners.

On a nut and bolt the best way to determine correct preload is by measuring stretch you will see Drag racers do this on rod bolts.

I should have been more specific in my question.

When I land a conductor on a breaker lets say 200 amp frame size and 4/0 CU the torque spec seems low, it is much less then the torque you would use for a mechanical connection.

And much less torque then it would take to sever or mash the conductors.

I am not saying that I think its wrong, I guess what I want to know is how that torque is determined by the maker

Steel has two strenght ratings.
Yield strength which is where one can stress the metal up to that point and it will "bounce back". Over the yield point the metal is never the same.
Ultimate strength is where the designer will design within these parameters and the steel will have infinite life. It is always under yield strength.

Aluminum on the other hand has no ultimate life. From the day that it is manufactured and exposed to stress, it will fail at some point in time depending on the amount of stresses applied to it. That is why aluminum bicycle frames eventually crack and have to be scrapped.

Copper, depending on what alloy and hardness, has an Ultimate Strength and a Yield Strength. Manufacturers design the torque settings so they remain in the ultimate strength range.

For a description of both let's use an antenna on a car.

Ultimate strength range is when you bend it and it returns to its original position with no deformation in shape and position.

Yield strength range is exceeded when you bend it and it develops a "kink" and you cannot ever bring it back to its original position and also have the same strength at that point in the metal. On a molecular level, the lattice in the metal has been distorted.

Copper is mallable and depending on the alloy used can have an impact on the life of the connection when heat, mechanical and torsional stresses are applied.

Hopes this makes sense to all who wondered about this topic.

[This message has been edited by Currently (edited 01-26-2003).]

Not relating to electric but relating to torque, for years it has always bugged me that mechanics will tighten lug nuts with an impact wrench without having any idea how much torque they are applying. Once the bolts have been overtorqued they are stretched or distorted and it has always been my belief that properly torqueing them after they have been stretched is now useless.

A given is overtorque yielding to deformation, splitting or thread stripping can occur with any metal. A characteristic of termination quality that used to discussed with the earlier "EC1350" aluminum alloy was coldflow. Apparently the modern "AA8000" alloy is compounded with a small amount of iron to reduce expansion and contraction in thermal cycling. Copper is not as likely to have coldflow problems.

With reasonable care, nonreversible hydraulic-compressed splices and terminals—copper or aluminum, are far and away the most reliable, but finding compatible equipment may take more time and planning. They can take up a little more space, but aluminum H-taps are inexpensive and close to bulletproof for Al-Al cable splices and Al-Cu transitions. They are suitable for a wide range of cable sizes with relatively few catalog numbers.