Torquing of lugs

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While I believe that under torquing results in a loose connection that is subject to overheating. I do not believe that over-tightening can result in the same problem. I would think some engineer found a cross threaded screw once that was over-heated when it was actually loose and incorrectly hypothesized this whole line of thought or maybe the overtightened lug was actually broken. But can anyone actually think a super-tight connection can over heat because the connection isn't loose enough? Any evidence from you guys that I'm wrong? Please, tell me, because I just don't see it.
 

Twoskinsoneman

Senior Member
Location
West Virginia, USA NEC: 2020
Occupation
Facility Senior Electrician
One of the big problems in over tightening lugs is stripping the lug threads. I'll never forget the first time my crew leader stripped a lug on a large breaker and tried to pretend it didn't happen. Shmuck.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
One thing that can happen with over torquing is the set screw can eat into the wires (stranded) actually cutting them which can then lead to over heating if they tend to arc at the cut areas. Notice I said can and not definitely will.
 

masterinbama

Senior Member
Different scenario but still applies to torquing. Over torquing bus bar connections will cause them to warp and they will actually lose contact area. I have seen the results of this and it's not pretty.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The correct torque takes into account the temperature expansion and contraction of the conductor. With a connection that is providing too much clamping force (over torqued) when the conductor heats up it "extrudes" some of the metal out of the area where the clamping force has been applied. This results in a higher resistance connection over time and possible connection failure. Re-torquing a mechanical termination also causes this same problem.
 

renosteinke

Senior Member
Location
NE Arkansas
Have any C-clamps?

Take a C-clamp, and tighten it down as much as you can on something solid, like a big chunk of steel. As you tightem the C-clamp, you'll feel a range of perhaps a turn or two where the handle gets progressively harder to turn. This is because the clamp frame is acting like a spring, and you'll feel the tension release as the handle is loosened.

Now ... using the same clamp ... get a pie wrench on it and tighten is some more, enough that you can see the frame distort and the screw bend. Ready? When you go to loosen the clamp, it will let free of its' grip as soon as you start to turn the handle. There's none of that 'spring' action.

That's what happens when you over-torque things. Aluminum is even worse in this regard than the iron of the C-clamp. In technical terms, the 'yield' strength isn't much different from the 'ultimate' strength. With little 'yield,' there's not much spring action- and a much greater chance of the aluminum lug body tearing or threads stripping.

That's the 'academic' view. Personally, IMO, this whole torque issue is way over-blown. Considering the ease with which aluminum screw heads are damaged, I doubt over-torqueing is much of an issue.
 

RICK NAPIER

Senior Member
Location
New Jersey
I have yet to receive a answer to this question. Which torque measurement do you use? The torque reading while your turning the screw or the static reading of a still screw? The readings can vary.
 
I have yet to receive a answer to this question. Which torque measurement do you use? The torque reading while your turning the screw or the static reading of a still screw? The readings can vary.

Torque is a measure of turning force. If the object isn't turning, you can't measure torque.You may have indicated force, but you don't have torque. (Like power, if no current flows, you only have voltage.) When the screw/bolt isn't turning, you're measuring the sticktion (static friction) between the device and whatever's holding it. When that friction brakes and the device starts to turn, then your measuring a different quantity because movement is involved.

This also means that if you're using a clicker torque wrench, put it on a bolt, pull, and it immediately clicks, you don't know what the bolt was torqued to. Only that the sticktion was greater than the wrench setting. (This was discussed here maybe a year ago.).

The wikipedia article on torque is pretty good.
 
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Chamuit

Grumpy Old Man
Location
Texas
Occupation
Electrician
Stiction is the amount of force required to get an object to move against another from a stationary position. I thought it had to do with molecules.
 

gndrod

Senior Member
Location
Ca and Wa
More Torque Info

More Torque Info

Stiction is the amount of force required to get an object to move against another from a stationary position. I thought it had to do with molecules.

Stiction = Locktite when used for high vibration prone equipment. Most lug torque force value ranges use the NEMA-CC1-1975 Specifications or better. For small screw torque values (i.e. 4-40 to 1/4-20) the differences between brass, steel and stainless are well documented in the SAE Manufacturers specs.

Screw torque Lb-in values for breakers vary between manufacturers. (i.e. breaker torque rating for a 15 Amp SQ D is 36 Lb-in, where GE and C-H have a 20 Lb-in listing.) Most torque values for breakers are generally 60-70% of the factory design limit. (Data source: Electrical Terminator Dwelling Wiring Construction)
 

brian john

Senior Member
Location
Leesburg, VA
Over torquing stresses the hardware, damages conductors and can strip hardware resulting in a loose connection.

Have you ever seen the videos of the stresses placed on buss during a fault. You want to be the guy responsible for the fault escalating due to damaged hardware?

Had a job where we load tested UPS batteries the electrician over torqued, when we hit the battery with a load the heads popped off several bolts.
 
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