NEC "Tug Test"?

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Every one is saying the inspector is wrong, I disagree if heavy cables can be pulled out they are loose. I teach my apprentices to pull the wire back and forth then retorque.

Even torquing relaxing and torquing 3 times how I was taught 40 years ago with my “arm strong” torque is not enough (notice each time the screw trend further) heavy gauge wires will burn off as they relax the pulling on the wires gets them to relax under the clamp so they are no longer tight.

As for the flex I would disagree with pulling on that but for the heavy gauge stranded as all heavy gauge is stranded the round conductors can stack with vibration they become loose and burn off causing all kinds of problems.
 
sgtsparky said:
Every one is saying the inspector is wrong, I disagree if heavy cables can be pulled out they are loose. I teach my apprentices to pull the wire back and forth then retorque.

Even torquing relaxing and torquing 3 times how I was taught 40 years ago with my “arm strong” torque is not enough (notice each time the screw trend further) heavy gauge wires will burn off as they relax the pulling on the wires gets them to relax under the clamp so they are no longer tight.

As for the flex I would disagree with pulling on that but for the heavy gauge stranded as all heavy gauge is stranded the round conductors can stack with vibration they become loose and burn off causing all kinds of problems.
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First, there's no recognized procedure for conducting a "tug" test. Second, it's clear from the description that the inspector is applying force until he generates a failure.

Objection overruled.
 
qcroanoke said:
If it was I bet I can tell you his name. That inspector is something.....
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Yeah . . . I am wondering if there are some underlying mental health issues.

Have not met him yet, but I have 3 projects that have "problems" coming up to resolve for different contractors -- and this one is the common thread.
 
In the future I would video all of his inspections.
 
AC\DC said:
I have seen this and always wondered though never looked further.
Does that mean a “250in pound torque” is initially for the compression of the material, and they account for slacking so the final connection is not really “250 in pounds”
Plus if your using copper the compression going to be much different.

Seems unless they have a different value for copper along the the range of final torque value the material will end up at, torque is garbage!!
Unless like you said use crimp on. No one’s going to do that though for a small resi service though.
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What we really want is the compression force on the joint. Most of the “torque” we apply is just overcoming friction in the threads. Only 10-15% of the torque is applied to the joint. Plus torque wrenches are not as accurate as believed, And we can get into arguments about lubrication, surface roughness, corrosion, and so on but the fact is that tension accuracy just isn’t very good.

In electrical contacts the metal is not totally flat and it is covered in oxide. Squishing the joint cracks the oxide layer at the high spots forcing metal on metal contact called alpha spots. Further force smears the aloha spots so they grow in size as well as forming new ones but it’s a “diminishing returns” issue. The aloha spots are literally cold welds. So as force is removed we don’t uniformly lose contact…there is hysteresis. Less than 10% of the area if contact with bus bars actually makes contact.

One thing that this makes clear though is that if you aren’t mushing the copper so hard that it is highly distorted out of shape it’s not tight enough. Many times it bird nests so bad in the ends you have to cut it off and strip a fresh area to land it again.

Mechanically there is something called “dislocation theory” that shows that even though the metal crystals are solid at room temperature they will naturally relax and stress relieve themselves, permanentjy stretching the fastener. Just check for YouTube videos. The surface can also smear a little, allowing some loosening and what is called “fretting”.

Sure metallurgy plays a part, too. The stress/strain relations are different depending on the alloys but really we don’t care what is in the joint. We are torquing and stretching the bolt/screw/box. This puts tension in the electrical contact. The torque is on the steel or aluminum threads, not the copper. That is what the steel washers are for…keep the steel from digging in.

Fortunately this is all known. Electrical joints as far as I can tell are seriously over-rated and we know that because failures are not very often.

There is a web site, boktscience.com, that goes over this in depth. Also the NASA fastener technical manual is quite good. Among other things it explains why you will find “helical spring washers” in a Grainger catalog but not “lock washers” (there is no such thing).
 
I can't believe not one person has commented that the inspector should go tug on himself instead of those conductors. LMAO Maybe it is just me with a warped mind.

Any "tug" by a human is subjective and therefore to me completely invalid. We do "pull tests" here at work on crimped terminals to validate our crimp tools and dies. It requires a crimped terminal to be placed into a machine and the wire to be connected to a head. The head will pull straight and constant until the crimp fails or the wire breaks. Force is recorded and there are min/max for pass/fail. This just grab it and jerk on it is nothing shy of ridiculous to me. Seems to me that re-torquing in front of the inspector and putting torque marks on it would be more valid.
 
LarryFine said:
The clamping pressure actually comes from fastener stretch, which is approximately accomplished by tightening until the specified torque is achieved. When you see the plates with all the nuts'n'bolts that connect girders, the strength of the joint comes from the clamping pressure, not the bolts' shear strength.

I would imagine the relaxing that Paul mentioned in post #16 is taken into account when torques are specified for mechanical connections. Otherwise, we would be instructed to tighten in steps, the way we are when torquing cylinder head bolts when assembling engines (which I know also minimizes warping.).
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Stepping torque is for alignment reasons. If you don’t do it in steps you can cause two surfaces to wedge instead of being even. That’s why you alternate torquing almost everything, On cylinders you can damage a gasket. It will always leak after that, You can also crack or damage bushings, especially Taperloc and B-Loc brands.

See this on torque inspections and experience:

iaeimagazine.org

The Difference between Success and Failure — How a Torque Wrench Improves System Reliability

In NASCAR, pit crew members use impact wrenches with preset torque values to install lug nuts during a wheel change. A loose lug nut can mean the difference between winning and losing, as Greg Biffle found out in 2005 when loose lug nuts on the left rear tire caused vibration and forced him to...
iaeimagazine.org iaeimagazine.org
 
ptonsparky said:
I did a tug test today. Obvious overheating on a starter lug. Couldn't loosen it enough to get fresh conductor and barely tighten enough to make it work for awhile. Maybe. Sure could use those spare parts I got rid of 3 months ago.
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Once the lugs are so hit they are discolored and the wire and lug are fused, the lug needs replaced.


Sent from my iPhone using Tapatalk
 
Most jurisdictions give electrical inspectors full exemption from liability law suits, so the idea of suing the individual is out the window. At least in my state that is how it is. Not allowed to sue inspectors for liability. You probably can sue them for defamation or civil rights issues though if they violate those rights.
 
Phil Timmons said:
This is so bizarre. Had a solar crew member tell me that we had an inspector yanking (grab and pull) on feeder wires on a job site last week. And these were LIVE conductors. Is this Covid Brain Damage or what?

My hands GO-IN-MY-POCKETS on someone else's job site -- even if I am doing commissioning or inspection.

cbooker -- was this in Irving?
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Mckinney
 
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