verifying torque value NEC 70B ?

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hbiss said:
set your wrench on the high side of the torque specSo, really there is no way to verify to an inspector that the fastener or lug has been torqued to specification.

-Hal
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Really if you set your wrench to the high side of the spec, and some time goes by, the internal stresses in the fastener and related hardware that are caused by the initial torquing are going to relax some. So you will get some rotation before the click. This is especially true if it goes through some thermal cycles. Hence the importance of retorquing
 
Joethemechanic said:
...are going to relax some...
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Isn't that usually accounted for in the initial spec? If you get some rotation before the click, it probably either wasn't done right to start or the metal has stretched past the deformation point (not good). And to "retorque" you need to loosen first to get the rotation.

We're not talking about tower cranes or aviation with stringent specs and procedures- lots of electrical equipment works until it's demo'd with whatever tightening was done when it was first installed.
 
zbang said:
We're not talking about tower cranes or aviation with stringent specs and procedures- lots of electrical equipment works until it's demo'd with whatever tightening was done when it was first installed.
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Yeah I know, but since we are getting super technical lol. Actually I really never saw an electrician touch a torque wrench until recently.

And yeah a lot is accounted for in the initial spec, or at least it should be
 
zbang said:
Isn't that usually accounted for in the initial spec? If you get some rotation before the click, it probably either wasn't done right to start or the metal has stretched past the deformation point (not good). And to "retorque" you need to loosen first to get the rotation.

We're not talking about tower cranes or aviation with stringent specs and procedures- lots of electrical equipment works until it's demo'd with whatever tightening was done when it was first installed.
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It is accounted for and the cold flow of the material, after some time, will always permit additional rotation at the original torque spec. If you want to re-torque any connection where the threaded device puts direct pressure on the conductor, you need to cut the end off the conductor and start over.
 
don_resqcapt19 said:
you need to cut the end off the conductor and start over.
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I do monthly scans with a thermal camera of large panels. Sometimes I find terminations that are a little warmer then the other phases on the circuit. I tighten them and check after the temperature stabilizes and most times the problem goes away.

Are you saying this is incorrect practice?
 
Joethemechanic said:
I do monthly scans with a thermal camera of large panels. Sometimes I find terminations that are a little warmer then the other phases on the circuit. I tighten them and check after the temperature stabilizes and most times the problem goes away.

Are you saying this is incorrect practice?
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I am.
 
I remember this question coming up and found an answer in 70B. Sorry I don't have a copy to give the article number. Maybe someone else does. What I remember is you set the torque wrench to 90% of spec to see if the connection is "OK". If not, tighten to full torque spec. What brought the question up was customers with robo-work-orders that specified to check?/tighten? all connections. OK... just how should you do so? Answer - NFPA 70B!
 
TwoBlocked said:
I remember this question coming up and found an answer in 70B. Sorry I don't have a copy to give the article number. Maybe someone else does. What I remember is you set the torque wrench to 90% of spec to see if the connection is "OK". If not, tighten to full torque spec. What brought the question up was customers with robo-work-orders that specified to check?/tighten? all connections. OK... just how should you do so? Answer - NFPA 70B!
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see post #3
 
don_resqcapt19 said:
I am.
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Do you have something I can use for a reference? I have a hard time justifying downtime. Interrupting a process for a minute or two is usually no big deal. Ten minutes to remove, cut, strip and retighten means product is going to back up somewhere
 
In the Navy nuclear program use of torque wrenches on the reactor system was required. Wrenches had to be calibrated every 90 days and torque value was between 20 and 90 % of torque range. All fasteners were lubricated. And there was a QC person to witness the torque. The NEC requirements are simple by comparsion
 
tom baker said:
In the Navy nuclear program use of torque wrenches on the reactor system was required. Wrenches had to be calibrated every 90 days and torque value was between 20 and 90 % of torque range. All fasteners were lubricated. And there was a QC person to witness the torque. The NEC requirements are simple by comparsion
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There were presses at The Budd Company in Philly that they had hydraulic cylinders to tension the studs holding the top of the press on. The cylinders went over the studs first, and then ran the nuts down. Before you started the press you built up hydraulic pressure to put the studs into the correct tension.

There are a lot of better ways to control fastener preload than torque. Caterpillar used initial low torque and then degrees of rotation. Waukesha used a dial indicator to measure elongation.

IMO torque isn't a great way, but it's quick and easy
 
All fasteners were lubricated.
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This is really important. A dry sticky thread really cant be tightened correctly which is totally different than the torque required to turn a bolt.
I saw a test from a trade show among apprentice types. 25 guys, 1 got it right. Guys thought 7# was 25 and several were at 12. It used to be that engine work was routine, cars often needed valve grinds at 50K, all those parts were torqued and guys learned and got a good feel. Lots of kids did lots of engine work, 6 graders fixed mowers and now a kid hasnt used a screwdriver till the day he sign up for apprenticeship.
About the only time we use the wrench is engine work, something with a real spec and even those head bolts for small ones simply do by feel.
Wondered wtf the other day, someone read spec wrong and found it by feel in a double check.
 
Joethemechanic said:
There were presses at The Budd Company in Philly that they had hydraulic cylinders to tension the studs holding the top of the press on. The cylinders went over the studs first, and then ran the nuts down. Before you started the press you built up hydraulic pressure to put the studs into the correct tension.

There are a lot of better ways to control fastener preload than torque. Caterpillar used initial low torque and then degrees of rotation. Waukesha used a dial indicator to measure elongation.

IMO torque isn't a great way, but it's quick and easy
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Bolt elongation is the correct way. We went over fastner torque in Nuclear power school. The tensile strength with diameter is used to calculate bolt stretch which determines clamping force
 
And now for confusion, it’s newton meters for torque.
It is important to torque lugs, get a torque wrench and use it. Klein makes a 3/8 and 1/2 drive version
 
tom baker said:
Bolt elongation is the correct way. We went over fastner torque in Nuclear power school. The tensile strength with diameter is used to calculate bolt stretch which determines clamping force
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Yeah and then you have to take preload into consideration and add that to the load you expect the fastener to carry. But then again if your bolts are in shear, well it gets even more complicated
 
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