checking wire termination torque

[resqcapt19]

I have been getting conflicting information on checking or re-torqueing a wire termination. I have been taught that this should never be done due to the cold flow or relaxation that occurs over time. This relaxation has been accounted for in the original torque specification. If you check the torque by re-torqueing to the original value you will make the connection to tight and when the conductor heats up on high loads the pressure due to wire expansion will cause additional cold flow that may result in high contact resistance when the wire cools and had a low load.
Others have told me that the wire terminations should be re-torued to the original value at least once a year.
Anyone have any comments?
Don(resqcapt19)

 

[wbd]

What kind of terminations are you refering to? Screw type on terminal boards or bolted type connections. I have some info back at the office on torques but won't be back til Monday. I'll give a detailed answer then unless the ? is answered satisfactorly for you by then.

wbd

 

[resqcapt19]

The connections I'm talking about are the mechanical wire termination's where you tighten a screw against the conductor. I don't think that the same type of cold flow problem occurs on bus bar to bus bar bolted connections.
Don(resqcapt19)

 

[troyster]

Don,
It has been my experience over the years that in the case of aluminum conductors that this is done as a part of a maintenance program when the gear is shutdown such as a yearly turn around, however usually only on larger cable size ie; 4/0 and larger. However in the case of copper if the original installation was torqued properly the re-torquing when called for usually shows no "loosening" of the conductors.
Troy (troyster)

 

[resqcapt19]

Troy,
I know that it has been commonly done, but I've been told it shouldn't be done and actually makes the connection worse then it was before it was re-torqued. I'm trying to find out it what I've been taught is actually correct. If what I've been taught is correct, then there are a lot of PM programs that are making problems worse rather than better.
Don(resqcapt19)

 

[jbartos]

Suggestion: The higher ambient and/or conductor temperature differential the bigger chances for more frequent re-torquing. If you happen to have a stabilized room temperature and the conductor Tc temperature does not fluctuate due to the load current, then the re-torqueing is less likely needed.

 

[resqcapt19]

jbartos,
My point is that the original torque specs account for the relaxation due to ambient or load induced temperature changes. If the connection is re-torqued, it becomes too tight and there will be more cold flow from the temperature changes resulting in a connection with too little contact pressure to carry the load without excessive heating. A connection should never be re-torqued, unless there is evidence of a problem. If there is evidence of a problem the conductor must be cut off and the new end inserted into the terminal and then tightened to the original torque spec.

 

[jbartos]

Suggestion: If you need a constant force, and to eliminate or downplay the re-torqueing, then a spring washer or similar device shall be applied to counteract the thermal forces and dilatation or contraction impact on the electrical contact quality. Please, notice that any better quality electrical equipment has these provisions, e.g. terminal boards.

 

[resqcapt19]

jbartos,
Those types of devices are used on terminal to bus and bus to bus connections, but what about mechanical wire terminations where the set screw directly contacts the conductor?

Don(resqcapt19)