re-torquing? 2

[alehman]

What does anybody think about periodic re-torquing maintenance programs for bus joint and connector bolts in switchgear? Is it better or adequate to do periodic IR imaging and only retorque if a problem is suspected? I've heard arguments both ways.

When re-torquing should the bolts be completely loosened first, or just checked with a torque wrench?

Finally, is there a standard which specifies bolt torque for bus connections?

 

[DanDel]

Periodic re-torqueing or tightening probably does more harm than good. There are too many variables for each connection for torqueing to be applied consistently and accurately. And when was the last time you calibrated your torque wrench? In any case, you can't apply a torque wrench on a connection unless it starts loose.
IR is a good option, as long as it is performed frequently enough and when there is adequate current in each connection to show up a problem.
The best option is to use a DLRO(Digital Low Resistance Ohmeter) on each connection to determine its actual resistance. This instrument uses a constant current(usually 10ADC) and Kelvin-bridge-type leads and will show resistance in the micro-ohm range.

 

[busbar]

A number or NETA maintenance procedures state:

Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer's published data or Table 10.12.

 

[5P10]

alehman,
After reading dandel's comment, I am not so sure anymore whether re-torquing is good or not. But at present, we re-torque our busbars and cables every 2 years. No loosening was done, just re-verifying with a torque wrench. And in the process, we do find a few bolts that we slightly loose. Something to note, a couple of years back, when we were re-torquing a 132kV GIS earth bar, we broke a bolt and had a hard time replacing it. So, when you torque, make sure you have a torque wrench that is in excellent condition.

Regards,

 

[alehman]

Thanks for the replies. I'm involved in a discussion between a facility owner (my client) and a contractor who is recommending that torque on all switchgear bus bolts be checked on an annual basis. I agree with Dandel's point that checking torque without first loosening seems to have limited value. I suppose if the connection is very loose this might correct it. Otherwise if you apply enough torque to rotate the nut it will be over-tightened and may damage the bolt. If you loosen each connection, it seems like there's just as much chance of forgetting to re-tighten as during the initial install, perhaps more because the work will be conducted during a maintenance interval where time may be limited.

This contractor also believes the manufacturer's recommended torque values, which happen to match NETA, are far too low. Usually NETA standards are derived from other sources such as ANSI, but I can find no other electrical standards relating to bolt torque.

 

[busbar]

Aside — NETA numbers seem very close to other threaded-fastener torque tables for non-electrical use, so they are likely default/generic values from ASTM or fastener-manufacturer boilerplate.