to torque or not to torque

[jeremysterling]

...Someone needs to design a better lug...

I agree.

Y'all need to get this torque thing settled before the manufacturers do away with mechanical lugs and start specifying crimp lugs and macadapts.:grin:

 

[mivey]

Not true at all for bolted connections ANSI maintenence standards require checking torque with a calibrated torque wrench, the main reason for this is loosening of the bolts from cold flow.

Are you talking about re-torquing and are you saying there are standards that suggest that all bolted electrical connections be periodically re-torqued and will keep us in compliance with NEC 110.3(B)? I think we are required to follow manufacturer's instructions/recommendations.

While it is true that re-tightening is one way to maintain the contact force in an electrical joint that is experiencing stress relaxation and creep, there are other ways, like Belleville washers. Also, thermography might be a more compliant way to identify a loose joint in some applications.

It is often asked whether bolted connections require periodic re-tightening. The simple answer is NO. Once the connector is installed with the proper torque, repeated tightening could actually damage the connector and/or the conductor and eventually lead to a failure.

When terminating a conductor with a set-screw connector, the bare conductor should be wire brushed and an oxide inhibitor applied to the bare conductor. Unless included in the connector manufacturer?s instructions this is considered a best practice and is not a requirement. The screw should be tightened using a calibrated torque wrench to the appropriate torque value as recommended by the connector manufacturer. An over tightened screw?s performance can be as detrimental as an under-tightened screw. Over tightening can lead to damaged conductors and connection points. Proper tightening or torquing is necessary to attain a reliable connection. Once the correct torque is achieved, there is no need to go back and re-torque the lug with an AA-8000 series aluminum alloy conductor.

From an application guide for connections in nuclear power plants prepared by the Nuclear Maintenance Applications Center:

3.3.3 Inspection of Bolted Electrical Connectors
Upon assembly, utility personnel typically check electrical connections by hand or with a wrench to ensure that bolts are not loose. The applied torque is not checked after assembly. Re-torquing the joint after assembly is not an appropriate method to confirm installation torque since repeated tightening could actually damage the connector and eventually lead to failure.

3.3.4 Maintenance of Bolted Electrical Connectors
Bolted connections do not require periodic re-tightening. As described above, once a connector is installed with the proper torque, repeated tightening could actually damage the connector and eventually lead to failure. Hardware from disassembled connections can be reused if determined to be in good condition. However, some nuclear power plants have procedural restrictions on reuse of Belleville washers due to concerns with possible fatigue damage.

Re-torquing - A Screwy Practice: Although ?re-torquing? as a way of checking tightness is common in many PM procedures, it violates basic mechanical principles and does more harm than good. A screw has maximum clamping power at a torque value specific to its size, shape, and composition. Exceeding that torque value permanently reduces the clamping power of that screw by reducing its elasticity and deforming it. Loosening and then re-torquing still reduces elasticity, which still means a loss of clamping power. Doing this to a lock washer results in a permanent 50% loss.

 

[zappy]

Torque specs generally are about 40 to 50% of stripping torque. So if you apply 3 times the proper torque you will strip the threads.

Where do I find this "torque stripper" tool you speak of?

 

[mivey]

Where do I find this "torque stripper" tool you speak of?

Sears, Lowes, Home Depot, Snap-On, Harbor Freight, Northern Tool, ...

 

[Pierre C Belarge]

Reading this thread and listening to the experienced people here has just confused me even more than I was confused before.


I believe that best practice when first terminating is to follow the installation instruction, regardless of the different practices mentioned here.

In regards to maintenance, that is where I am now confused the most. I understand that there are many different types of terminations, and each one may need it's own method. I am not sure now which is the best method, because all of the different explanations seem sound.

 

[zog]

Are you talking about re-torquing and are you saying there are standards that suggest that all bolted electrical connections be periodically re-torqued and will keep us in compliance with NEC 110.3(B)? I think we are required to follow manufacturer's instructions/recommendations.

While it is true that re-tightening is one way to maintain the contact force in an electrical joint that is experiencing stress relaxation and creep, there are other ways, like Belleville washers. Also, thermography might be a more compliant way to identify a loose joint in some applications.

ANSI/NETA specs require 1 of 3 methods to be used for bus connections
1. Check with calibrated torque wrench
2. IR scan under load
3. Check with microhmeter (DLRO)

 

[mivey]

Reading this thread and listening to the experienced people here has just confused me even more than I was confused before.


I believe that best practice when first terminating is to follow the installation instruction, regardless of the different practices mentioned here.

In regards to maintenance, that is where I am now confused the most. I understand that there are many different types of terminations, and each one may need it's own method. I am not sure now which is the best method, because all of the different explanations seem sound.

Re-torquing is more damaging to aluminum connectors & wire. The manufacturers mitigate the loss in contact pressure by using materials with similar expansion characteristics and by using spring washers, etc.

It may be worth noting that there may not be a direct correlation between loss in contact pressure and contact resistance. An aged connector can show less contact resistance than a fresh connection with the same contact pressure.

Anyway, like you said, refer to the manufacturer's recommendations.

Would I re-torque an aluminum connector on a routine basis? Probably not. If it keeps working loose, I would think I need a different solution. I would be less worried about re-tightening a brass connector.

I don't see as big of a problem checking connector tightness at less than install torque, but would not necessarily re-torque to original install specs unless it was an approved method.

Besides, tightening a loose connector may not give you the reduction in contact resistance you expect. Some think it is better to disconnect, clean and reconnect with a clean surface when you find a troublesome connection.

 

[mivey]

ANSI/NETA specs require 1 of 3 methods to be used for bus connections
1. Check with calibrated torque wrench
2. IR scan under load
3. Check with microhmeter (DLRO)

It would seem a lot of manufacturers would not allow method #1.

 

[zog]

It would seem a lot of manufacturers would not allow method #1.

Why not? It is standard practice for bolted connections and recommended by all the OEM's. For example here is GE's specs.

http://www.gepower.com/prod_serv/serv/repair_sol/en/switch_serv/substation/mv_switchgear.htm

 

[mivey]

It is standard practice for bolted connections and recommended by all the OEM's.

No, it is not. See the quotes in post #42.

 

[zog]

No, it is not. See the quotes in post #42.

Those are for screws and conductors, which I agree with and said so earlier, My posts have all been directed at bolted bas bar connections as I thought I had made clear. We are talking apples and oranges.

 

[mivey]

Those are for screws and conductors, which I agree with and said so earlier, My posts have all been directed at bolted bas bar connections as I thought I had made clear. We are talking apples and oranges.

Re-torquing is not recommended by every manufacturer and this is not limited to just conductor terminations. The relaxation & creep problem also applies to terminals bolted to a bus bar.

The method recommended by some manufacturers is to install at the specified torque and to check for loosening or heat before making any adjustments. If the joint needs adjustment, it is better to disassemble the joint, clean, and re-install.

How about a quote from the EPRI & Nuclear Maintenance Applications Center's Bolted Joint Maintenance & Applications Guide:

8.2 Inspection of Electrical Bolted Joints
Inspect bolted joints for evidence of overheating, signs of burning or discoloration, and indications of loose bolts. The bolts should not be re-torqued unless the joint requires service or the bolts are clearly loose. Verifying the torque is not recommended. The torque required to turn the fastener in the tightening direction (restart torque) is not a good indicator of the preload once the fastener is in service. Due to relaxation of the parts of the joint, the final loads are likely to be lower than the installed loads. However, this load reduction has little effect on electrical conductivity or joint performance. Check the joint resistance of bolted joints using a low range ohm meter.

When they are talking about the relaxation and final load, they are telling you that the relaxation and tightening curve have different contact resistance paths. The joint can relax to a lower contact force but still maintain relatively the same contact resistance. You might accomplish nothing more than connector damage by re-torquing to the original value, not to mention the issues with restart torque.

 

[Pierre C Belarge]

This is from the GE link:
"
Bolted pressure connections will be checked for proper torque (tightness) where possible
- Bolted connections, where accessible, will be checked for proper torque as per applicable standards.

Ground bus connections and supports will be checked for proper tightness"

What are the applicable standards.
I think, the applicable standards are different for the type of termination. At least this is what I am starting to think, since reading this tread.



Whoever said this industry was going to be easy? But...I am glad it is not. I would be bored and looking for something else to do.

 

[zog]

Re-torquing is not recommended by every manufacturer and this is not limited to just conductor terminations. The relaxation & creep problem also applies to terminals bolted to a bus bar.

The method recommended by some manufacturers is to install at the specified torque and to check for loosening or heat before making any adjustments. If the joint needs adjustment, it is better to disassemble the joint, clean, and re-install.

How about a quote from the EPRI & Nuclear Maintenance Applications Center's Bolted Joint Maintenance & Applications Guide:
When they are talking about the relaxation and final load, they are telling you that the relaxation and tightening curve have different contact resistance paths. The joint can relax to a lower contact force but still maintain relatively the same contact resistance. You might accomplish nothing more than connector damage by re-torquing to the original value, not to mention the issues with restart torque.

Arggg, EPRI is saying to check the connections with a DLRO, as I mentioned as one fo the 3 options. How many EC's have DLRO's? Not many, in which case either a torque spec should be checked with a calibrated device (Per the OEM instructions I mentioned) or an IR scan (By a cetified operator who knows what they are looking at) should be completed.

EPRI is a very anal group and while they are usually technically correct they are not always practical, I deal with thier standards every day. How many people on this site have to comply with EPRI's nuclear procedures besides me? I am guessing not very many.

 

[mivey]

How many people on this site have to comply with EPRI's nuclear procedures besides me? I am guessing not very many.

No argument there.

But I'm sure there are many that use Burndy products and that is their perception as well.

 

[Kdog76]

No, but you can cut 8' off of Wheatland's 20-foot EMT.

Is it deja vu, or have you mentioned that full stick of 20' wheatland before? Just thought i'd heard it once before here on the forum.

 

[bradleyelectric]

Is it deja vu, or have you mentioned that full stick of 20' wheatland before? Just thought i'd heard it once before here on the forum.

It's deja vu all over again.

 

[mlnk]

I like the idea of cutting the wire and doing a new connection. If it is loose it probably also was arcing and damaging the conductivity. ...Writing the torque value and date on the panel is also a great idea. How do you follow the Manuf. specs on old equipment? It can be a nightmare to find out about even about new equipment. "Follow the manuf specs" means nothing when you do not have the specs in front of you.

 

[One-eyed Jack]

After a 1 hour long discussion/debate at an IAEI siminar on the importance of "checking" for proper torque of terminal connections the UL rep had this to say when pinned down. If you don't witness it you can not verify after the fact. You will overcompress.