Busbar Mechanical Fasteners

[TBDaniel]

Is a there an IEEE code that regulates the materials used for mechanical fasteners connecting a copper busbar? At this newly constructed power station we have experienced a short between two phases of the 4160 V busbar, which apparently initiated at a loose bolted connection on one phase. The copper busbar is connected with what appears to be stainless steel bolts and nuts. There is installed a flat washer on the bolt head end and a locking washer on the nut end. Upon inspection, there were numerous bolted connections found loose, and numerous seized bolts. Could there be a problem with a galvanic couple that removes metal from the washer and thereby loosens the joint? Is there any torque criteria for these mechanical fasteners?

 

[peebee]

Usually the manufacturer provides torquing data. But if not, possibly UL 486? I'm not personally familiar with 486 so I can't say for sure.

How old is the switchgear? Has it been periodically maintained, including periodic torquing of all joints? If not, have you at least done periodic thermal scans to identify loose joints? You should be doing one or the other.

I doubt galvanic action is the culprit. Copper and stainless should be pretty happy together. More likely is thermal cycling.

 

[rbulsara]

See my thread238-69135 for a similar question.

Normal practice is to use Grade 5 steel bolts with a flat washer on the hex side and a flat washer and locking washer and nut on the nut side.

As mentioned in other responses regular thermoscannig or retorquing(at least once a year) is a good way to track and find out any loose connections before they become a real problem. NETA/ATS also lists the torquing values. Also switchgear manufacturers publish the data. You may also search the net.

 

[busbar]

Limited supporting information in NETA ATS §7.1.6

Specification info at

http://www.fciconnect.com//products/technical_01_01.asp

 

[RAMConsult]

Avoid the retorquing problem by using Belleville washers in the following order: bolt, belleville, flat washer, busbar, flat washer, belleville, nut. The belleville washers have well defined compression/torquing values.

 

[rbulsara]

I beleive, for some reason belleville washers are not recommended for copper buses. Only good for aluminum. (or is it reverse?) Look up T&B catalog. (If get time in my office I will get some quote on this).

They have their issues.

 

[busbar]

FWIW, Key Bellevilles sells Phosphor-Bronze / Beryllium-Copper disc springs.

 

[alehman]

I think the problem with the poor connections was poor torquing (to little or too much) during assembly. That many connections don't just loosen up by themselves unless there is some extreme environmental condition. There should be a flat washer between the lockwasher and the bus.

Normally bellevilles are considered to be required only for connections involving aluminum conductors unless vibration is a concern. Many engineers specify them for copper also, but I don't believe it necessary unless vibration is a concern. Bellevilles are normally installed under the head of the bolt. The most common bolt material is zinc-plated grade 5 steel.

See thread238-61717 for discussion of bolt torque and re-torquing. There seems to be no consensus.

 

[jbartos]

Suggestion: The busbar installations should be checked for stray currents and a need for cathodic protection.

 

[jbartos]

Suggestion: Visit

http://www.moeller.co.uk/documents/busbar_brochure.pdf

for low voltage busbar details
and References:
1. ANSI/UL 857 Safety Standard for Busways and Associated Fittings
2. NEMA BU-1 Busways
3. NEMA BU-1.1 General Instruction for Proper Handling, Installation, Operation, and Maintenance of Busway Rated 600Volts or Less
4. ANSI/IEEE Std C37.23 IEEE Standard for Metal-Enclosed Bus and Calculating Losses in Isolated-Phase Bus
5. Etc.