Infromation Needed About Bird Caging in ACSR Conductors

Please see today's earlier thread on this same topic. There is some good information there.

I don't have direct experience with birdcaging along spans of aerial cable, but it would appear that the tension on the outer (aluminum) strands was not kept the same as the center steel strands. This could result from improper tensioning when installed, improper handling while unrolling & stringing the cable, improper termination of the cables allowing the outer strands to slip, unusual stress on the cables (severe cross-winds or heavy gusts), or possibly manufacturing defect. For example, if the cable was not strung using proper shieves and then tensioned appropriately, the cable stranding could have been affected before it was finally terminated.

If there were heavy, gusty winds, it could cause the cable to stretch and spring back, and the different strand materials would react differently. See the following article for some info on this. It refers to wire rope, but the principle they mention of what causes birdcaging is the same:

http://www.rigging.net/wire.html

That's an excellent site Peter and a GA.

Reminded me of an incident when as the electrician on a multi-discipline team I was the crane driver for the mechanical bods. I had just started hoisting a 38T load as a "birdcage" pasted the drivers cab, I soon put the load down!

Besides issues related to installation high temperature can cause bird caging.

ACSR is composed of an outer core of aluminum strands and inner core steel strands. Bird Caging can be casued by aluminum compression in ACSR when the conductor temperature exceeds 194F or 90C. The aluminum has twice the thermal expansion rate of the steel inner core. The aluminum starts to lose strength and transfer its load to the steel inner core and can go into compression and push on the steel core causing bird caging in the aluminum strands.