The NESC recommends limits on the tension of bare overhead
conductors as a percentage of the conductor’s rated breaking strength. The
tension limits are: 60% under maximum ice and wind load, 33.3% initial unloaded
(when installed) at 60°F, and 25% final unloaded (after maximum loading has
occurred) at 60°F.
It is common, however, for lower unloaded tension limits to
be used. Except in areas experiencing severe ice loading, it is not unusual to
find tension limits of 60% maximum, 25% unloaded initial, and 15% unloaded
final.
This set of specifications could easily result in an actual
maximum tension on the order of only 35 to 40%, an initial tension of 20% and a
final unloaded tension level of 15%. In this case, the 15% tension limit is
said to govern.
Transmission-line conductors are normally not covered with
ice, and winds on the conductor are usually much lower than those used in
maximum load calculations. Under such everyday conditions, tension limits are
specified to limit aeolian vibration to safe levels.
Even with everyday lower tension levels of 15 to 20%, it is
assumed that vibration control devices will be used in those sections of the
line that are subject to severe vibration. Aeolian vibration levels, and thus
appropriate unloaded tension limits, vary with the type of conductor, the
terrain, span length, and the use of dampers.
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