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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