Page 21 of 23 Guide for Installation and use of Southwire C
7
Overhead Conductor
Sagging Methods
Conductor sagging involves the use of stringing tables to determine the required sag or tension
at a specific conductor temperature. Conductor temperature should be measured on the day of
sagging using a conductor thermometer placed at or near the conductor. The conductor
thermometer should be installed prior to sagging to allow for temperature stabilization.
Conductor is often tensioned to the correct sag/tension using one of the following three
methods: the stopwatch method, transit method, or dynamometer method.
Stopwatch Method
The stopwatch method involves pricking the conductor and measuring the time it takes the
vibration wave to be reflected back a certain number of times. Three or five return waves
usually provide an accurate measurement of sag. The governing equation for the stopwatch
method can be seen below.
= 122.7
2
D = conductor sag, cm
t = time, sec.
n = number of return waves
Transit Method
The transit method includes three types of sagging methods: calculated angle of sight,
calculated target, and horizontal line of sight. Choice of the best transit sagging method to use
is determined by the terrain of the span in the right-of-way and span length. Tall structures on
flat terrain and short spans indicate the calculated target or horizontal line of sight method
would be most applicable. Steep slopes, long spans, and large sags indicate the calculated angle
of sight method would be best.
Dynamometer Method
In the dynamometer method, a dynamometer is inserted in-line with the sagging equipment to
get a direct measurement of line tension in the line. There should be minimal sheaves between
the dynamometer and span being measured. This method works best on smaller conductors,
shorter spans, and ruling spans containing one or two spans.
Vibration Dampers
C
7
, due to its light weight, is more prone to vibration issues than steel core conductors under
the same tension. Therefore, in most applications, dampers are necessary to protect the
conductor from fatigue damage from Aeolian vibration. Recommendations on type and
placement of dampers on C
7
lines should be made by the damper manufacturer. The
mechanical impedance characteristics of C
7
are different from conventional conductors.
Dampers qualified for service on C
7
are the only ones suitable. To date, only AFL and PLP have
qualified dampers for C
7
overhead conductor.
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