Creep is Permanent Elongation at Everyday Tensions. Conductors permanently elongate under tension even if the tension level never exceeds everyday levels.

This permanent elongation caused by everyday tension levels is called creep. Creep can be determined by long-term laboratory creep tests.

The results of the tests are used to generate creep-versus-time curves. On the stress-strain graphs, creep curves are often shown for 6-month, 1-year, and 10-year periods.

Figure 14-27 shows these typical creep curves for a 37-strand 250- to 1033.5-kcmil AAC. In Fig. 14-27, assume that the conductor tension remains constant at the initial stress of 4450 lb/in2.

At the intersection of this stress level and the initial elongation curve, 6-month, 1-year, and 10-year creep curves, the conductor elongation from the initial elongation of 0.062% increases to 0.11%, 0.12%, and 0.15%, respectively. Because of creep elongation, the resulting final sags are greater and the conductor tension is less than the initial values.

Creep elongation in aluminum conductors is quite predictable as a function of time and obeys a simple exponential relationship. Thus, the permanent elongation due to creep at everyday tension can be found for any period of time after initial installation.

Creep elongation of copper and steel strands is much less and is normally ignored. Permanent increase in conductor length due to heavy load occurrences cannot be predicted at the time a line is built.

The reason for this unpredictability is that the occurrence of heavy ice and wind loads is random. A heavy ice storm may occur the day after the line is built or may never occur over the life of the line.

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