Sag is the vertical displacement of a steel messenger wire between support points, determined by cable weight, span length, tension, and environmental loads.

Definition of Sag

In aerial fiber optic installations, sag is defined as the vertical distance between the steel messenger wire and an imaginary horizontal line connecting the attachment points on poles, assuming no elevation difference between poles . It represents the natural droop of the wire under its own weight and additional loads such as ice, wind, or the lashed optical cable . Sag is typically measured at the midpoint of the span.

Steel Messenger Wire

The steel messenger is a stranded steel cable that supports the weight of the optical fiber cable . It usually consists of six wires wrapped around a central wire, often made of carbon steel with a zinc or zinc-aluminum coating for corrosion protection . The diameter and tensile strength of the messenger are selected based on span length, cable weight, and environmental conditions.

Factors Affecting Sag

1. Tension: The axial tensile force applied to the messenger inversely affects sag; higher tension reduces sag, while lower tension increases it .
2. Span Length: Longer spans naturally result in greater sag due to the increased unsupported length .
3. Cable Weight: The combined weight of the messenger and lashed optical cable contributes to sag .
4. Environmental Loads: Ice accumulation, wind pressure, and temperature changes can increase sag and must be considered in design .
5. Creep: Over time, steel wires experience permanent elongation under sustained load, slightly increasing sag .

Installation Considerations

During installation, the messenger wire is first tensioned and sagged to a predetermined value, then the optical cable is lashed onto it . Proper sag ensures compliance with ground clearance requirements and prevents excessive tension that could damage the cable. Calculations often follow standards such as the National Electrical Safety Code (NESC) and may use software tools to simulate multiple spans and load conditions .

Summary

Sag in steel messenger wires is a critical design parameter for aerial optical cable systems. It balances tension, span length, and environmental loads to maintain structural integrity, ensure safety, and protect the optical fibers. Accurate calculation and proper installation are essential for long-term reliability of the communication network .