Aerial Span With a Mid-Span Slack Problem
A contractor is stringing a 48-fiber ADSS cable on existing utility poles over a 1.1 km run. The poles are spaced 60 m apart on average. One span crosses a road at 7 m clearance; the rest are over a field. The contractor reports that after tensioning, one span has visible sag that looks excessive compared to neighbors, and the customer is worried about clearance. The cable has a rated maximum tension and a specified installation sag. The contractor used a come-along and a dynamometer but did not record the tension values. There is no messenger wire; the ADSS is self-supporting.
1. What is the most likely cause of the excessive sag in one span, and how should it be verified?
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Most likely cause: insufficient tension on that span - the contractor did not pull to the specified installation tension/sag for the span length and temperature.
Verification:
- Measure actual sag at mid-span against the design sag for that span length and conductor temperature (sag varies with temperature; use the temperature at time of measurement).
- Measure tension with a calibrated dynamometer and compare to the cable's specified installation tension.
- Check for a slipped dead-end or grip at either pole - a partially slipped wedge or grip will show as sag even if tension was correct at the pull.
- Confirm span length - if the actual span is longer than 60 m, the design sag changes.
Do not accept "looks like the neighbors" as verification; sag is a measured value.
2. The road-crossing span is the one with excessive sag. What is the immediate risk and the corrective action?
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Risk: reduced clearance over the road. The design clearance is 7 m; excessive sag can drop the cable below the required road clearance, creating a vehicular strike hazard and a code/utility violation.
Corrective action:
- Treat as an immediate safety issue - notify the utility and, if clearance is below the minimum, restrict or flag the crossing until corrected.
- Re-tension the span to the specified sag using a calibrated dynamometer, recording the tension value this time.
- Verify clearance after re-tensioning with a measured height, not visual estimate.
- Document the tension and clearance for the as-built record.
If re-tensioning cannot achieve both the design sag and the required clearance, the pole height or span must be changed - not the clearance requirement.
3. What should the contractor have done differently during installation?
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- Record tension values at every pull with a calibrated dynamometer, and compare against the cable's specified installation tension.
- Measure and record sag per span at a known temperature, using the design sag table for the cable and span length.
- Check dead-ends and grips for proper seating before and after tensioning.
- Verify clearances (road, ground, other utilities) with a measuring device after stringing.
- Follow the ADSS manufacturer's installation guide for span length, tension, and sag - self-supporting ADSS has specific limits that differ from messenger-supported cable.