Aerial build: clearance, sag, and bonding disputes
A co-op is building 2.4 km of aerial FTTH on 38 poles along a county road. The strand is existing 5/16 inch galvanized, originally installed for a 50-pair copper cable that has been removed. The contractor proposes to lash a 48-fiber loose-tube cable to the strand with 1.2 m sag in a 60 m ruling span, place slack coils at every third pole, bond and ground the messenger only at the first and last poles, and cross a private driveway at 4.6 m. The joint-use agreement requires 4.9 m over driveways, 5.5 m over the county road, and bonding at intervals not exceeding 300 m and at every pole with a supply ground. The contractor says the 4.6 m crossing is close enough because the driveway is private and the fiber is dielectric. The co-op inspector must approve or reject the work.
1. How should the inspector rule on the 4.6 m driveway crossing?
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Reject it. The joint-use agreement requires 4.9 m over driveways, and a private driveway does not exempt the span. The fact that the cable is dielectric is irrelevant to clearances: the lashed cable and messenger still create a physical and electrical hazard, and the attachment height is governed by the agreement and the applicable safety code.
The contractor must raise the attachment, re-tension the span, or relocate the crossing so that the final sagged clearance is at least 4.9 m. The inspector should measure clearance after the cable is installed and at final sag, not before lashing. The county road crossing must also be verified at 5.5 m. Document the measurement point, temperature, and date. Approving a 0.3 m shortfall invites liability and a future rework order at the co-op's expense.
2. Evaluate the proposed 1.2 m sag in a 60 m span and the use of the existing 5/16 inch strand.
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For a rough check, with a combined cable and lashing weight of about 0.25 kg/m, tension is approximately T = wL²/(8s) = 0.25 × 3600 / (8 × 1.2) ≈ 94 kgf. That is low tension, so the strand is not being overstressed by the fiber alone. The concern is the opposite: excessive sag reduces clearance and may indicate that the strand was not re-tensioned after the copper was removed, or that the design sag was never verified.
The inspector should require the contractor's pole-loading and sag calculation, verify the 5/16 inch strand size, grade, and corrosion at clamps and anchors, and confirm that the added 48-fiber cable plus ice and wind loading remains within the strand's rating. If the design sag is 0.6 m, the installed 1.2 m must be corrected by re-tensioning before acceptance. Do not accept a strand merely because the fiber is light; the messenger must still satisfy the joint-use and safety requirements.
3. Is bonding only at the first and last poles acceptable? What should be required?
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No. The proposal violates the joint-use agreement, which requires bonding at intervals not exceeding 300 m and at every pole with a supply ground. A lashed cable has a metallic messenger, and even an all-dielectric fiber cable can become energized by contact with supply conductors or lightning. Bonding only at the ends leaves long ungrounded sections and exposes technicians to hazardous potential differences.
Require bonding clamps and grounds at the first and last poles, at least every 300 m, and at every pole with a supply ground. Bond splice closures and terminal enclosures to the messenger. Verify the ground resistance and inspect connections for corrosion. The inspector should also confirm that the bonding does not create a parallel path that bypasses the supply grounding system. Record each bond location on the as-built.