Manhole Entry, Cable Bend Radius, and Pull Tension
A two-person crew arrives at a 1.2 m diameter, 4.5 m deep manhole to splice a 48-fiber cable. The gas detector shows 8% LEL and 12 ppm H2S; oxygen is 20.4%. A 120 V portable pump is running with its cord through the manhole cover opening. The crew has a tripod and harness but no retrieval line attached, and the attendant is also the person who will enter. Inside, cable with a minimum bend radius of 260 mm has been tied in a coil with a 150 mm radius. Traffic cones are set at 30 m on a 60 km/h road; the posted work zone requires 100 m advance warning. The cable is to be pulled later that day with a calculated tension of 3.2 kN, and its short-term tensile rating is 2.7 kN.
1. Should entry proceed? Identify the permit-required confined-space issues and controls.
Read the worked answer
Entry should not proceed. 8% LEL and 12 ppm H2S are above normal entry thresholds, and the atmosphere must be ventilated and retested before entry. The 120 V pump is an electrical and ignition hazard in a potentially flammable atmosphere; remove it and use air-driven equipment, or provide GFCI protection outside the space. The attendant cannot also be the entrant; a non-entrant attendant must maintain communication. Attach a retrieval line to the harness, use the tripod, monitor gas continuously, and have a rescue plan. If ventilation cannot bring readings to acceptable levels, do not enter; supplied air or respiratory protection is only for trained personnel under a permit.
2. What workmanship and traffic-control problems are present, and what corrections are needed?
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Workmanship: the 150 mm coil radius is below the 260 mm minimum bend radius. Reroute the coil to at least 260 mm and secure it without tight ties. Placement: the calculated pull tension of 3.2 kN exceeds the cable short-term rating of 2.7 kN, so the pull cannot proceed as planned. Traffic: cones at 30 m are inadequate for a 60 km/h road with a required 100 m advance warning; extend signage and coning and use proper flagging. Also secure the manhole cover and check that the work zone is lighted.
3. If the pull tension cannot be reduced below 2.7 kN, what options exist and what must be checked afterward?
Read the worked answer
Options include lubricating the duct, using a larger duct or gentler sweep, pulling from both ends, pulling in shorter sections with an intermediate handhole and splice, or using a cable with a higher tensile rating. Do not treat the 3.2 kN calculated value as acceptable; verify tension with a dynamometer and stop if the limit is approached. If the cable is pulled above its rating, fibers may be strained or the jacket may be damaged. After installation, OTDR-test the cable and inspect for jacket damage; reject or replace if loss increases or damage is found.