An electric contractor contacted us about a problem that they had with installing the initial pull string (fish line) that is used to pull the primary service entrance wires to a power transformer through underground conduit. For short runs, they use a regular “fish tape” with no problem. However when it comes to longer runs (say over 50 ft), the solution becomes a little more difficult.
In the past, the contractor said that they used a small electric vacuum to try and pull the string through but that was a real hassle, took a bit of time and once the string made it to the suction point, it just kept going on into the vacuum cleaner itself and got tangled quite easily.
Another issue they face is actually clearing the conduit if it has been in the ground for any period of time. Small rodents, birds, snakes, water, etc all can find their way into the conduit over time. All of that has to be cleared before the local utility can pull the primary wires through.
After discussing the usual conduit sizes, we eventually settled on a 3” Aluminum Line Vac that would be able to produce a high enough airflow velocity to clear the debris and also pull the fish line through. Following are some photos of the Model 6086 3” Aluminum Line Vac attached to the conduit. Also shown is a fabricated conduit “pig”, a round piece of foam that is secured to the end of the fish string and inserted into the pipe. This piece allows the Line Vac to exert considerable pulling force onto the line. In this application, the conduit was over 120 ft. long and the job of blowing the string through took less than 30 seconds. Problem solved.