Rigging for rescue uses a factor of 10-15. For instance, components carrying one person need a capacity of 4500 lbs, which agrees pretty well with OSHA's 5000 lb requirement for fall protection anchorages. The reason for this are many, including the unknowns, the effect of impact loads, and the aforementioned safety factor.
When you say "horizontal lifeline", what is the application? (i.e., between two fixed anchor points with one or more people tied off to it. In which case, is it to keep them away from an edge or could it be loaded with impact when they actually fall off something?)