In my opinion if Horizontal lifelines not properly installed can be one of the most dangerous fall protections issues an Ironworker may face .          Among the terms used in the field are "cantenary lines" and "ankle wires". These ARE Horizontal lifelines. Its been my experience tha most all of the applications I have seen have NOT been designed, installed, and used under the supervision of a qualified person.  Horizontal lifelines may, depending on their geometry (see diagram on right) and angle of sag, be subjected to greater loads than the impact load imposed by an attached component. When the angle of horizontal lifeline sag is less than 30 degrees, the impact force imparted to the lifeline by an attached lanyard is greatly amplified. For example, with a sag angle of 15 degrees, the force amplification is about 2:1 and at 5 degrees sag, it is about 6:1. Depending on the angle of sag, and the line's elasticity, the strength of the horizontal lifeline and the anchorages to which it is attached should be increased a number of times over that of the lanyard.  Extreme care should be taken in considering a horizontal lifeline for multiple tie-offs. The reason for this is that in multiple tie-offs to a horizontal lifeline, if one employee falls, the movement of the falling employee and the horizontal lifeline during arrest of the fall may cause other employees to also fall.  Horizontal lifeline and anchorage strength should be increased for each additional employee to be tied-off.  For these and other reasons, the design of systems using horizontal lifelines must only be done by qualified persons. Testing of installed lifelines and anchors prior to use is recommended. Horizontal lifelines shall have a tensile strength capable of supporting a fall impact load of at least 5,000 pounds (22.2 kN) per employee using the lifeline, applied anywhere along the lifeline. 296-155-483(7)(c) (e) Horizontal lifelines are designed, installed, and used under the supervision of a qualified person, (SEE BELOW) and only used as part of a complete personal fall arrest system that maintains a safety factor of at least two. Note: The system strength needs below are based on a maximum combined weight of employee and tools of 310 pounds.  If combined weight is more than 310 pounds, appropriate allowances must be made or the system will not be in compliance. Qualified Person" means one who, by possession of a recognized degree, certificate, or professional standing, or who by extensive knowledge, training, and experience, has successfully demonstrated their ability to solve or resolve problems relating to the subject matter, the work, or the project

Note the extreme difference in forces that apply to sag angle on a lifeline. We all assume we will "never" fall... but as we all know it happens. If your anchorage points withstands the impact of the fall, then total fall distance, striking a lower level is also critical.