Immediate means of Retrieval
OSHA 1910.146 states "Harnesses and retrieval lines
Authorized entrants who enter a permit space must wear a chest
or full body harness with a retrieval line attached to the center of
their backs near shoulder level or above their heads. Wristlets may
be used if the employer can demonstrate that the use of a chest
or full body harness is not feasible or creates a greater hazard.
Also, the employer must ensure that the other end of the
retrieval line is attached to a mechanical device or a fixed point
outside the permit space. A mechanical device must be available
to retrieve someone from vertical type permit spaces more than
five feet (1.524 meters) deep."
Anytime someone breaks the plane, enters a confined space you must have a retrieval line attached. It states nothing of the mainline. In addition, anytime someone enters said space, YOU MUST HAVE a "means of immediate retrieval. So if I lower someone on the mainline and have a belay line attached as well I have to have a M/A anchored ready to go. That M/A could be anything. You could attach the M/A to the belay line if needed (mainline incapacitated) and haul.