Caution: stacked rappel with a Grigri(s)
There is a caution on the Petzl website against doing a stacked rappel with a Grigri.
From the website:
“With the GRIGRI + from 2017 and on, and the GRIGRI from 2019 and on, unblocking and a consequent fall can occur if the rope is heavily loaded below the user. Unblocking can occur if the rope is loaded with a weight equal to or greater than that of the person rappelling on the GRIGRI.
Examples of dangerous situations:
Multiple people rappelling in sequence, GRIGRIs pre-installed on the rope. If the person waiting in turn to rappel has their GRIGRI pre-installed on the rope, it can be inverted (and thus unblocked) by the weight of the person rappelling below them. So it will not be operational when the second person wants to start their rappel.
Bottom belay: the person performing the belay maneuver must not hang on the rope.
Rescue from below: The rescuer must not ascend the rope of a person who is stuck on rappel.”
Below is a stacked rappel with a Grigri. When the first person down starts loading the rope, (apparently, in some cases) it can cause the rope to invert on the top person. (I tested this briefly and could not cause failure, but I certainly trust the Petzl engineers on this one, so don't do it!
Related to this, Petzl also cautions against a firefighter belay, or someone ascending a rope when someone above them has a Grigri.
