Do you need to use a harness whilst working in SAE?
A full body harness and appropriate lanyard would be part of standard PPE for most installations.
Can one person work alone in a two person cradle?
SAEMA recommend that there should be two people in the cradle and one person assisting from the roof or at ground level.
Do you need a ground or roof marshall whilst using SAE?
This depends on site conditions and needs to be considered as part of the risk assessment which should include emergency descent operations.