Fall arrest (PPE) is used to stop a fall that is already happening.
t’s only legally permitted to use this type of equipment when it’s not reasonably practicable to avoid or prevent falls, so fall arrest PPE must only be used a last resort.
Anyone using work at height PPE (including both fall arrest and fall prevention equipment) must have extensive practical and theoretical training and must be properly supervised when using it.

Once all the other steps of the hierarchy have been exhausted, fall arrest (PPE) can be considered.