Using a harness should a worker go through training and or certification, or both per OSHA

Workers who may be exposed to fall hazards must be trained to recognize these hazards and understand the procedures to minimize them. Training must occur before workers begin tasks where they could be exposed to fall hazards or before they use fall protection systems. [1]

The training program should cover:

• The nature of fall hazards at the worksite
• Procedures for erecting, maintaining, and disassembling fall protection systems
• How to use and operate fall protection systems
• The role of each employee who may be affected by a safety-monitoring system
• The restrictions that apply to mechanical equipment used during roofing work
• The procedure for handling and storing materials and for erecting protection from falling objects
• The applicable requirements of Subdivision 3/M for the type of fall protection systems used

[3] Employers must verify training compliance by preparing a written certification record that includes the employee's name or identity, training date(s), and the signature of the trainer or employer. The latest training certification must be maintained. [2] [2]

Retraining is necessary when employees do not recognize fall hazards, do not follow safe practices for using fall protection systems, or when changes in the workplace or fall protection systems make previous training outdated. [1]

The trainer must be a competent person who can identify worksite hazards and has the authority to take prompt corrective measures. [3]