When working at height, there are two main elements that help keep a person safe: the body-worn component, and the component that stops the fall when it occurs.
Safety Harnesses
Full body harnesses are preferred as they distribute the impact of a fall across the thighs and shoulders, as opposed to a fall that focuses at a single point. This is the difference between being seriously injured as a consequence of the fall, and being able to walk away from the incident. The right fit is just as important as the harness itself. A harness that is loose will fall to the waist when the harness is loaded, while a harness that is too tight will impede movement and will be sub-optimally worn, and will have the same effect as not wearing the harness at all.
Wherever the body-worn harness component is secured is determined by the task being performed. The most common fall arrest harnesses have a D-ring located at the middle of the shoulder blades, which ensures that the body falls upright and the head is out of the impact zone. A D-ring located at the chest is appropriate for harnesses worn when ascending ladders, as the connection is required to be at the front. D-rings situated at the hips are used for work positioning harnesses, which allow a worker to be held against a structural element while still allowing both hands free.
Fall Arrest & Restraint Gear
Where restraint gear is used, a person cannot physically reach the edge of the fall hazard, because the lanyard has been designed to keep them well away from that edge. Where fall arrest gear is used, the opposite is true. It has been designed with the safety of a fall in mind and is used to safely stop a fall once it has occurred. Because these two systems are the complete opposite of one another, using a restraint-length system where fall arrest is required (or vice versa) is one of the more common errors made on a project.
A shock-absorbing lanyard is the most common component to fall arrest systems. In the case of a fall, the webbing or stitching designed to absorb the shock extends and ruptures to reduce the impact force that would otherwise be transmitted through the harness to the person. A twin-leg lanyard allows the person to stay connected as they move from one anchor to another.
Fall arrest blocks (or inertia reels) serve a similar purpose, but over a longer reach. These work much like a seatbelt in a car. A reel of webbing or wire pays out as a person moves and then solidly locks in place when it detects a fall.
None of this can be utilized without knowing how far a person can fall before the process of stopping their fall begins. The fall clearance is the distance measured from the anchor point down to where the fall can be safely stopped. This is measured based on the lanyard length, the height of the fall arrest mechanism, and the height of the person.
The use of a fall arrest harness is only as good as its most recent inspection. The webbing of the harness may degrade over time due to prolonged exposure to UV rays, chemicals, and abrasion, and this may not be obvious to the operator. When a harness is put on, the operator must inspect the stitching, and buckles, and ensure that the D-rings and each strap of the harness are structurally sound. This preuse inspection is a requirement every time the harness is used.