Safety Harness FALL Clearance Calculation

To calculate safety harness fall clearance,sum the distances of free fall, deceleration, harness stretch, and a safety factor.Free fall is the distance the user falls before the fall arrest system activates, while deceleration is the distance it takes the system (like a shock absorber) to slow the fall.Harness stretch is the added distance due to the harness and the user's body stretching, and the safety factor is an additional buffer to prevent hitting the surface below.

Breakdown of the Calculation

The following components are added together to determine the required fall clearance distance:

1.Free Fall Distance (FFD):

This is the distance from the anchor point to the worker's dorsal D-ring.It can be affected by the length of the lanyard and the height of the anchor point relative to the D-ring.For example, a 6ft lanyard anchored overhead would allow for a 6ft free fall before the fall arrest system activates.

2.Deceleration Distance (DD):

This is the distance the energy absorber (shock pack) deploys to gradually stop the fall, preventing excessive stress on the worker.A standard shock-absorbing lanyard typically has a deceleration distance of about 3.5 feet, according to OSHA.

3.Harness Stretch:

As the user falls, the harness and their body will stretch.This adds to the total distance the user travels downwards.A typical harness may stretch about 1 foot, but stretch-style harnesses require an additional amount to be added.

4.Safety Factor:

An additional clearance distance is added as a precaution to ensure a safe buffer zone between the worker and the surface below.A minimum safety factor of 2 to 3 feet is generally recommended.

Example Calculation

Here is a sample calculation for a worker using a shock-absorbing lanyard:

Free Fall Distance:6 feet (assuming a 6-foot lanyard anchored overhead)

Deceleration Distance:3.5 feet (standard for a shock-absorbing lanyard)

Harness Stretch:1 foot (for a conventional harness)

Safety Factor:2 feet

Total Fall Clearance = 6 ft + 3.5 ft + 1 ft + 2 ft = 12.5 feet

Therefore, there must be at least 12.5 feet of clearance between the anchor point and the ground or nearest obstruction for this setup.

Key Considerations

Manufacturer Instructions:

Always refer to the fall protection equipment manufacturer's instructions for specific details on activation distance and other variables.

Work Scenario:

Consider the worker's position and the height of the anchor point, as this can change the free fall distance.

Obstructions:

Ensure that any obstructions below are accounted for in the calculation to prevent contact.

Regulations:

Familiarize yourself with relevant regulations, such as those from OSHA in the US, to ensure compliance with maximum free fall distances and other requirements.