How to calculate fall distance
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Preparing for a fall arrest situation
Nobody likes to think that they are going to suffer a fall and the thought of a fall from height is even more frightening. This is why we use fall protection. Fall arrest systems such as anchor points, static lines and rigid rails, combined with fall arrest harnesses and lanyards, are all common types of fall protection.
Fall arrest systems do not stop the operator from falling, but will arrest the fall once it has occurred. This means that the operator may fall some distance before the system arrests their fall. If the fall arrest system is used correctly, the operator should not get into a fall situation.
When choosing the design, install and use of fall arrest systems it is vital that correct fall distance clearance has been calculated, to ensure that persons using the system do not suffer any injury resulting from impact with lower level hazards.
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What is fall distance clearance?
Fall distance clearance is the measurement which is calculated by totaling the distance that a person falls from the working surface to the position they end up in when their fall has been arrested. It also factors in a clearance distance to account for any stretch in the harness and lanyard.
Correct calculation ensures that the operator's fall arrest system will activate so they are not injured by coming into contact with any obstructions below.
We now have a brand new fall distance clearance calculator which determines fall distance clearance and lanyard length required when using overhead mount systems.
How to calculate fall distance clearance?
The fall distance clearance required is dependent on the following factors:
Elevation of anchorage
Line deflection, if applicable*
Lanyard length
Lanyard elongation on deceleration pull out (personal energy absorber)
Operator height
Fall distance residual clearance (allowance for stretch in the lanyard and harness)
*Line deflection is only added if the operator is attached to a horizontal lifeline to allow for deflection of the cable in the event of a fall, which may increase the fall distance. Check the manufacturer's guidelines.
Fall Distance Clearance Diagram
(To be used as a guide only.)
In the example shown, with a allowance of 1800mm for the operator, a line deflection* of 500mm, a 2000mm lanyard length and a 1700mm energy absorber extension, the operator would experience a total fall distance of 6000mm. Into this equation add in the additional 1000mm residual clearance which factors in the stretch of both harness and lanyard.
Working on this example the fall distance clearance is 7000mm.
Some equipment may have different clearance requirements. Refer to Australian and New Zealand Standard AS/NZS 1891.4 for further information.
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*Line deflection is only added if the operator is attached to a horizontal lifeline to allow for deflection of the cable in the event of a fall, which may increase the fall distance. Check the manufacturer's guidelines
Fall arrest positioning
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Diagram 1
NOTE: This is to be used as a guide only. Calculations must be based on the conditions and Australian Standards / Codes of Practice Regulations.
Diagram 2 - Measurement of free-fall distance - fixed length lanyard
Extract from AS/NZS 1891.4.2009.8.2
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NOTE: Some configurations may result in a potential free-fall distance in excess of 2.0m (see Clause 8.3 in AS/NZS 1891.4).
More Information
System Requirements
Workers must wear a full body harness when connected to any fall arrest system, including a personal energy absorber, compliant with Australian Standard AS/NZS 1891.2:2001 and AS/NZS 1891.4:2009 limiting the force on the anchor and operator to a maximum of 6kN.
Harness connectors must support at least 15kN. Non-compatible connectors may unintentionally disengage (roll-out) so always ensure that there is no mixing of different manufacturers. Karabiners supplied with proprietary systems must not be removed or substituted with any other component.
Inspection and Maintenance
Inspection and recertification of fall arrest systems and equipment is required at least every 12 months by competent person in accordance with manufacturer’s specifications and requirements of Australian and New Zealand Standard AS/NZS 1891.4:2009 Section (9).
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