November 2020
Chris Peace
Lecturer in OHS, Victoria University, Wellington
An imagined conversation.
“And with the net in that position it stops a worker hitting the concrete.”
“But the net won’t stop them falling.”
So how do you stop a worker falling from a height? Can we design construction work and workplaces to minimise the chance of falling, or, as some research describes, avoid loss of balance while working at a height?
Research evidence
For most of this year I have been loading into bibliographic software information about UK Health and Safety Executive (HSE) research reports to make them easier to find, to aid OHS teaching and applied research at Victoria University. A report about safety nets caught my eye and led me to other research, including the following.
Hsiao & Simeonov (2001) identified some causal factors for loss of balance at a height, including: lack of visual clues about height and the environment; “confined” or inclined support surfaces and changes in roof surface friction; physical exertion and fatigue due to load handling; task complexity; distractions; individual differences, including work experience and training.
These factors might be both practicable and reasonable to change and help eliminate or reduce the chance of falling.
Common protection equipment (controls) identified in this research included: covers and guardrails; warning systems; personal fall arrest systems; safety nets; safety monitoring systems; and fall protection plans.
But no mention was made of safety by design of the work or workplace.
Safety nets as the “ambulance at the bottom of the cliff”
Safety nets are used in a harsh environment and can be damaged by overloading, dust and dirt, welding, ultra violet (UV) degradation, mechanical damage, etc.
McCann (2001) dropped a dummy and a 100kg LPG cylinder into safety nets but testing was limited by the availability of test nets; the tests became a comparison between used and unused, polypropylene and nylon, and knotted and knotless nets.
At the time of the tests there was little available information on workers falling into nets safely.
The testing considered a wide range of issues, including: net type; different impact locations (ie edge and corner); attachment point spacing; repeated dynamic loading at one position in the net; multiple falls; effect of sag in the net; presence of defects; effect of differently shaped objects falling into the net; and effects of aging and degradation on service; the effectiveness of monitoring degradation.
During the tests, the test dummy moved erratically within the net, suffering considerable damage, suggesting that workers falling into a net from height may be injured.
Recovery of net properties tended to reduce as testing progressed, indicating a reduction in elasticity on repeated dropping. The knotted polypropylene nets gave significantly lower recoveries, perhaps due to tightening within the knots, and should be scrapped after a fall into a net.
While small defects could be temporarily repaired on site they were a potential source of failure if the defects were at the drop point and subject to repeat loading.
Premature failure resulting from other material (including the 100kg cylinder) entering the net was an issue.
What next?
Do we understand the effectiveness of safety nets in New Zealand?
The above and other information can be used to develop a bowtie analysis for “loss of balance while working at a height” to help show control gaps, a possible activity for students I teach in 2021.
If this note is of importance to you please download and read the research reports listed below.
Sources used
Bentley, T., Hide, S., Tappin, D., Moore, D., Legg, S., Ashby, L., & Parker, R. (2006). Investigating risk factors for slips, trips and falls in New Zealand residential construction using incident-centred and incident-independent methods. Ergonomics, 49(1), 62-77.
Cameron, I., Duff, R., & Gillan, G. (2005). A technical guide to the selection and use of fall prevention and arrest equipment. Health and Safety Executive [Research Report RR0302]. https://www.hse.gov.uk/research/rrhtm/index.htm
Hsiao, H., & Simeonov, P. (2001). Preventing falls from roofs: a critical review. Ergonomics, 44(5), 537-561.
McCann, P. (2011). Evaluation of safety nets by experiment. Health and Safety Executive [Research Report RR0835]. https://www.hse.gov.uk/research/rrhtm/index.htm
Contact Chris by email at: [email protected]
PHOTOGRAPHY CREDIT: Our thanks to Safety Nets NZ for use of the image
