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Drop Zones vs Tool Tethering - Is tool tethering necessary when we use exclusion or drop zones?

by Toby Salisbury 04 May 2023

If you’re in the in the process of researching tool tethering options for your at-heights projects, you’ve no doubt asked yourself, or been asked by your team whether tool tethering is necessary when you use drop zones.

This is a great question and one that is often raised when we meet onsite crews and demonstrating the GRIPPS® range. The short answer is, in a perfect world, no.

If a drop zone could be set-up in such a way that any risk of injury or damage was eliminated there would be little need for additional control measures. The reality is that objects dropped from height almost instantly become projectiles containing enormous amounts of dynamic energy, that is out of control and dangerously unpredictable, particularly if the object ricochets during its fall.

Drop zones and exclusion zones are essential components of most dropped object prevention policies. However, it is important to recognise the 5 key limitations of drop zones.

Objects don’t fall straight down

The single biggest challenge with establishing any effective drop zone is the fact that objects don’t fall straight down. Even without deflecting, air movement and object momentum can cause items to stray significantly from their anticipated fall path. As a rule of thumb, the drop zone radius should be approximately one third (33)% of the working height. However, as a general rule, a minimum drop zone radius of 4m should be established (where practicable).

Construction hit by falling object visual

This obviously provides a lot of scope for human judgment and site variables, however practically it is often not possible and is also often not adequate. We have been involved in incidents where the final landing place of dropped objects, particularly in windy conditions with lighter items such as phones and small tools, is up to 75% of the actual drop height.

Deflections

Deflection of falling items is where the lethal and unpredictable nature of dropped objects is highlighted the most. A study conducted by Dean Pratt at the University of Central Queensland, recorded every drop incident that occurred across a range of projects over a two-year period. The findings in terms of distance items were projected away out the initial fall points are as below:

Dropped Object Study Results Dean Pratt University of Central Queensland

Distance out from drop point % of dropped objects

0-20M
88%
20-40m
6%
40-100m
3%
100m+
2%!

While 88% of dropped objects were recorded as landing within 20m from the initial drop point, the remaining 12% of incidents were almost guaranteed to be outside the standard drop zone. It’s worth noting that these incidents are also almost invariably dropped from the greater heights, resulting in…

Greater drop height = Greater danger = Less effective drop zone!

Management

The most effective drop zone becomes ineffective as soon as it is either purposefully or inadvertently ignored. Drop zones require constant monitoring and management for as long as they are being relied on. This policing does obviously involve considerable expense, particularly for long-term height works projects.

Message to the workforce

Drop zones, by definition, are a secondary control measure. They are put in place to hopefully minimise the damage that will be cased by a falling object that is is totally out of control.

While secondary control measures are necessary, if they are being relied on with no other control measure, what message does this communicate to the workforce? Reliance on secondary measures alone for personal height safety is unthinkable. Imagine a height worker removing their harness and relying solely on a catch net for safety. If we only have secondary measures in place for dropped object prevention, does this send a negative message to the workforce about out committal to their safety?

Construction Workers At Height

Luck

As demonstrated in the study on dropped objects and deflections, approximately 12% of dropped objects are going to fall outside of a drop zone due to space limitations. This does raise the question as to what we’re relying on for avoiding these incidents causing, damage, injury or even fatality? The answer is LUCK!

Once an object has been dropped, and happens to deflect outside the drop zone, the only safety measure we have left to rely on is luck. Unfortunately, luck does run out and there are countless incidents, injuries and even fatalities that have been incurred by objects that have fallen outside of drop zones.

Drop zones are an essential component of a dropped object prevention strategy. However, it is critical that other more effective measures are implemented to effectively eliminate the critical risk of dropped objects and to save workers lives.

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