There is a misconception, and for good reason, that the CSA Z259.11 Class E6 personal energy absorber (PEA) requires more clearance to arrest a fall than the Class E4 PEA. Also, most users steer away from the Class E6 PEA as it deploys at a higher arrest force. However, the benefits to the worker using the Class E6 PEA in a Personal Fall Arrest System (PFAS) greatly outweigh these two reasons for not using the Class E6 PEA.
I will explain the benefits by examining the two concerns noted above. First, let’s consider the higher deployment force. The Class E6 PEA is designed to deploy at a force no greater than 6 kN (kilonewtons) (or 1,350 lbs.), in accordance with the CSA Z259.11 standard. However, due to variations in the manufacturing process of the tear-away webbing mechanism that creates the energy absorbing device, there will always be some variation in the deployment force of the device. Unfortunately, the only way to determine what the deployment force of an energy absorber will be is to deploy the energy absorber, destroying it in the process, and measuring the average deployment force. Now, there is empirical data regarding the average deployment force of the energy absorber and with some statistical analysis the manufacturers, with confidence, can ensure the device meets the requirements of the CSA Z259.11 standard. The average deployment force will always be less than the rated or Class deployment force. The big question now is, what is the average statistical deployment force of a CSA Class E6 personal energy absorber? In my experience, most energy absorbers sold on the market by the major manufacturers will have an average deployment force from 65% to 85% of the rated value. This goes for either CSA Class personal energy absorber, E4 or E6. Therefore, the actual arrest force for the CSA Class E6 PEA is between 3.9 kN (877 lbs.) and 5.1 kN (1,147 lbs.). This force isn’t much higher than the rated deployment force of the Class E4 PEA. And by comparison, the actual arrest force for the CSA Class E4 PEA is between 2.6 kN (584 lbs.) and 3.4 kN (764 lbs.). I will come back to the forces towards the end of this discussion.
Now let's address the concern that the CSA Z259.11 Class E6 energy absorber requires more clearance for fall arrest. Let me explain why I commented earlier that this was for good reason. Most end users are taught to assume full deployment of an energy absorber when calculating required clearance. The Class E6 PEA has a maximum deployment length (elongation) of 1.75m (5.74 ft), in accordance with the CSA Z259.11 standard. If we compare this to the Class E4 PEA, which has a maximum deployment length of 1.2m (3.94 ft), in accordance with the CSA Z259.11 standard, we see that the E6 will require 0.55m (1.8 ft) more clearance to arrest the fall of a worker. Assuming full deployment of the PEA, the worker will always calculate a more conservative required clearance for the Class E6 PEA. Unfortunately, it would appear that more clearance is required for the Class E6 PEA than for the Class E4 PEA.
Now, let’s review both of these factors together, force and distance. When multiplied together, these values calculate the capacity of the energy absorber. If we think of the capacity as the total amount of fall energy the device can absorb, all other factors being equal, we can make a practical comparison between the two CSA Classes of personal energy absorber.
First, let’s calculate the capacity of the Class E4 PEA, using the minimum average arrest force and maximum elongation. 2.6 kN x 1.2m = 3.12 kN.m (584 lbs x 3.94 ft = 2,300 ft.lbs) energy absorption capacity.
Second, let’s calculate the capacity of the Class E6 PEA, using the minimum average arrest force and maximum elongation. 3.9 kN x 1.75m = 6.83 kN.m (877 lbs x 5.74 ft = 5,033 ft.lbs) energy absorption capacity.
By examination it can be seen that the Class E6 PEA has more than twice the capacity of the Class E4 PEA, all other factors being equal. This will always be true as the Class E6 PEA deploys at a higher force than the Class E4 PEA.
E4 capacity kN.m (ft.lbs)
E6 capacity kN.m (ft.lbs)
@ 65% rated arrest force
@ 85% rated arrest force
@ Rated arrest force
Now, let’s look at a real fall scenario. A worker weighs 100 kg (220 lbs.), including tools, clothing and other PPE. Let’s assume the worker has a fall using each of the Classes of energy absorber examined above. Let’s also assume that the worker has his fall arrest system configured that he has exactly a 1.8m (6 ft) free fall (FF), before the fall arrest system begins to arrest the fall of the worker.
The total amount of fall energy the worker will generate during the fall, regardless of Class of personal energy absorber, will be 100 kg x 9.81 m/s2 x 1.8m = 1.77 kN.m (220 lbs x 6 ft = 1,320 ft.lbs). But, since the worker will continue to fall as the fall arrest system arrests the fall (deceleration phase), the actual distance required to stop the worker falling, (i.e. arrest the fall), is dependent on the Class of personal energy absorber used.
The actual formula used to calculate the deployment distance (elongation) of a personal energy absorber to arrest a fall is called the Personal Energy Absorber Equation and can be found in section 18.104.22.168 of the CSA standard Z259.16-04 Design of Active Fall-Protection Systems. Without getting too technical, below is a comparison of the elongation of the Class E4 and the Class E6 PEAs for the above scenario. Using the minimum average arrest force will result in the greatest PEA deployment (elongation or Deceleration Distance, DD).
Energy Absorber Equation: Xpea = mgh/(Favg – mg)
PEA elongation (DD)
Arrest distance (FF + DD)
E4 @ 65% rated arrest force,
2.6 kN (584 lbs)
1.09m (3.58 ft)
2.89m (9.48 ft)
E6 @ 65% rated arrest force,
3.9 kN (877 lbs)
0.60m (1.97 ft)
2.40m (7.87 ft)
The Class E4 PEA actually deploys more, and requires 0.49m (1.60 ft) more clearance than the Class E6 PEA due to the lower average arrest force.
In conclusion, because the Class E6 Personal Energy Absorber has a greater capacity to absorb fall energy, it is the better device to use for any worker weighing above 90 kg (200 lbs) using a Personal Fall Arrest System (PFAS).
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