FALL ARREST SYSTEM REQUIREMENTS
Vertical Fall Clearance:
The Tether Track® trolley will follow a worker along the length of the Swing Arm and remain overhead
as long as the worker remains directly under the Swing Arm. However, as the worker moves
horizontally away from the Swing Arm, the arm will likely remain until the worker gently moves it or a fall event occurs and the
arm moves to the leading edge. If a SRL is being used, the extra payout of the lifeline must be accounted for in the fall clearance
calculation. The worker must never exceed a 30 degree vertical offset angle with the trolley. The extra fall clearance distanced
required is provided in the following calculation:
ODH = (TS - WS - DR) * (1/cos
Ө
- 1) (
diagram 1
)
Where: ODH = the offset distance height required to be included in the fall
clearance
calculation
TS = the trolley saddle height
WS = the working surface height
DR = the lowest height of worker’s harness dorsal D-ring from
working surface
Ө
= the vertical offset angle the worker is allowed (30 degrees
maximum)
Here’s a typical example:
•
Worker Height (WH) = 6 ft.
•
Minimum SRL Length including connectors (SL) = 2 ft.
•
ANSI Z359.14 Class B SRL, Maximum Activation and Deceleration
Distance (Arrest Distance, AD) = 4.5 ft.
•
Safety Factor used in this example (SF) = 1.5 ft.
•
Working Surface Height above nearest obstruction, ground or floor in this
case (WS) = 10 ft.
•
Worker may be required to kneel or crouch.
•
Worker is allowed to work with a vertical offset angle of 20 degrees of his
or her SRL line.
What is the lowest Trolley Saddle the Swing Arm may have? SRL’s typically
require that there be no slack in the line (see manufacturer’s specific usage requirements). To achieve this, the trolley
saddle must be the minimum SRL length above the workers dorsal ring.
Minimum TS = WS + WH - 1 ft + SL = 10 + 6 - 1 + 2 = 17 ft.
•
Here 1 ft is taken as the distance from the dorsal ring to the worker’s full height.
The trolley saddle may also be calculated to limit the worker’s horizontal travel away from being directly under the Swing
Arm. Additional information is SRL has a maximum length (SML) = 8 ft.
TS = WS + DR + SML * cos
Ө
= 10 + 3 + 8 * cos (20 degrees) = 20.5 ft.
•
Here 3 ft is used as the minimum height of the dorsal ring above the working surface for a worker that
is crouching or kneeling (DR = 3).
Is there sufficient clearance below the working surface to arrest a fall?
ODH = (TS - WS - DR) * (1/cos
Ө
- 1) = (20.5 - 10 - 3) * (1/cos (20 degrees) - 1) = .5 ft.
Required Clearance below working surface (RC)
RC = ODH + AD + SF + 1 + .33 + 2
•
Here 1 ft is allowed for harness stretch, .33 ft or 4” is maximum allowed deflection for a rigid system,
and 2 ft is allowed for additional leg clearance for a crouching person.
RC = .5 + 4.5 + 1.5 + 1 + .33 + 2 = 9.83 ft
•
9.83 feet is less than 10 ft, therefore there is sufficient clearance for this application.
This example shows the importance of including the additional clearance requirements for allowing work horizontally away from
the Tether Track® trolley. If the maximum allowed offset from vertical angle of 30 degrees was used, the required clearance
would have exceeded the available clearance.
It is recommended to work directly below the anchorage trolley. However for some applications this may be difficult to achieve.
The above calculation is identical to a swing fall calculation. However, in this case the Swing Arm will move to keep the lifeline
vertical instead of the worker swinging.
Note:
even with the drive or friction brake option, the Swing Arm may move to the vertical
lifeline position during a fall event.
17
10/12 Rev. E
WARNING
A worker must not travel beyond the length of the Swing Arm.
ODH
?
L
L
(MAX. 30°)
Ө
Diagram 1.
