FREE FALL CALCULATIONS
FULL-BODY HARNESS AND PFAS
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FALL CLEARANCE AND CALCULATING
FULL-BODY HARNESS AND PFAS
BEFORE THE START OF WORKING AT HEIGHT
FALL HAZARDS AND CONTROLS
The
measurement for assessing the fall hazards and controls before the start of
working at height activity shall be considered. A few basic measurements and
equations can aid in evaluating a Personal Fall. Fall Arrest System FAS will be
sufficient to prevent workers from contacting a lower level. This provides
information on evaluating:
FULL-BODY HARNESS
Full body
harness the human body and the attachment system. The Full Body Harness shall
be suitable and appropriate to withstand the weight of personnel wearing it
including the weight of tools to be carried at work at height. The full-body
harness belt system shall be equipped with shoulder straps and leg straps, a
sub-pelvic assembly, adjustable buckles or fasteners, and one or more D rings
to connect to shock-absorbing lanyards a complete set of harness. Full body Harnesses
shall be selected to ensure they are suitable for the intended use at height. Ensure
it shall have a combined body and tool weight of less than 310 pounds.
Total Fall Clearance Distance for PFAS
The total
fall clearance distance is the minimum vertical distance between the worker and
the lower level that is necessary to ensure the worker does not contact a lower
level during a fall from height. The total fall clearance distance shall be
calculated before a decision is made to use a FAS before the fall arrest system.
Ensure If the available distance is not greater than the total fall clearance
distance, it is inappropriate to use the FAS fall arrest system, and a fall
restraint system might be used instead. Total fall clearance distance
calculations to perform based on several factors at work at height, including:
1.
Lanyard
length
2.
The
height at which the lanyard is anchored relative to where the other end
attaches to the worker's full-body harness.
3.
The
distance the worker will travel as the deceleration device absorbs the energy
from the fall from the height.
4.
The
worker's height
5.
D-ring
shift
6.
A
safety factor
Calculate the total fall clearance
distance:
Free fall distance: This is the distance the worker falls
before the PFAS fall arrest system begins to slow the fall. When using a PFAS,
this distance shall be 1.8m or less and also prevent the worker from contacting
a lower level by the following limit:
· Make sure the maximum arresting force
on an employee to 900 pounds (4 KN) when used with a full-body harness belt
· Make sure the maximum arresting force
on an employee to 1,800 pounds (8 KN) when used with a full-body harness.
· Free fall distance varies depending on
the lanyard's length and where the anchor is set relative to the back D-ring on
the full-body harness.
Deceleration distance:
This is the distance the lanyard stretches to arrest the fall.
Ensure the deceleration distance shall be not greater than 3.5 feet
(1.07m).
D-ring shift:
This is the distance the D-ring moves and the harness shifts when they
support the worker's full weight of a worker. The line tugs upwards, and the
harness can shift so the D-ring location is higher on the worker than it was
before the fall from the height. This shift is often assumed to be one foot,
but it can vary, depending on the equipment design and the manufacturer of the D-ring.
Back D-ring height:
The D-ring height is measured as the distance between the D-ring and the
worker's shoe sole while the worker is wearing the full-body harness. This
height is often standardized as five feet for six-foot-tall workers shorter
workers may also be protected using this default distance. It is necessary to
adjust the back D-ring height for workers exceeding six feet of height.
Safety factor:
A
safety factor is an additional distance added to the total fall clearance
distance to ensure there is enough clearance between the worker and the lower
level after a fall from the height. It is typically 2 feet of safety distance.
Calculating Total Fall Clearance Distance for
Fall Arrest Systems with a Shock-Absorbing Lanyard
The following
common assumptions are to be taken:
· Deceleration distance: 3.5 feet
· D-ring shift: 1 foot
· D-ring height shoe sole to point
between shoulder blades: 5 feet
· Safety factor: typically 2 feet
Calculate the
total fall clearance distance for the fall arrest system. The fall arrest
system will not protect a falling worker if the calculated clearance distance
is greater than the actual distance available below the elevated work area measured
as the distance between the point at which a worker would be anchored and any
lower surface.
Calculating Free Fall Distance
The following
cases shall be considered to calculate the distance:
· D-ring (above Anchor): Free Fall
Distance = Lanyard Length + Distance from D-ring to anchor
· D-ring (below Anchor): Free Fall
Distance = Lanyard Length - Distance from D-ring to anchor
· D-ring (level with Anchor): Free Fall
Distance = Lanyard Length
The fall
distance applies to a worker using a shock-absorbing lanyard. SRL typically
activates, and thus limits free fall distance, within 2 feet. Ensure to refer
to the manufacturer’s recommendation for activation details of the D-ring.
Typical Example (D-ring below Anchor):
A competent and
experienced person checks that the trusses in the adjacent section have been
installed, fastened in place, sheathed, and are stable enough to serve as an
anchorage point. An anchor point is installed 2 feet above the back D-ring on
the worker's full-body harness.
What is the total fall clearance
distance?
· Free fall distance = 6-foot lanyard -
2 feet between the anchor and D-ring = 4 feet
· Deceleration distance = 3.5 feet
· D-ring shift = 1 foot
· Back D-ring height = 5 feet
· Safety factor = 2 feet
Evaluating
the Swing Fall Hazard
The swing
fall hazard is created by the pendulum effect, which can swing a fallen worker
into a nearby surface, such as a wall or protruding beam of the wall. In
addition to calculating the total fall clearance distance before beginning work
on an elevated level, it is important to evaluate the swing fall hazard at the
edges where a worker might fall from the height. A worker who falls while
connected to an anchor point unless it is directly overhead will swing back and
forth like a pendulum.
Workers or
technicians can be seriously injured if they strike objects during a swing fall
from a height. Ensure installing the anchorage point directly above the work
area i.e., connected to an overhead attachment point with sufficient strength
will help prevent injury or death.
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