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OSHA Safety Distance Calculation Formula
The OSHA safety distance formula as specified in CFR Subpart O
1910.217 is as follows:
Ds = 63 X T
S
Ds
Safety Distance in inches
63
Is the OSHA recommended hand speed constant in inches per
second
Ts
Is the total stop time of all devices in the safety circuit, measured in
seconds. This value must include all components involved in
stopping the hazardous motion of the machinery. For a
mechanical power press it is the stopping time measured at
approximately the 90º position of the crankshaft rotation.
The T
number must include the response times of all devices, including
S
the response time of the safety light curtain, the safety light curtain
controller (if used), the machine's control circuit and any other devices
that react to stop the hazardous motion of the machinery. Not including
the response time of a device or devices in the stop time calculation will
result in insufficient safety distance for the application. This may result
in operator injury.
The ANSI Safety Distance Formula
The ANSI safety distance formula, which is the Rockwell Automation
recommended formula, is as follows:
D
= K x (T
+ T
+ T
+ T
) + D
r
bm
S
S
C
Ds
Minimum safety distance between the safe guarding device and
the nearest point of operation hazard, in inches.
K
Hand speed constant in inches per second. The ANSI standard
value is 63 inches per second when the operator begins reaching
toward the point of operation hazard from rest. NOTE: ANSI
B11.19 1990 E4.2.3.3.5 states "The value of the hand speed
constant, K, has been determined by various studies and although
these studies indicate speeds of 63 inches/second to over 100
inches/second, they are not conclusive determinations. The
employer should consider all factors, including the physical ability
of the operator, when determining the value of K to be used."
Ts
Stop time of the machine tool measured at the final control
element.
Tc
Response time of the control system
T
and T
are usually measured by a stop time measuring device.
s
c
Tr
Response time of the presence sensing device (safety light curtain)
and its interface, if any. This value is generally stated by the device
manufacturer or it can be measured by the user.
Tbm Additional time allowed for the brake monitor to compensate for
variations in normal stopping time.
Dpf Depth penetration factor. It is an added distance to allow for how
far into the protective field an object, such as a finger or hand, can
travel before being detected. D
curtain's object sensitivity. Object sensitivity is the smallest
R
pf
is related to the safety light
pf
Original instructions
GuardShield™ Safe 4 Safety Light Curtain User Manual
diameter object which will always be detected anywhere in the
sensing field.
Example:
In opto-electronic safeguarding, such as with a perpendicular safety light
curtain applications with object sensitivity (effective resolution) less than
2.5 inches, the D
can be approximated based on the following formula:
pf
D
(inches) = 3.4 × (Object Sensitivity – 0.276),
pf
but not less than 0.
European Safety Distance Formula
A safety distance must be maintained between the light curtain and the
point of danger. This safety distance ensures that the point of danger can
only be reached after the dangerous state of the machine has been
completely removed.
The safety distance as defined in EN ISO 13855 and EN ISO 13857
depends on:
• Stopping/run-down time of the machine or system. (The
stopping/run-down time is shown in the machine documentation
or must be determined by taking a measurement.)
• Response time of the protective device, e.g. GuardShield Safe 4
(for "Response Time" see page 6).
• Reach or approach speed.
• Resolution of the light curtain and/or beam separation.
Safety distance S (D
s
Protective field height
Distance to avoid standing behind
the safety curtain ≤75mm
Figure 3: Safety distance from the point of danger
How to Calculate the Safety Distance S for GuardShield Safe 4 Systems
with a resolution ≤40 mm. According to EN ISO 13855 and EN ISO 13857:
-> First, calculate S using the following formula:
S = 2000 × T + 8 × (d – 14) [mm]
Where ...
T = stopping/run-down time of the machine
+ response time of the protective device [s]
d
= resolution of the light curtain [mm]
S
= safety distance [mm]
The reach/approach speed is already included in the formula.
-> If the result S is ≤ 500 mm (19.6 in.), then use the determined
value as the safety distance.
)
Point
of
danger
7
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