Rockwell Automation Publication 445L-IN005A-EN-P - May 2018
7
GuardShield Micro 400 Safety Light Curtains
Determining the Safety Distance
The light curtain must be mounted with proper safety distance
• From the point of danger
• From reflecting surfaces
US Safety Distance Formula
This distance, referred to as the safety distance, must be properly
calculated before determining the safety light-curtain protective height
and mounting the light curtains on the machine. Failure to calculate this
safety distance can result in operator injury.
In the United States, there are two formulas that are used to calculate
the safety distance. The first, the OSHA formula, is the minimum
requirement for the calculation of the safety distance. The second
formula, the one recommended by Rockwell Automation, is the ANSI
formula, which incorporates additional factors to be considered when
calculating the safety distance.
OSHA Safety Distance Calculation Formula
The OSHA safety distance formula as specified in CFR Subpart O
1910.217 is as follows:
D
s
= 63 X T
S
D
s
Safety Distance in inches
63 Is the OSHA recommended hand speed constant in inches per
second
T
s
Is the total stop time of all devices in the safety circuit, which is
measured in seconds. This value must include all components that
are involved in to stop the hazardous motion of the machinery. For
a mechanical power press, it is the stopping time that is measured
at approximately the 90° position of the crankshaft rotation.
The T
S
number must include the response times of all
devices. For
example:
• Safety light curtain,
• Safety light curtain controller (if used),
• Machine control circuit,
• 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 results in insufficient safety distance for the application.
This insufficient distance can 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
S
= K x (T
S
+ T
CS
+ T
r
+ T
bm
) + D
pf
D
s
Minimum safety distance between the safeguarding 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.
T
s
Stop time of the machine tool that is measured at the final control
element.
T
cs
Response time of the control system.
A stop time device measures T
s
and T
cs
.
T
r
Response time of the presence sensing device (safety light curtain)
and its interface (MSR4x and MSR45E). The device manufacturer
states this value or you can measure it.
T
bm
Additional time that is allowed for the brake monitor to
compensate for variations in normal stopping time.
D
pf
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
pf
is related to the safety light-
curtain object sensitivity. Object sensitivity is the smallest
diameter object, which is always detected anywhere in the sensing
field.
Example:
In safeguarding, such as a perpendicular safety light-curtain application
with object sensitivity (effective resolution) less than 63.5 mm (2.5 in.),
the D
pf
can be approximated based on the following formula:
D
pf
(inches) = 3.4 × (Object Sensitivity – 0.276), 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 confirms that the point of danger
can only be reached after the dangerous state of the machine has been
removed.
IMPORTANT
The GuardShield Micro 400 safety light curtains must be
mounted at a sufficient distance from the pinch point or point
of operation hazard. The machine has to stop before a finger,
hand, arm, or body reaches the hazard.
IMPORTANT
Regardless of the calculated safety distance, GuardShield Micro
400 safety light curtains are never be mounted closer than
152.4 mm (6 in.) from the point of operation or pinch point
hazard.
IMPORTANT
Determining Stop Time: The measurement of stopping time
(Ts) must include the stopping times of all devices in the stop
circuit. Not including all device and control system elements
when you calculate Ts results in an inaccurate safety distance
calculation.
IMPORTANT
ANSI B11.19 1990 E4.2.3.3.5 states “The value of the hand
speed constant, K, is 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 must consider all factors,
including the physical ability of the operator, when
determining the value of K to be used.”