Rockwell Automation Publication 450L-UM001F-EN-P - March 2022
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Chapter 6 Installation and Wiring
OSHA Safety Distance Formula
The OSHA safety distance formula as specified in CFR29 Subpart O1910.217 is
as follows:
Ds = 63 × T
ANSI Safety Distance Formula
The ANSI B11.19 safety distance formula, which is the formula we recommend
for USA, is as follows:
Ds = K × (Tst + Tc + Tt + Tbm) + Dpf
Ds
Safety Distance in inches (1 in. = 25.4 mm)
63
Is the OSHA recommended hand speed constant in inches per
second (63 in/s = 1600 mm/s)
T
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 to stop the hazardous motion of the
machinery. For a mechanical power press, the stopping time is
measured at approximately the 90° position of the crankshaft
rotation.
IMPORTANT
The “T” number must include the response times of all devices,
including the response time of the safety light curtain, the safety light
curtain controller (if used), the machine 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 results in insufficient safety distance for the application,
which results in operator injury.
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.
IMPORTANT
ANSI B11.19 1990 E4.2.3.3.5 states “The value of the hand-speed constant,
K, is determined in various studies. Although these studies indicate
speeds of 63 inches/second to over 100 inches/second, they are not
conclusive. The employer can consider all factors, including the
physical ability of the operator, when determining the value of K to be
used.”
Tst
Stop time of the machine tool that is measured at the final control
element.
Tc
Response time of the control system
Tt
Response time of the presence sensing device (safety light
curtain) and its interface, if any. The device manufacturer states
this value or you can measure it.
IMPORTANT
A stop time device is used to measure Tst + Tc + Tr.
