70 Rockwell Automation Publication 450L-UM001D-EN-P - December 2019
Chapter 6 Installation and Wiring
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 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.
Tbm Additional time that is allowed for the brake monitor to compensate
for variations in normal stop 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. Dpf is related to the safety light-curtain object
sensitivity. Object sensitivity is the smallest diameter object that is
detected anywhere in the sensing field (commonly also known as
resolution).
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