Allen-Bradley PowerFlex 525 Application Technique page 7

Safety Distance Calculations
Detailed calculation of a proper safety distance is beyond the scope of this document, but some considerations follow.
Safeguarding systems must make certain that a person cannot reach a hazardous motion before the safeguarding system has
brought that hazardous motion to a halt. This is addressed in safety standards relevant to this application:
• IS0 14119 (Safety of machinery - Interlocking devices associated with guards - Principles for design and selection)
• ISO 18355 (Safety of machinery - Positioning of safeguards with respect to the approach speeds of parts of the
human body)
• ANSI B11.19 (Performance Criteria for Safeguarding)
Safety Distance and Access Time
Safety distance is the distance between the guarded access point and the hazardous motion necessary to make certain that a
person cannot access a hazardous motion before it is stopped, that is, the hazard has ceased.
This document uses, as an example, an interlocking device (SensaGuard switch) monitoring a partial-body access gate.
Imagine that this access gate allows a person time to reach their arm 762 mm (30 inches) into the potentially hazardous area
to perform an occasional, necessary task.
ISO 14119 3.22 defines access time as the time taken by a person to reach the hazard zone after initiation of the stop
command by the interlocking device (the SensaGuard actuator moving beyond sensing range), as calculated on the basis of
an approach speed of the body or part of the body, in our case, a hand.
ISO 13855 defines the approach speed of a hand as 1600 mm per sec. Using this value, we calculate the access time:
762 mm/1600 mm per sec or 476 ms
ANSI B11:19 defines the approach speed of a hand as 63 in. per sec. Using this value, we calculate the access time:
30 in./63 in. per sec or 476 ms
Overall System Stopping Performance
ISO 14119 6.2.1 stipulates that the overall system stopping time for a hazardous machine safeguarded by an interlock must
be less than the access time. If the overall system stopping performance is equal to or greater than the access time, an
interlock with guard-locking must be used, the distance from the safeguard to the hazard must be increased, or a different,
more suitable method must be used to safeguard the hazard. In this document, the overall system stopping performance of
our application, using an interlock must, therefore, be less than 476 ms.
The overall system stopping performance of a safeguarding system must be determined by actual system testing and
IMPORTANT
measurement. The worst-case, overall system stopping performance from these tests and measurements must be used to
evaluate the safety distance requirements.
The overall stopping performance of these applications is the sum of the response time of the input device (SensaGuard
switch), the response time of Guardmaster dual-input safety relay, the response time of the Guardmaster multifunction-
delay expansion module, the delay configured in the Guardmaster multifunction-delay expansion module, the safety
reaction time of the drive used (PowerFlex 525 or PowerFlex 527 drive), and the coast-to-stop time of the hazardous
motion. The response and reaction times can be taken from the product support literature.
Safety Function: Actuator Subsystems – Stop Category 1 via the PowerFlex 525 and PowerFlex 527 Drives with Safe Torque-off
Rockwell Automation Publication SAFETY-AT140A-EN-P - May 2015 7