78 P/N 133487
Banner Engineering Corp. • Minneapolis, U.S.A.
www.bannerengineering.com • Tel: 763.544.3164
SC22-3 Safety Controller
Instruction Manual
Appendix A
Category 3: A dual-channel hookup switching +24V dc is
typically a Category 3 application, because a single failure does
not result in a loss of safety. Loss of the switching action in one
channel is detected by the actuation of opening and closing the
guard, allowing the monitoring function of the safety inputs to
detect the discrepancy between the channels. However, a short
circuit between input channels or safety outputs may not be
detected. It should be noted that an accumulation of faults may
cause loss of the safety function. The principle of fault exclusion
must be incorporated into the design and installation to either
eliminate, or reduce to an acceptable (minimal) level of risk, the
possibility of undetected faults or catastrophic/common mode
failures that could result in the loss of safety function.
Figure A-3. Category 3 circuit
Category 4: The self-monitoring safety inputs can be interfaced
to achieve a Category 4 level of safety. The principle of fault
exclusion must be incorporated into the design and installation
to either eliminate, or reduce to an acceptable (minimal) level of
risk, the possibility of catastrophic/common mode failures that
could result in loss of the safety function.
Figure A-4. Category 4 circuit
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