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Pete Archer - Product Specialist
June 2018
Why do I need dual-channel safety?
To answer this, we need some basic background information.
First, why is safety needed? Here are 4 good reasons:
1. To Protect Employees
2. To Reduce Insurance Costs
3. To Satisfy Government Regulations
4. To Minimize Liability and Claims Exposure
Second is the concept of Control Reliability.
From ANSI B11.20-1991Paragraph 6.13, The control system shall be designed, constructed, and installed such that a single component failure within the system does not prevent stopping action from taking place - but will prevent successive system cycles until the failure has been corrected.
• Any single fault shall be detected and shall not lead to a loss of the safety system. (Multiple faults can result in a loss of the safety system). • Successive machine cycles shall be prevented until the fault is corrected. • This strongly implies: o Redundancy o Cross Monitoring o Fault Detection
Redundancy leads to dual-channel safety.
The duplication of control circuits and/or components such that if one component or circuit fails, the other (redundant) unit will still be able to assure machine shut-down.
Risk Assessment
For those who do not want to go deep into Risk Assessments (will go a little deeper at the end of this discussion).
• If an injury will heal to its original state then the risk is probably Safety Category 1. • If an injury results in permanent damage then the risk is probably Safety Category 3.
**** Please note: The above 2 bullet points are not a substitute for a formal risk assessment. **** For Safety Category 3 and above, you will need Control Reliability Redundancy Dual Channel
Category 1 All conditions of category B apply, but the safety related system must use “well-tried” principles and components: o Avoid certain faults o Reduce probability of faults o Detect faults early o Assure the mode of a fault o Limit the consequences of a fault
Category 2 o All conditions of Category B apply. In addition, the machine shall be prevented from starting if a fault is detected upon application of machine power. o Single-channel monitoring is permitted provided input devices are periodically checked. This check may be manual.
Category 3 o All conditions of Category B apply. In addition, the safety control system must be designed such that a single fault will not lead to the loss of the safety function, and, where practical, the single fault will be detected. An accumulation of undetected faults may lead to a loss of safety function. In addition, successive machine cycles shall be prevented until the fault is corrected. o This requires redundancy and self-checking in the interface relay and dual-channel monitoring of the input devices.
Category 4 o All conditions of category B apply. In addition, every single fault must be detected at or before the next demand on the safety system. If this is not possible, then the accumulation of undetected faults must not lead to the loss of the safety function. o The number of allowable multiple faults will be determined by the application, technology, and system structure.
A bit on Safety Circuits:
A simple safety circuit: Safety Category B if components selected properly.
Possible failure modes:
• Switch contact weld • Contact spring failure • Short in wiring • Break-in wiring • Actuator failure/Key break • Switch manipulation • Motor contactor/control relay weld
Improved safety control system: Safety Category 1
Improved reliability by adding another positive break contact to the switches---Redundancy
• Lacks fault detection
• Still not control reliable
Control Reliable
• Fault recognition
• Cross monitoring
• Self-checking
Black Box (Safety Controllers - really should be discussed in a separate discussion)
• Safety system fault detection and control modules that detect (and locate) open machine guards and safety system component faults. • Typically feature a redundant, self- checking circuit design and positive-guided control relays.
**Drawings and Diagrams are complimentary of Schmersal Safety Solutions http://www.schmersal.com/en/home/
