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| Control Reliable Interlocking |
What is the advantage of a control reliable gate interlock, for example, over a conventional device, given that OSHA considers gate interlocks to be control circuit devices regardless of their rating? If I upgrade an interlock to a SIL 4, control reliable device, I am still only permitted to perform routine service tasks associated with the production cycle of the machine. I cannot set up, unjam, etc. w/out de-energizing and installing what OSHA considers a physical lockout device.
Omron STI Answer: There is no simple answer to this one, because the question itself contains a number of references that are commonly used, but frequently misunderstood. Let's try to sort through the issues.
First is the issue of a control reliable gate interlock versus a conventional one. The use of conventional, i.e., non-safety-rated, components for safety applications is not recommended. Two of the main reasons are:
a. conventional switches are easily bypassed allowing access to the hazardous area.
b. conventional switches may fail in a dangerous condition, allowing exposure to the hazard.
The use of safety-rated components ensures that any failure that occurs will be to a safe condition, and bypassing a safety-rated switch is more difficult.
Achieving a control reliable interlocking solution is a result of the design of the entire safety-related control system through the use of redundancy and monitoring of the system.
Second, SIL is an acronym for Safety Integrity Level which comes from the standard IEC 61508, Functional Safety of Electrical/Electronic/Programmable Electronic Safety-Related Systems where it is discussed in detail. SIL 3 is currently the highest level for machine guarding applications. SIL 4 is typically not used when discussing machine guarding. Regardless, of whether a SIL 3 level is implemented, the requirements under OSHA for set-up, clearing jams, etc. are unaffected. There are, however, a variety of techniques that can be applied that may provide some relief from the stringent lock-out rules.
Without knowing more about the specific application it is not possible to provide a more definitive answer, but we suggest having a machine guarding assessment done on the machine to ascertain the various risks, and develop a comprehensive guarding plan that will provide both the safety and productivity that is required.
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