Is one layer of a system or subsystem considered adequate protection, in whole or in part, against a specific hazard. An AISL: Is generally one of a number of AISL required as protection against a specific hazards. Cannot be contaminated by the failure of another AISL. Is totally independent of any other AISL. May be a chemical or mechanical design alternate. Must be an "approved mature" EMR, PC, DCS, or microcomputer if electrical controls are used. Must have EMR, PC, DCS or microcomputer with the necessary safety features. Must be able to pass tests for today's process hazard analysis. Is generally one of a number of AISL required as protection against a specific hazards
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