Approved Independent Safety Layer (AISL)

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 (See Table 2).
  • Must have EMR, PC, DCS or microcomputer with the necessary safety features.
  • Must be able to pass tests for today's process hazard analysis.