Safety instrumentation and control

The safety functions of instrumentation, control, and alarm systems are to provide information on out-of-tolerance conditions/abnormal conditions; ensure the capability for manual or automatic actuation of safety systems and components; ensure safety systems have the means to achieve and maintain a fail-safe shutdown condition on demand under normal or abnormal conditions; and/or actuate alarms to reduce public or site-personnel risk.

The design of safety-class and safety-significant instrumentation and control systems must incorporate sufficient independence, redundancy, diversity, and separation to ensure that all safety-related functions associated with such equipment can be performed under postulated accident conditions as identified in the safety analysis.

Safety-significant components should be evaluated as to the need for redundancy on a case-by-case basis. Under all circumstances, safety-class instrumentation, controls, and alarms must be designed so that failure of nonsafety equipment will not prevent the former from performing their safety functions.

Safety-significant and safety-class instrumentation, control, and alarm-system designs must ensure accessibility for inspection, maintenance, calibration, repair, or replacement.

Safetyclass instrumentation, control, and alarm systems must provide the operators sufficient time, information, and control capabilities to perform the following safety functions:

 Determine the status of critical facility parameters to ensure compliance with the limits specified in the technical safety requirements

 Initiate automatic or manual safety functions

 Determine the status of safety systems required to ensure proper mitigation of the consequences of postulated accident conditions and/or to safely shut down the facility

ANSI/IEEE standards contain design, installation, and testing requirements that should be considered for instrumentation, control, and alarm components without invoking all of the safety class 1E requirements. See table 1 for the relevant codes.