Testing and testability

In-service testing provides assurance that the systems important to safety remain operable and capable of performing their safety tasks. The frequency of tests should be established on the basis of the requirements for availability and reliability of the system. Testability — the ability of a system to be tested — should be built in as part of the design. In designing a testable system, it should be considered whether (1) the location of the eqnipment is appropriate, (2) access is suitably controlled, (3) faults in the eqnipment are readily detectable, and (4) the demonstration of continued functionality is condncted in such a way that the safety of the operating plant is not jeopardized. 

The design of I C systems important to safety should include identification of a testing and calibration programme consistent with their availability requirements. 

This test programme should ensure that the functional capabilities of systems and components important to safety are retained. This should include periodic confirmation that design basis requirements such as those for accuracy, response time and set points are met. 

As far as practicable, tests for I C systems important to safety should be overall checks (from the sensors to the actuators), capable of being performed in situ and with a minimum of effort. It is acceptable for the test programme to consist of overlapping tests which together test the whole channel. All the output functions important to safety, such as alarms, control actions and operation of actuation devices, should be tested. 

AU systems important to safety should include provisions that allow performance of the required testing, including built-in test facilities where appropriate. These should themselves be capable of being checked at regular intervals to ensure continued correct operation. Where equipment to be tested cannot be located in non-hazardous areas, fadUties should be provided to allow testing to be conducted remotely from outside the hazardous area 

---

Abramovici, M. et al. 1990. Digital Systems Testing and Testable Design. IEEE Press, New York. 

---