As testing and design progress, some modules may still not meet specifications. At this point, consider the same four alternatives that were discussed at the end of the previous section. Assuming all is progressing satisfactorily, testing processes and result documentation provide valuable and necessary information for validating redesigns and maintenance. Final testing A test plan should verify that the newly designed system meets all specifications. It is desirable that testing is time invariant, which means that once a system tests as valid, it is valid for all time.
Software is time invariant.
Excluding the software virus, any software bugs that show up after the equipment has spent time in the field were originally shipped in the new equipment. Hardware, however, is not time invariant since mechanical, thermal, and electrical stresses eventually wear out equipment. Unfortunately, testing can only identify existing faults. It cannot verify the complete absence of faults. Power systems are far too complex to permit exhaustive testing from both a time and an economic perspective.