Engineers should work closely with relay vendors to determine what relay functions are not checked by relay self-tests and how those functions should be checked in the field. In the case of the processing section, there are typically no special tests required. Many of the maintenance features are executable by remote command and often could replace routine maintenance altogether. Also, consider the analysis of digital relay fault data comparable to routine relay maintenance. Those relays which do not encounter faults may require more thorough routine maintenance checks. Digital relay routine testing practices should verify relay functions that cannot be fully verified by the relay self-testing. Figure 9.16 shows how all relay failures can be detected using a regime consisting of: