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Old 26th Sep 2008, 19:52
  #1999 (permalink)  
Join Date: Dec 2005
Location: Finland
Posts: 242
Some speculations about R2-5, inspired by the new information about previous RAT heater problems.

From a systems reliability point of view the fact that the four relay contacts control totally different functions implies that a fault indication affecting any of those functions may have side effects on the other functions. This implies that, for example, any fault tracing instruction of the RAT heater system should include a test that can discover if the R2-5 relay has either failed completely, which may have been the case here, or only in the RAT heater circuit.

More generally, I wonder if the electrical fault isolation procedures are generally designed only top-down (i.e. if a system does not work, which components to suspect) or also bottom-up (i.e. if a component fails, which systems are affected). For this particular case, it seems both methods should be used.
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