A DEHAVILLAND 8-100 IN CRUISE AT 13000 FT HAD A TRANSFORMER RECTIFIER CIRCUIT BREAKER TRIP FROM AN ICING CHKLIST TEST. CAPT DECIDED CIRCUIT BREAKER COULD BE SAFELY RESET. OP NORMAL AFTER RESET.

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FLT OF A DHC100; EXPERIENCED A CAUTION WARNING LIGHT FOR THE L TRU (TRANSFORMER RECTIFIER UNIT) IMMEDIATELY AFTER COMPLETING THE 'BEFORE ENTERING ICING CHKLIST.' THE CREW REVIEWED THE ABNORMAL CHKLIST FOR L TRU CAUTION LIGHT ON. THE CHKLIST CALLED FOR THE L TRU CIRCUIT BREAKER TO BE PULLED. THE FLT CREW OBSERVED THAT THE L TRU CIRCUIT BREAKER WAS ALREADY TRIPPED. IN THE CAPT'S JUDGEMENT; THE TRU CIRCUIT BREAKER HAD TRIPPED DUE TO THE HVY ELECTRICAL LOAD RELATED TO THE EQUIP TURNED ON DURING THE 'BEFORE ENTERING ICING CHKLIST.' ALTHOUGH THE FCOM (FLT CREW OPERATING MANUAL) PROVIDES THAT A CIRCUIT BREAKER CAN BE RESET ONE TIME FOR ESSENTIAL EQUIP ONLY AND CONSIDERING THAT 1 TRU CAN BE MEL'ED AND THEREFORE COULD BE CONSIDERED NON-ESSENTIAL; IN THE CAPT'S JUDGEMENT THE TRU'S WERE AN EXTREMELY IMPORTANT COMPONENT OF THE DHC8 ELECTRICAL SYS AND FLYING WITH 1 TRU DISABLED BY LEAVING THE CIRCUIT BREAKER TRIPPED WOULD PLACE A POTENTIALLY UNNECESSARY RISK ON THE FLT (IN CASE THE REMAINING TRU FAILED). THE CAPT DECIDED THAT THE L TRU CIRCUIT BREAKER COULD BE SAFELY RESET ONE TIME IF THE SUSPECTED CAUSE; THE ELECTRICAL LOADS RELATED TO THE 'BEFORE ENTERING ICING CHKLIST;' COULD BE ELIMINATED. INCLUDED IN THE CAPT'S DECISION WAS THE KNOWLEDGE THAT THE ACFT HAD RECENTLY EXPERIENCED A R TRU CAUTION LIGHT ON. THE R TRU HAD BEEN MEL'ED RECENTLY AND THE MEL HAD BEEN CLRED PRIOR TO FLT'S RELEASE THAT MORNING. IN FACT; THE CAPT HAD JUST DISCUSSED THE TROUBLESHOOTING AND DIAGNOSTIC TESTING AND STATUS OF THE TRU'S AND THE RECENT MEL WITH MAINT PRIOR TO OUR DEP. WE WERE CRUISING IN CLOUDS AT 13000 FT AT AN SAT OF APPROX 3 DEGS C. ICING CONDITIONS FOR THE FLT COULD BE DEFINED AS 5 DEGS C OR LOWER PLUS VISIBLE MOISTURE. THE CAPT WAS ABLE TO ESTABLISH THAT A QUICK DSCNT OF ONLY 1000 FT WOULD TAKE THE ACFT OUT OF ICING CONDITIONS AND ELIMINATE THE NEED FOR THE ICING EQUIP ELECTRICAL LOADS. THE CREW DISCUSSED THE SIT BTWN THEMSELVES AND INCLUDED THE FAA INSPECTOR WHO WAS ON BOARD IN THEIR DECISION-MAKING PROCESS. THE CAPT CONFIRMED VISUALLY THAT THE ACFT WAS NOT EXPERIENCING ANY NOTICEABLE AIRFRAME ICING AND THE CAPT ELECTED TO DSND; LOAD SHED THE ELECTRICAL LOAD FROM THE ICING EQUIP; AND THEN RESET THE L TRU CIRCUIT BREAKER ONE TIME. FLT CONTINUED TO ITS DEST WITHOUT FURTHER INCIDENT AND THE L TRU CAUTION LIGHT DID NOT COME ON AGAIN AND THE L TRU CIRCUIT BREAKER DID NOT TRIP AGAIN.