A B737-300 DURING CLBOUT AT FL270 EXPERIENCED LOSS OF #1 ELECTRONICS BUS. RETURNED TO DEP ARPT. CAUSE OF BUS FAILURE UNKNOWN.

Narrative

WE HAD DEPARTED ZZZ AND DURING THE CLBOUT; WE EXPERIENCED A FAILURE OF THE #1 115 VAC ELECTRONICS BUS; AS INDICATED BY A TRIPPED CIRCUIT BREAKER AND LOSS OF THE #1 ADC; TCAS; AND EGPWS. THIS FAILURE IS INTERESTING BECAUSE THIS BUS CONSTITUTES PART OF THE #1 XFER BUS; BUT ITS FAILURE YIELDS NO WARNING ON THE ELECTRICAL PANEL. SINCE THE WX IN ZZZ1 WAS 3/4 MI WITH SNOW; WE ELECTED TO RETURN TO ZZZ WHERE IT WAS VFR. THE REMAINDER OF THE FLT WAS CONDUCTED WITHOUT INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: WHEN THE #1 INSTRUMENTATION BEGAN FAILING THE CIRCUIT BREAKER PANEL WAS CHKED AND #1 ELECTRONICS BUS 3 PHASE BREAKER WAS FOUND TRIPPED. WAITED 2 MINS AND RESET THE BREAKER AND IT DID NOT RESET. THE QRH DOES NOT COVER THIS FAILURE. THE ELECTRONICS BUS IS POWERED BY THE XFER BUS BUT THE REMAINDER OF THE XFER BUS WAS POWERED SO NO XFER BUS 'OFF' LIGHT WAS ILLUMINATED. WX AT THE DEST ARPT WAS NOT GOOD SO THE DECISION WAS MADE TO RETURN TO THE DEP ARPT. THE CAUSE OF THE FAILED BUS WAS NOT RPTED BY MAINT.