B737-800 FLT CREW HAS A 'HYD SYS B' CAUTION LIGHT; DECLARES EMER AND COMPLETES THE QRH CHKLIST.

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DURING DSCNT ON ARR TO CVG; MASTER CAUTION LIGHT CAME ON WITH HYD SYS LIGHT. CAPT STARTED QUICK REACTION CHKLIST FOR 'B' SYS FAILURE. WE CONTINUED TO UNEVENTFUL LNDG AT CVG. AFTER LNDG WITH ACFT SHUT DOWN ON TXWY; WE DISCUSSED THE EVENTS. CAPT ASKED IF DURING THE FLT I HAD TURNED OFF THE ENG #2 AND #2 ELECTRIC HYD PUMPS. I TOLD HIM I HAD NOT. HE STATED THAT DURING THE QRC CHKLIST PROC HE HAD FOUND BOTH PUMP SWITCHES IN OFF POS AND HAD ASSUMED I HAD TURNED THEM OFF. PREVIOUS TO THE MASTER CAUTION WARNING LIGHT; WE HAD DSNDED THROUGH A CLOUD LAYER AND HAD TURNED ON ENG HEAT/ANTI-ICE SWITCHES. THESE SWITCHES ARE IDENTICAL TO; AND LOCATED DIRECTLY ABOVE; THE 'B' SYS HYD PUMP SWITCHES. MOST LIKELY; INSTEAD OF TURNING ENG ANTI-ICE OFF; I INADVERTENTLY TURNED OFF BOTH B SYS HYD PUMPS; CAUSING THE 'B' SYS LOW PRESSURE. SUPPLEMENTAL INFO FROM ACN 696798: I GOT OUT AND COMPLIED WITH THE LOSS OF SYS 'B' NON NORMAL HYD CHKLIST. WHEN I GOT TO SYS 'B' HYD PUMP SWITCHES OFF; I FOUND THAT THE PUMPS WERE ALREADY OFF. I ASSUMED THE COPLT HAD TURNED OFF THE PUMPS WHILE I HAD MY HEAD DOWN IN THE CHKLIST. EVIDENTLY HE HAD TURNED THE PUMPS OFF BY MISTAKE INSTEAD OF THE ENG ANTI-ICE AFTER COMING OUT OF IMC.