THESE ARE SIEMENS BX231M CIRCUIT BREAKER LOCKS TO KEEP IMPORTANT CIRCUITS ( LIKE ALARM, OR SUMP PUMP) CIRCUITS FROM INADVERTENTLY BEING TURNED OFF. THEY WILL NOT STOP THE BREAKER FROM TRIPPING JUST MAKE SOMEONE THINK TWICE ( AND BRING A SMALL SCREWDRIVER BEFORE TURNING OFF A PROTECTED BREAKER ) THEY ARE GREAT FOR PROTECTING ALARM CIRCUITS OR SUMP PUMP CIRCUITS OR ANY OTHER BREAKER YOU DON'T WANT ACCIDENTALLY TURNED OFF. THEY FIT MOST ITE / SIEMENS AND MURRAY BREAKERS, BRYANT, GE, SQUARE D QO, CUTLER HAMMER, T B, CHALLENGER HOME LINE BREAKERS. ABOUT THE ONLY BRAND OF STANDARD BREAKER THEY DON'T FIT IS THE TAN HANDLE CUTLER HAMMERS FEDERAL PACIFIC STAB LOC'S. IF YOU WANT TO KNOW IF THEY FIT A PARTICULAR BREAKER MESSAGE ME AND I WILL TRY AND CHECK IT OUT. THIS IS THE THINNER TANG MODEL WHICH IS PERFECT FOR GFCI, AFCI, TWIN AND OR TANDEM BREAKERS, SEE PICTURES THEY WILL ALSO WORK JUST FINE ON A FULL SIZE HANDLE BREAKER!