THESE ARE SIEMENS ECQL1 CIRCUIT BREAKER LOCKS DESIGNED 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 OR EVEN LIGHTING CIRCUITS OR FOR THAT MATTER ANY OTHER BREAKER YOU DON'T WANT ACCIDENTALLY TURNED OFF. THEY FIT MOST ITE / SIEMENS AND MURRAY BREAKERS, BRYANT, GE, CUTLER HAMMER BR ( not tan handles) T B, CHALLENGER HOME LINE BREAKERS (std ones not Sq D) ABOUT THE ONLY BRAND OF STANDARD BREAKER THEY DON'T FIT IS THE FEDERAL PACIFIC STAB LOC'S AND SQUARE D QO. IF YOU WANT TO KNOW IF THEY FIT A PARTICULAR BREAKER MESSAGE ME AND I WILL TRY AND CHECK IT OUT.