Logical consideration on lockout and trapped key interlock for machine

"Lockout" is an important method for hazardous energy control to protect humans working at a place where they may be injured by unexpected release of hazardous energy. Actually, this administrative control is used in order to compensate for the incompleteness of the ZMS (Zero Mechanical State). This paper proposes the basic requirements for the "Lockout" used for machine maintenance work by applying the "principle of safety confirmation". In view of the above, the relation of "locking up the power switch in the OFF position", "withdrawing and possessing the key for hostage control" and "unlocking the movable guard for accessing to the working space" of the "trapped key interlock", which is alternative to "Lockout", should be made unate in terms of system. This paper formulates these interrelations, presents them in the form of interlock structure, and shows an example of configuration that can meet the required safety functions.