A trip unit for use with a plurality of different types of circuit breaker apparatus, each of the plurality of different types of circuit breaker apparatus having a personality module and into which the trip unit is insertable, the trip unit comprising: a communication unit configured to communicate with the personality module of each of the different types of circuit breaker apparatus and to thereby receive from the personality module key breaker personality parameters that relate to operational characteristics of the circuit breaker apparatus; and a controller, including: a memory on which a register, previously stored key parameters relating to general operational characteristics of trip protection, and the received key breaker personality parameters are stored, the controller being configured to control the communication unit to receive the key breaker personality parameters of the circuit breaker apparatus from the personality module thereof in accordance with information determined from the register upon insertion of the trip unit in the one of the plurality of circuit breaker apparatus, and components to execute trip protection when at least one of a current and a voltage within the circuit breaker apparatus exceeds a respective predetermined threshold as defined by the previously stored key parameters and the received key breaker personality parameters, wherein the register comprises numerical data set to a specified value and the controller is configured to recognize the specified value of the register as identifying the trip unit as such. a communication unit configured to communicate with the personality module of each of the different types of circuit breaker apparatus and to thereby receive from the personality module key breaker personality parameters that relate to operational characteristics of the circuit breaker apparatus; and a controller, including: a memory on which a register, previously stored key parameters relating to general operational characteristics of trip protection, and the received key breaker personality parameters are stored, the controller being configured to control the communication unit to receive the key breaker personality parameters of the circuit breaker apparatus from the personality module thereof in accordance with information determined from the register upon insertion of the trip unit in the one of the plurality of circuit breaker apparatus, and components to execute trip protection when at least one of a current and a voltage within the circuit breaker apparatus exceeds a respective predetermined threshold as defined by the previously stored key parameters and the received key breaker personality parameters, wherein the register comprises numerical data set to a specified value and the controller is configured to recognize the specified value of the register as identifying the trip unit as such. a memory on which a register, previously stored key parameters relating to general operational characteristics of trip protection, and the received key breaker personality parameters are stored, the controller being configured to control the communication unit to receive the key breaker personality parameters of the circuit breaker apparatus from the personality module thereof in accordance with information determined from the register upon insertion of the trip unit in the one of the plurality of circuit breaker apparatus, and components to execute trip protection when at least one of a current and a voltage within the circuit breaker apparatus exceeds a respective predetermined threshold as defined by the previously stored key parameters and the received key breaker personality parameters, wherein the register comprises numerical data set to a specified value and the controller is configured to recognize the specified value of the register as identifying the trip unit as such.