A residual current operated circuit breaker (RCBO) is a device that combines the functions of a residual current device (RCD) and a miniature circuit breaker (MCB). The primary function of an RCD is to protect against electric shock by quickly disconnecting the circuit if it detects a leakage of current to the ground, whereas an MCB is designed to protect against overloads and short circuits by interrupting the current flow when it exceeds a certain threshold.

However, an RCBO without over-current protection means it lacks the MCB function. In other words, it only provides residual current protection (RCD functionality) without the ability to trip in case of overloads or short circuits. This configuration might be suitable for specific applications where only protection against electric shocks or leakages is necessary without concern for over-current situations.