RCBO=MCB+RCD, so its working principle is actually RCCB, RCD combine with MCB.
The working Principle of RCCB RCD:
When electrical equipment with leakage current, there are two abnormal phenomena: A fault occurs the current balance of line and neutral did not matches (imbalance occurs, as the fault current, finds another earthing path of current). The second is that the non-charged metal shell has a voltage to the ground (under normal conditions, the metal shell and the earth are at zero potential).
The basic operating principle lies in the Transformer shown in the diagram containing three coils. There are two coils say Primary (containing line current) and Secondary (containing neutral current) which produces equal and opposite fluxes if both currents are equal. The RCD obtains the abnormal signal through the current transformer detection and transfers it through the intermediate mechanism to make the actuator operate, and the power supply is disconnected through the switch device.
The structure of a current transformer is similar to that of a transformer. It consists of two coils that are insulated from each other and wound on the same core. When the primary coil has a residual current, the secondary coil will induce a current.
The current flowing back and forth in the transformer is equal in magnitude, opposite in direction, and positive and negative cancel each other). Since there is no residual current in the primary coil, the secondary coil will not be induced, and the switching device of the leakage protector is operating in a closed state. When leakage occurs to the equipment shell and someone touches it in time, a shunt occurs at the fault point. This leakage current passes through the human body, The earth.
The work is grounded and returns to the neutral point of the transformer (without a current transformer), causing the current flowing in and out of the transformer to appear unbalanced (the sum of current vectors is not zero), and the primary coil produces residual current. Therefore, the secondary coil is induced, and when the current value reaches the operating current value limited by the leakage protector, the automatic switch trips and cuts off the power supply.