An RCCB compares the current flowing out on the live wire with the current returning on the neutral. If they differ by more than its sensitivity, some current must be escaping to earth, perhaps through a damaged cable, water in a junction box or a person touching a live part. It then disconnects the circuit in a fraction of a second.
For protection against electric shock, RCCBs with a 30 mA sensitivity are widely used. They are especially relevant in commercial kitchens, where floors are wet and equipment is cleaned with water. An RCCB does not replace an MCB; the two work together, one for overload and short circuit, the other for earth leakage.
Refrigeration equipment occasionally causes RCCB trips because of moisture in heaters, a failing defrost element or water around electrical connections. If an RCCB trips repeatedly, do not bypass it. Unplug equipment one item at a time to find the source and call an electrician or refrigeration technician.