The main reasons are as follows:

1. Poor Installation 

Loose connections at various terminals can cause them to heat up, oxidize, and damage the insulation of the wires. This can produce sparks and a burning smell, leading to undervoltage in the circuit and tripping the circuit breaker.

2. Mismatch Between RCCB and Load 

The actual household load exceeds the rated current of the low-voltage circuit breaker. This often occurs after installing new high-power appliances like air conditioners or electric water heaters. The circuit breaker needs to be replaced with one that matches the load.

3. Electrical Appliances or Circuit Leakage/Short Circuit 

If an appliance is leaking electricity, unplugging it and resetting the circuit breaker can restore power. If the issue is with the circuit, an electrician should address it. To troubleshoot, turn off all branches, then restore power to each branch one by one. If the breaker trips again, the fault lies in that branch. Fix the issue and then restore power.

4. High Incoming Line Voltage 

This is very dangerous and typically occurs in three-phase four-wire residential buildings. Check if both wires are live, if neighbors are experiencing the same issue, and measure the incoming voltage with a multimeter. Do not forcibly reset the breaker as it can damage appliances or cause a fire.

5. Defective RCCB First, check the quality of the RCCB. 

Use the test button to see if the breaker trips properly. If it fails to close when unloaded, the RCCB is faulty and should be replaced (self-repair is not recommended).

6. Incorrect RCCB Wiring 

Check if any appliance’s live wire is connected before the RCCB, causing an imbalance in the neutral current and tripping the breaker.

7. Electrical Equipment Leakage 

If equipment has a live metal casing, use a test pen to identify the fault. 

8. Reversed N and PE Wires at the Socket 

Reversing these wires creates a zero-sequence current, causing the RCCB to trip.

9. Lighting Circuit's N Wire Connected to the Socket Circuit 

Modern homes have separate circuits for lighting and sockets. If an incandescent light’s N wire is connected to the socket circuit, it causes a zero-sequence current, tripping the RCCB. To check, turn off the socket power (both live and neutral wires). If a light does not work, its N wire is connected to the socket circuit. Locate and correct the wiring issue.

10. Moisture in the Circuit Causing Leakage and Tripping 

Check the junction boxes in the kitchen and bathroom. If they are damp or have water droplets, this is due to a damaged waterproof layer above. Open all associated boxes for natural drainage and drying, re-insulate all connections, and allow 1-2 days for the circuit to dry. Resolve any waterproofing issues to prevent future problems.

After addressing these issues, if the household RCCB (which combines overload and short-circuit protection with the circuit breaker) still does not reset, check the terminals and incoming voltage. If these are fine, disconnect each group of outgoing wires to check for internal faults. If no faults are found, the RCCB may need to be replaced.