To understand why the RCCB rips when the lighting occurs, we have to go through the normal workings of an RCCB.
A residual current circuit breaker is generally called an RCCB, and it works by sensing the residual current or leakage current in the circuit.
It is a device designed to protect devices and people from electrical earth faults. An RCCB-connected electrical circuit continuously monitors the incoming and outgoing current through it.
This means that the RCCB always checks that the current flowing in the phase line (s) and neutral line are the same.
If there is any variation or unbalance in the current value, the RCCB will work and the circuit will be isolated.
The variation in the current between the two lines means that there is some leakage current anywhere in the circuit.
In a healthy circuit, the phase current flows through the load and returns to the RCCB through the neutral line.
If the phase line comes in contact with any earthed path, including humans, there will be current flow through the earth.
RCCB senses this current flow and disconnects the circuit to protect the device or life from electrical shock or any other hazards.
Here we have seen that whenever there is an unbalanced current between the phase line and the neutral line of the RCCB, it will function as a safety device.
Then let us understand why it trips when lightning strikes.
When lightning occurs heavy voltage is developed in overhead electrical lines. Most of the time, it produces transient or surge voltages and currents in the transmission and distribution lines.
It may affect the phase and neutral lines simultaneously. But it should be noted that on the distribution side, the neutral lines are always earthed to keep their zero ground potential.
So whenever electrical lightning occurs on a neutral line, it will be directly grounded to the earth. But when it occurs on the phase line, it tries to flow to the earth through the nearest available earth path.
Suppose lightning occurs on the phase line in your area. It may flow to earth through your earthing circuit.
Here the lightning current (surge current or transient) in the phase line is not returning back to the neutral line as it is bypassed to earth. So RCCB senses an unbalancing of current flow and it starts to operate.
Thus, the devices connected to the electrical circuit will be isolated and protected from the high lightning surge currents.
But you should note that an RCCB is not a suitable device to handle huge lightning transients or surge currents.
For this, we have to use a dedicated device called a surge protection device (SPD) or surge protector.
So we can conclude that when lightning occurs, some transient current or voltage develops in the electrical circuit, through which leakage or residual current flows to the earthing circuit.
RCCB identifies this residual current and functions to disconnect the circuit.
I hope that now you have an idea why the RCCB trips when lightning strikes.
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