short circuit

Because it is made that way!

RCD or Residual Current Device is provided in electrical circuits to protect the user from electrical shocks or to trip off the system when there is a leakage current in the system . When the leakage current exceeds pre set value of 4-6 milliamps for 25 -40 milliseconds the device trips off the supply . This works with a differential current transformer which measures the differential current between phase and neutral. So if earth is connected to neutral the neutral current becomes zero and the differential current trips off the RCD device.

Yes indeed, with a fuse and thermal magnetic trip there is no disconnection if the fault occurs after that circuit impedance (neutral earth), as there is no effect on the circuit impedance causing excess current.

In an RCD the Live conductor and its associated neutral both pass through a toroidal current transformer, provided the current flowing in the Live is the same as the current returning in the neutral, then no current is induced in the transformer. If the current is not the same then a current is induced in the transformer by the out of balance state and as the transformer is connected a trip solenoid then the trip operates causing the disconnection.

It does not matter where the lost current is going - it could be a neutral earth wiring fault, or it could be you getting an electric shock - the RCD just detects and imbalance. So the circuit or equipment does not necessarily require an earth conductor to operate an RCD and it can protect Class 2 equipment.

With a neutral earth fault the return current is split between the neutral and earth paths with the full current not flowing back through the RCD

The imbalance state will also occur if a neutral is borrowed from another circuit as in this case the current retuning internal would be greater than that in the Live.

So for a 30mA RCD on a circuit with a 10A load, it will trip when the neutral current falls to 9.97A

For a 100mA RCD it will trip when the neutral current falls below 9.9A

An RCD is designed to detect differences between the live conductor current and the neutral conductor current downstream of it. By inadvertently connecting the neutral to earth at the appliance, some of the current that would otherwise go along the neutral is going along the earth conductor instead, resulting in a neutral current that is lower than the live current. The device sees a difference between neutral and live larger than its setting and, quite correctly, trips off as it is designed to do so as to provide protection.

The device is highlighting that there is something wrong downstream, as indicated in the posted question. It is placed there because the risk to personnel downstream is sufficient for a higher level of protection than a simple fuse or over-current circuit breaker can provide.

Don't bypass the RCD to get things going again, as this act could expose personnel to a high level of risk of electric shock, which might prove fatal. Take the time and trouble to find the faulty neutral-to-earth connection and eliminate it, so that the RCD doesn't trip.

Just share some experiences, I had been worked in one plant which whole plant neutral and earth were connected together. I do not know whether they used any RCD or ELCB but the plant is running well. This make me confused those day, as until this moment i still do not understand the advantages and disadvantages of the joinning of neutral and earth. Can any advice on this method?