Is there any real connection between the place where the earth fault takes place and the neutral of transformer?
How can earth fault current take the earth as a path to the neutral of transformer?
Is it due to ground grid?
Electrical – Does earth fault current return to transformer’s neutral via earth
mains
Related Solutions
As you are in Europe, the breaker is most likely an RCD ("Residual Current Device") triggered by approx 20ma imbalance between Live and Neutral.
As the Neutral wire has some impedance, it will show some small voltage due to currents returned from other circuits in the house back to the substation, even though the circuit you were working on was disconnected (It was, wasn't it? :-)
So a momentary short from neutral to earth would divert some neutral current to earth, introducing the imbalance which tripped the breaker.
It seems unlikely that you actually shorted a phase to earth, as you did check for "live" voltages. However as the neutral current could be quite large and the earth wire has a low impedance, that imbalance could have been several amps despite the low voltages involved. So it is worth taking the whole house supply off before doing this sort of work.
Here is a simplified version of AC power lines:
simulate this circuit – Schematic created using CircuitLab
The two lines at lower right represent you holding the neutral. Since the neutral is connected to ground elsewhere, as you agree you only feel a small voltage. Note that the load is fairly low resistance, so it can produce a lot of power.
Now let's break the neutral
Current will now flow through the upper hot wire, through the load, through you and then the ground in order to get back to the transformer neutral. And since the load has a low resistance, you are the big resistor in the circuit, and you will take most of the voltage.
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Best Answer
Power distribution practices may vary between countries.
In my area (and probably most of US and Canada) the power company's Neutral conductor is connected to an earth ground rod at each step-down transformer, and at the service entrance for each customer. With this system, a leakage from a hot wire in my home to earth (through a water pipe, for example) will get back to Neutral via the Neutral/Earth connection at my service entrance, and also through the Neutral/Earth connections at nearby power poles.
I consider the name "Ground Fault Circuit Interrupter (GFCI)" used in North America to be slightly misleading - the GFCI will trip if the hot wires (or Neutrals) of two circuits are shorted. Anything that causes the Neutral and Hot currents to be different will trip the GFCI. Earth leakage need not be involved.