At single phase system, when there is a contact point between neutral and earth, would this cause RCCB/RCD tripped? RCCB/RCD current sensitivity is 300mA. Is it a normal behavior and why?
RCCB detects the current flow from Hot line to earth by measuring the current difference of inpout Hot and NULL
in normal operation HOT and NULL inputs has the same amount of current in opposite direction so the difference is zero
but in case of Ground fault the difference is not zero showing that an amount of current has flow from Hot to earth and didnt come back from NULL.
I tested this many times before. You have a good chance that it trips.
But it depends on some details.
hamidmoallemi explained the working principle (almost) correctely.
To make it trip:
* you need current on NULL, best from a load connected between HOT and NULL (as usual). The higher the current the higher the chance to trip.
* a low ohmic connection between NULL and EARTH.
The current follows the path with lowest impedance (Indeed it divides). So if you have a good connection between NULL and EARTH, then (a part) of the load current travels through EARTH...obviously this current is missing when returning via NULL through the RCCD.
The difference (indeed sum) of currents in the RCCD is not zero anymore...if the difference is more than 300mA the RCCD will trip.