RCCB tripping while checking the current between neutral and ground
This is normal. There can be a slight potential difference between the neutral and ground at a receptacle. (This would be due to a load on the circuit.) If you provide a current path to ground, then current will flow in the neutral path and there would be no corresponding current in the hot. A difference in the current between the neutral and the hot will cause the RCB (GFCI in the US) to trip.

EDIT Another way to look at it is that connecting the neutral and the ground at a receptacle divides any current flowing in the neutral path thereby reducing its value below that flowing in the hot path. This causes a difference in the current flowing in the hot and neutral paths. This will trip a RCB if it is in the receptacle or in the circuit breaker for the circuit or the master breaker.

EDIT2 There has to be enough current flowing through the RCB for a connection between neutral and ground to trip the RCB, that is, the circuit must have a load somewhere causing current to flow through the RCB.

If a circuit is completely unloaded and a low resistance path is established between the neutral and the ground (such as a connection with an ammeter), then the RCB would not trip.

In the case of RCB receptacles they can be in a loaded circuit, but on spur (as they would be if they are connected by pigtailing) or they can be "distal to" (beyond) the location where the neutral and ground are connected. I think the RCB receptacles would trip in those cases, but I doubt this is relevant to the OP's question here. He is probably asking about an RCB breaker.