RCCB tripped after addition of a 4p 2way knife switch

[winayetun]

Problem : addition of a 4P 2 way knife switch and the main house 1st floor RCCB is tripped if the switch changes position.

The simplified and relevant wiring diagram is like this (before addition of a 4p switch):

meter -> main breaker -> RCCB (sub main breaker) for 1st floor -> 3p MCB -> 4P RCCB (pump room) -> MCBs -> 2p knife switch for pump, a light, a chlorinator.

This setup has been working for years and I am able to turn a pump on or off using the 2p knife switch. Recently, I decided to add a 4p swith and the diagram becomes this:

meter -> main breaker -> RCCB (sub main breaker) for 1st floor -> 3p MCB -> 4p two way knife switch -> 4P RCCB (pump room) -> MCBs -> 2p knife switch for pump, a light, a chlorinator.

At this moment, the 4p switch is connected only to the 3p MCB. So, in theory, nothing has changed except the addition of the 4p switch that is not yet connected to a second power source. However, this new setup works only if the 4p switch is already switched to connect to the 3p MCB. This way, I can still control the pump using the 2p knife switch and things work like before. If, however, I want to control the pump using the 4p switch, the sub main RCCB breaker will be tripped.

What could go wrong? Grounding wire problem? The brand new 4p two way knife switch is a simple mechanical device. I doubt it'd be the culprit.