To isolate you need to switch off
All live conductors that includes line and neutral with a domestic consumer unit this means using the Isolator or RCD the MCB or RCBO will not isolate.
The RCD is operated by current not voltage so with the neutral to earth fault there is likely not enough voltage to feel but there is enough current to trip a RCD. With a TN earth system specially a TN-C-S the neutral and earth are bonded very close to the house so very little current will flow
the circuit shows what is happening when the RCD trips open the switch and although fault is still there no current flows.
Oddly we use the same consumer unit for TN and TT supplies however reading the regulations it would seem with a TT supply really we should have twin pole MCB's but this all comes down to history with a fuse we could not link two fuses and we found fusing the neutral was dangerous so we did not fuse the neutral and when we moved to the MCB we followed the same idea and did not use twin pole MCB's however looking at the regulations it may be that to follow to letter with a TT supply we should use twin pole.
In fact when one reads the regulations it does become interesting as it raises the question of what is a circuit which was academic until the Part P regulations said you need to notify the LABC when a new circuit is created.
Our regulations make interesting reading with a TN system each MCB forms a new circuit but with a TT system because the neutral is not switched the MCB does not form a new circuit. However the regulations to the common man are like gobbledegook I have listed some of them below. Can you understand them?
Regulations 314.1 (vi) prevent the indirect energizing of a circuit intended to be isolated, and 521.8.2 The line and neutral conductors of each final circuit shall be electrically separate from that of every other final circuit, so as to prevent the indirect energizing of a final circuit intended to be isolated. Table 53.2 Note (5) In an installation forming part of a TT or IT system, isolation requires disconnection of all the live conductors. See Regulation 537.2.2.1.
However 537.1.2 Combined protective and neutral (PEN) conductors shall not be isolated or switched except as permitted by Regulation 543.3.4.
Except as required by Regulation 537.1.4, in a TN-S or TN-C-S system the neutral conductor need not be isolated or switched where it can be regarded as being reliably connected to Earth by a suitably low impedance. For supplies which are provided in accordance with the Electricity Safety, Quality and Continuity Regulations 2002, the supply neutral conductor (PEN or N) is considered to be connected to Earth by a suitably low impedance.
And 537.2.1.1 Every circuit shall be capable of being isolated from each of the live supply conductors. In a TN-S or TN-C-S system, it is not necessary to isolate or switch the neutral conductor where it is regarded as being reliable connected to Earth by a suitably low impedance.
Provision may be made for isolation of a group of circuits by a common means. if the service conditions allow this.
537.4.1.2 Except as provided in Regulation 537.1.2. where a risk of electric shock is involved the emergency switching device shall be an isolating device and shall interrupt all live conductors.
Except as required by Regulation 537.1.4, where the neutral conductor can be regarded as being reliably connected to Earth in a TN-S or TN-C-S system the neutral conductor need not be isolated or switched.
551.6.2 For a TN-S system where the neutral is not isolated. any RCD shall be positioned to avoid incorrect operation due to the existence of any parallel neutral-earth path.
NOTE: It may be desirable in a TN system to disconnect the neural of the installation from the neutral or PEN of the system for distribution of electricity to the public to avoid disturbances such as induced voltage surges caused by lightning