In European power grids passive, electro-mechanical RCDs can do different jobs:
1. in so called "TT-systems" they are mandatory for the whole of the service. In Europe xformers can be pretty far away. TT means that xformer (always star point)ground electrode and your local ground electrode are not bonded. In this case a ground fault will almost never cause an important current to ground able to trip a breaker as the ground resistances will be too high. That's why RCDs are in this case integral part of the system (example: France 2 RCD in series (often 500mA/30mA).
2. Furthermore in any location with an increased danger of fire a 300mA is supposed to prevent fires. Here you have to understand that in Europe there is almost mandatorily an isolated ye/gr ground run alongside every hot wire. In actually Germany we mainly use cables not wires, but that differs. A melting isolation will create a ground contact before external isolation/(plastic!!)conduit is destroyed.
3. For additional protection of persons the same technical principle with a nominal maximum ground fault current of 30mA (Standard)or even 10mA is mandatory for bathrooms, garages , outside in one country, for everything in the next, this depends on local rules and pocos.
With regard to grid forms it is important to understand that the 230 V can either consist of 1 hot and 1 grounded conductor,
or of 1 hot and 1 one conductor only grounded at a xformer a mile away (not accepted as really grounded)
or just 2 hot wires (f.i. Belgium often).
This is why we do not really care about poles , as any of the both "active" conductors in a receptacle can be "not" cold. And therefore a detection of a "bad neutral" is not helpful at all.
