Need of ground cable?
The grounded (Neutral) conductor normally carries return current.
The grounding (Ground or Earth) conductor does not.
If a wiring fault opens the Neutral, the separate Earth/Ground conductor provides a return path for fault current. If you only had the Neutral, a neutral fault would lead to the entire thing becoming energized.
Exactly this &quot;logic&quot; was used for electric stoves and dryers in the USA years ago - they had (two Hot or Live, being the US system of 120V-0-120V with 240V between the lines) Hot, Hot, Neutral and no separate Ground. Enough actual problems resulted that code was changed to require Hot, Hot, Neutral AND Ground. There are still stoves and dryers connected that way (since the code system does not require replacing things installed to code when they were installed until you are changing the circuits, usually when remodeling.) But any new work is required to follow the safer standard.
Additionally, if fault current is returning on the Neutral, your RCD (which I understand as a &quot;whole house ground fault circuit interrupter&quot; - terminology differences) won't see a fault, because the current out on Hot/Live will match the current in on Neutral. Those devices look for a mismatch between current out on Hot/Live and current returned on Neutral (or for the US system Neutral or the other Hot) and trip when there is a difference between them, because current is going where it should not. We do use single circuit GFCI's to upgrade older circuits with no grounding conductor since they will trip if current passes out through a person or otherwise not in the circuit, but for the case of a fault that would go to Earth/Ground on the circuit, combining that with neutral would fool the RCD.