Without RCD protection, you are looking for a low Zs that will, in the event of an earth fault, give rise to a large enough current to trip the MCB in a timely manner - in other words in the event of an earth fault you are trying to maximise the fault current that is allowed to pass through the circuit (and any hapless soul who happens to be providing a fault path), which will then trip the MCB more quickly. So what you are doing there is limiting the amount of time that someone is experiencing an electric shock. You are also limiting the amount of time that the cable is subjected to current above its natural current-carrying capacity, thus preventing irrepairable damage. Worst case is a slowly rising earth fault current due to high impedance, which may just trip the MCB at some point, but certainly not quickly enough to save someone's life.
When you add RCD protection, you are limiting the voltage on that fault path from rising above 50V. Fifty volts is considered a relatively safe voltage for the human body to handle, which is based on two facts:
1. the natural resistance of the human body with wet or broken skin (worst case scenario) being typically 1000ohms.
2. the human body can handle 50 milli-amps but any more than that may cause fibrillation and kill you
Using ohms law, V = 0.050A * 1000ohms = 50V
In other words at 50V, the body has 50 milli-amps passing through it, which would hurt but would not kill you (typically). The current is proportional to the voltage, so if the voltage rises above 50V, then the current will rise above 50 milli-amps.
So where does the 1667ohms come in?
For a 30mA RCD, V=0.030A * 1667ohms = 50V
Any Zs reading less than 1667 will give a smaller potential voltage, and because the resistance (typ 1000ohms for the human body) is constant, then the current passing through that body will be proportionally smaller with the voltage.
