The basic idea is you should not see a dangerous voltage for longer than an acceptable time. Both 'dangerous' and 'acceptable' depend on the circumstances but for something like a sub-main not in a wet/agricultural area it would be your 50V and 5s.
So if you know the current that will disconnect in no more then 5s (say the 170A for a 40A fuse) you can compute the maximum earth impedance such that it is not exceeding the 50V danger level during that period (50V / 170A = 0.294... ohms).
Under a harder fault you would see more than 50V, but the disconnection time would drop rapidly so again you are still keeping within an accepted window of risk.
For something like an RCD of course it might be 30mA (or 100mA for an incomer) trip level with very little time-dependence in most cases, hence the 1667 ohm (or 500 ohm) limit often seen. But there you would still want lower so it is likely to be
reliable, which is why the guidance is 200 ohm max for an earth rod.
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Just to add, if you have measured or computed Zs then typically it contains some of the supply impedance (the R1 part) as well as the earth impedance (R2 + Ze) so if Zs is no more than your computed max Ze value from the above steps then:
- You can always disconnect in under 5s
- During that period your earth voltage must be less than the 50V (as the voltage drop on any finite R1 part serves to reduce the voltage seen over R2 + Ze)