Safety Rule: Sum of wire lengths over two phases may not exceed X. What is this for?
It is about disconnect times. As the loop impedance rises due to the higher resistance of longer wires the time it takes for a fuse or circuit breaker to open the circuit in the event of a fault increases because there is less current available. 

The UK standards say for example that if the fault can cause a touch voltage larger then 50V then the protection must disconnect the circuit within 0.5 seconds, which requires a fairly large current to be able to flow. 

For this reason a rule of thumb over here is that an extension should not be longer (in meters) then 10 times its CSA (in mm^2), so a 1.5mm^2 cable is good for up to ~15m and a 2.5mm^2 one 25m. Past that, pull in something heavier and break it down near the point of load. 

The German generator thing is probably another case of trying to ensure that touch voltages stay sane, but really with a three phase machine you should be driving in some ground stakes (and measuring the ground impedance).