ELCB tripping is multiple circuits at same time
Anytime a single fault trips two ground fault detectors, the cause is that the circuits are crossed.  

Either the neutrals have been tied together somewhere; the hots have been tied together somewhere; or a particular load is drawing hot from one circuit and returning neutral to the other circuit.  This can and should trip both ground-fault detectors.  

It should be completely OK to share or cross grounds.   Of course a neutral-ground fault will also cause a ground-fault detector trip.  

I go into more detail in my answer over here.  Though the questions seem quite different, they are the same core issue.