Tripping the equivalent of a circuit breaker incorporated GFCI

In a Middle eastern location my relative was checking out a potential fault that occasioanlly tripped the 'whole house' GFCI/circuit breaker of his living accomodation.
Poorly built 'villas'; often by semi-skilled cheap labour. No fault of theirs because workers are imported, low paid, untrained and with little no proper training or equipment and then sent back when work is finished. Also those temporaraily imported workers have no hope whatever of ever building a future in a new location or being accepted as permanent citizens; their purpose being to earn enough to send back some sort of living to their families in their countries of origin.
There a UK style 230 volt system; two wire (plus ground/earth) with one wire being live at 230 volts and the other neutral (not the North American 115 - 0-115 system) is used.
What my relastive found was the cause of the problem was that at least one of the neutrals in his living unit was someehow shared with something in the next living unit and under certain conditions unbalance could trip 'his' protective equivalent of a whole house GFCI!
Being a technical person he was able to figureit out; although not sure if he was able to cure it.
BTW there was real 3 phase (3 phases at 230 volts to a common neutral coming in to that 'villa' the size of a typical 3 bed, 2.5 bathroom 'town house' with a total of 8 largish 230 volt Air Conditioners on the two flooors!

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