According to the national electrical codes prevailing at the location of the flats?
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"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
If local codes permit connect each apartment to a common earth bar and earth it. Or connect apartments in each floor to a common bar and connect them to a bar at ground floor/basement and earth it. Consult local authorities regarding telephone/antenna earthing.