Thanks for your Adds;
Well, our case I think, is caused by other things than water, as we hadn't got any leak of water at all, also the heads are brand new so their tags should be OK. But, just my guessing is that:
1 - The system is earthed to so many points, as the cabling are old Pyro in which the screen is earthed to a lot of metal trunkings....
2 - Pyro cabling run inside electric cupboards and risers close to high voltage cabling of at least 230ac... Where interference between cabling is likely to occur...
The customer refused at this time to change the whole cabling. Although, to find out just the suspicious parts, and replace them is not that easy...etc
Just wondering if we can make earth faults not monitored by the system? Also not easy :/
I am still not sure really