With all respect to Ankou........ I don't want to be a know-all, but I'm afraid that won't do it.
The "earth" conductor (which is obviously missing) would make a circuit to carry a fault current back to Earth at the board, and operate the fuse/mcb/trip or whatever.
With no earth wire, a switch could remain live in the event of a fault .......... definitely unsafe.
If there's no way of connecting back to earth, I think your only option is to fit a non-conductive (plastic) swichplate Sandy :o)