The answer to Breezer's question could be the nature in which the wires get pulled. When bunching the wire, all but the earth wire get cut away and are then taped from the cut/s back. The earth wire/s are then used to create a loop that the draw wire is connected to. Depending on the lay of the earth wire, it could be that over time the liquid residue runs down this as opposed to the other wires. Also, it may be that the other wires are effected, but is more apparent on the earth wire. Sorry if this sounds all a bit tentative, but without actually looking at the situation it's difficult to be more specific. The assumption here is, conduit is in use. As for the sockets, these rarely tend to have so many cables in one piece of conduit as to create the same problem.