I don't understand the etiology of the problem myself, but that is what we heard back after the district sent in several electricians. Several possibilities: breakdown of insulation or impurities in insulation, mechanical failure due to stressed parts, or transmission error (district says: shoddy wiring, [they even used the] cheap plugs - the part in brackets gets dropped out, etc.) Anyway, there is a very noticeable difference between the cheapie wiring supplies and the nicer "commercial grade" stuff, similar to the difference between a modern cheap PeeCee and a real computer.