It is interesting to see your News letter on "Earth fault box. Back in 1957 I worked ias an electrician n south Africa for a swedish born electrician by the name of Charlie Olsen. He experimented with a device using a Big & I mean BIG black box in conjunction with a Miniture Circuit breaker to disconnect a circuit from the mains if a current passed from the live "hot" or Neutral to earth. His aim was that this current should be disconnected before it could hurt a Fly. We Constructed a cage of pins wired alternatly to the two wires with a cork floor and roof & put a fly inside. He kept making adjustments to the Coils,transformers,resistors etc in his box.He kept all these a secrete from all. eventually he had the correct settings. The Fly shorteed out the pins and the circuit Breaker tripped and the Fly was snone the worse. Charlie had attained his goal. Unfortunatley he got no regognition for his work. He wanted to sell the device to the various manufacturers of the Circuit Breakers for a really low royalty. He did not want an outright sale as he prdicted that this device would be a long lasting and high volumne seller. In this prediction he was spot on.They have sold many millionsover the years and willdefinitley sell for many years to come in ever increasing volumns He always said that he wanted it to provide for his family long after his death. The various manufacturers did not want to play ball and despite the numerous copywrites he had he could not patent the idea. These Earth Leakage units were eventually manufactured by the various firms . Today it is compulsory to fit these EarthLeakage trip switches (RCD's) to all electric installationsin South Africa. Perhaps he should of sold out & at least got some small compensation for his forsight.