A Residual Current Device, or RCD, is a device which disconnects electrical current whenever a dangerous situation such as an electrical leak is detected. This could be caused by the current passing by the body or a person, in which case shutting off the electricity by an RCD can often be a life saver.
The first system to protect people from the hazards of coming into contact with a live conductor was a high sensitivity earth leakage system and was an item known as a Magamp. The Magamp was developed by Henri Rubin for a company called FWJ Electrical Industries in South Africa, where the electrical hazards of the gold mines with their miles of cables in damp wet conditions made life difficult.
Rubin initially developed a cold cathode system in 1955 which operated at 525 volts and had a tripping sensitivity of 250mA. Before this, the best chief balance earth leakage protection systems operated at sensitivities of 10 Amp. The cold cathode system was installed in a number of gold mines and worked dependably, but Rubin was not satisfied and began to work on a new idea. By early 1956 he had produced a prototype second harmonic magnetic amplifier kind chief balance system, which he duly patented. The prototype magamp was rated at 220V 60A and had an internally adjustable tripping sensitivity of 12.5 to 17.5 mA.
Very rapid tripping times were achieved with this new system. This combined well with the high sensitivity and was well within the range of safe current time envelope for ventricular fibrillation. These safety standards had been established by Charles Dalziel of the University of California, who had estimated electrical shock hazards in humans. The prototype was able to trip at a much lower sensitivity, consequently providing additional fire protection which would prove to be very helpful within the mines.
An accident in the Stilfontein gold mining village near Johannesburg where a woman was accidentally electrocuted in a domestic accident resulted in 20 mA earth leakage protection systems being fitted to the homes in 1957. Rubin truly considered using transistors in his design, but with transistors being unreliable at the time, the idea was ditched. In a way, Rubin was a little ahead of his time in terms of the developments he wanted to unprotected to. With the eventual arrival of reliable transistors, Rubin and other companies designed units with this characterize included. However it was Charles F Dalziel in 1961 who can claim fame for developing the unit which we base our residual current units on today. Working for the Rucker Manufacturing Company in the USA, he originally developed a unit known as the Ground Fault Circuit Interrupter, and his original design has been continued and maintained as Residual Current Devices for decades.