A "Fail-safe" circuit principle in semiconductor and solid-state systems

This paper deals with the problem of securing greater safety from shock hazard due to leakage to ground in installations involving portable tools, swimming-pool wiring, and hydrotherapy equipment, and proposals for "built-in" protective systems. Criticism has been directed at the use of transistors on grounds of reliability, and it has been required that such a system must "fail safe." The construction and performance of a compact test model, a protective element embodying magnetic amplifiers and a single diode, which fails safe if the diode fails either as open circuit or as short circuit, is described. This device will handle the starting current of a half-horsepower motor and will disconnect a load with leakage current to ground of less than two milliamperes.