GFI – A 1970 Article by Charles Dalziel

By Charles Dalziel
Written in 1970 for IEEE Spectrum

Transistorized Ground-Fault Interrupter Reduces Shock Hazard

Even the home outlet can deliver a lethal electric shock. Yet for a decade there have been simple sensitive devices for ridding us of such dangers.

The ground-fault interrupter is the most successful device for eliminating the hazard from low-voltage electric shocks in the home, on the farm, and in industry. It is the newest of four recognized means – isolation, insulation, grounding, and shock interruption – for reducing the danger from electric shock and it is, by far, the most radical.

The ground-fault interrupter, or GFI, is a device that interrupts an electric circuit when the fault current exceeds a predetermined value less than that required to operate the overcurrent devices of the circuit. (In Europe this device is called an earth-leakage circuit breaker.) Such apparatus have been used to protect high-voltage power lines since the 1920s, and they were set to operate at 10 to 20 percent of the minimum operating current, or trip value, of the associated overcurrent devices.

The operating time of these devices is so fast, and the corresponding shock energy so small, that the modern GFIs virtually eliminate electrocutions, burndowns, and fires due to currents flowing to ground. However, it must be recognized at the outset that the sensitive ground-fault indicating mechanism does not respond to line-to-line or three-phase faults unless zero phase-sequence currents are involved.

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Charles Dalziel (1904-1986) was a professor of electrical engineering and computer sciences at UC Berkeley. He was a pioneer in understanding electric shock in humans and animals, and wrote The Effects of Electric Shock on Man, a book in which he explains the effects of different amounts of electricity on human subjects. He is also the inventor of the ground-fault circuit interrupter or GFCI which he invented in 1961. He volunteered his services to many professional, advisory and public service groups, including duty as chairman of the San Francisco Section of the AIEE and vice chairman of the San Francisco Section of the IEEE Power Group.