Dalziel and the numbers
Charles Dalziel, a professor of electrical engineering at the University of California, Berkeley, published the studies that still underpin shock-protection standards: the let-go threshold (the current above which a person cannot release a conductor, roughly 10 mA for a man, less for a woman), and the fibrillation threshold, which he put at a level that a 5 mA device would keep well clear of. In 1961 he built a working ground-fault interrupter around a differential transformer and, with the manufacturer Rucker, brought the first commercial units to market a few years later.
Into the Code
- 1968 — the first NEC requirement: underwater lighting in swimming pools.
- 1971 — receptacles near pools; construction sites follow soon after.
- 1973 — outdoor receptacles at dwellings.
- 1975 — bathrooms.
- 1978 — garages.
- 1987 — kitchen countertop receptacles near the sink; 1996 extends it to every countertop receptacle.
- 1990 — unfinished basements and crawl spaces.
- 2014 — laundry areas and dishwashers.
- 2020 — 250 V receptacles up to 50 A, outdoor outlets generally, all pool pump motors.
Each entry followed injuries the earlier rules had not reached. The list in where the NEC requires GFCI is that history written as a table.
The device kept changing
The first receptacle GFCIs were expensive and famous for nuisance trips. Solid-state sensing brought the price down through the 1980s. In 2006 UL 943 required a receptacle to refuse power if the installer swapped line and load — the miswire that had left thousands of outlets unprotected while looking fine. In 2015 the standard required every receptacle GFCI to test itself periodically and lock out when it fails, ending the silent-failure problem (self-test). The portable devices we build carry the same Class A circuit in a plug head or an in-line module.
Abroad
Europe took a different route: the residual-current device at 30 mA, aimed at fire and gross faults as much as at people, installed in the panel rather than at the outlet. The two worlds are compared in GFCI vs RCD.