Charles Dalziel built a shock-proof circuit breaker

By the early 1960s, American homes already had circuit breakers and fuses on every outlet, and the assumption behind them was that overcurrent protection made a circuit safe: a breaker tripped whenever current on a line spiked past 15 to 20 amps, the point at which household wiring itself would start to overheat and risk a fire.

Charles Dalziel, an electrical engineering professor at UC Berkeley who spent years reviewing case files of fatal and near-fatal shocks for expert testimony, kept finding the same pattern: ordinary household ground faults that never came close to tripping a standard breaker, because the current involved was a tiny fraction of the amps a breaker was built to catch. A device engineered to protect the wiring from overheating was never going to be the device that protected the person touching it.

Built a ground-fault interrupter that trips at 4-6 milliamps leaking to ground, a thousandth of a breaker's fire threshold, using let-go current data from volunteers.

Standard breakers only trip at 15-20 amps, far above lethal currents.

Dalziel measured the let-go threshold at 10-16 milliamps.

He built a device sensitive to 4-6 milliamps, catching lethal faults.

The GFCI trips fast enough to prevent electrocution.

When a safety device guards one failure mode, check if a different, smaller-scale hazard needs its own detector.

Because the GFCI protected against a completely different, far smaller current threshold than any circuit breaker on the market, it could catch exactly the ground faults that had been electrocuting people in kitchens, bathrooms and around pools for years without ever registering as a fault to the existing wiring-protection system. The National Electrical Code began mandating GFCIs for underwater pool lighting in 1968, just three years after Dalziel's patent, and steadily expanded the requirement to bathrooms, kitchens, laundries, and outdoor outlets in the decades since.