AT&T telephone network mechanical switch failure risk

The 1 in 10,000 per 10 years FIT rate that drove the projection of system collapse. Origin of the "FITs" (failures in time) reliability terminology now used across semiconductors.

In about 1925, AT&T, which was called Bell Laboratories back then, the Bell System, was looking at all the mechanical switches they had. People always say that the transistor came out of basic research. No — AT&T in about 1925 was looking at how many mechanical switches they had in their system, and they started something, a reliability group, a bunch of statisticians who were looking at how to make the system more efficient. They projected that sometime by about 1960, 35 years hence, the whole system would come to a screeching halt because the reliability of mechanical switches was about 1 in 10,000 per 10 years. They call it FITs, failures in time. If you're in the semiconductor business, they talk about FITs, failures in time.

The failure-rate calculation (one in 10,000 hours) that drove the Bell Labs search for an electronic switch. Tom references the AT&T Reliability Handbook as a primary data source.

A lot of what we know about statistical process control was developed at Bell Labs in 1925. They wrote the book originally on statistical process control and statistics and manufacturing, because they had to make telephones. And they were looking ahead, because they could afford all these people to look ahead at six percent profit. They asked, what's the growth of the phone business? They had switchboard operators — people sitting there moving plugs around. Those are all mechanical switches. They looked at the failure rates. There's something called the AT&T Reliability Handbook, and it has all the statistics for failure of mechanical switches and bearings and motors and electrical switches.