In computer system administration, we often speak of the “cutover” from one system (hardware, software package, etc.) to a new one, in which the goal is to minimise the service outage and disruption to users. In the telephone business, sometimes when they say “cutover”, they really mean it. In 1984, a Pacific Telephone central office in Glendale, California upgraded from antiquated step-by-step (SXS) electromechanical switching gear to the then new Electronic Switching System (ESS). This was done by installing the ESS hardware in an adjacent location and physically connecting it to all of the 35,000 lines served by the office, while they remained connected to the SXS gear. When everything was ready, after verifying no emergency calls were in progress, all of the cables connecting the lines to the SXS equipment had to be physically cut, then the ESS was activated, taking over switching on all of the lines.
Downtime? Forty-seven seconds.