Google Cloud blames PLC misconfiguration for London outage

A cable failure at the wrong moment

Google says the root cause was a failure in an external underground cable run by its power supplier, which connects to feeder A. Feeder B was out of action at the time: its circuit breaker had been physically opened for repairs after problems found during a quarterly generator load test on 14 October 2026. Feeder A was therefore carrying the entire critical load.

When feeder A failed, the uninterruptible power supplies, which can hold the zone for five minutes, did their job. But a "voltage-sensing design flaw and PLC misconfiguration" meant the generator plant received no automatic start signal. According to Google, the PLC logic indicated a healthy utility source was available even though both distribution feeds were de-energized, so it issued no start command. The UPSes drained and the zone lost power.

Fixes and follow-ups

After reconnecting to the utility, Google reprogrammed the PLC to prevent a repeat. It says it has made multiple changes to the electrical setup at the zone and tested that they work.

Two action items follow. A "Communications Enhancement" will build an automated alert-to-communication mechanism to speed up notifications for location-impacting incidents. Google also plans to expand platform-wide testing outcomes for infrastructure-level events.

For customers, the incident is a reminder that redundancy on paper depends on control logic working under unusual conditions, such as maintenance windows that reduce resilience. The faster-notification pledge also suggests customers were not told quickly enough as the outage unfolded.