Google says a PLC misconfiguration stopped its generators starting in the 19 October UK cloud outage, letting UPS batteries run down

What changed

Google has published the cause of the 19 October outage in its europe-west2-b zone, DCD reported on 9 November, first reported by The Register. The UK site has a 2N power design with two utility feeds and a central high-voltage generator bank under dual-redundant programmable logic controllers (PLCs), with UPS batteries giving about five minutes of runtime. The site was temporarily on a single feed for generator load testing when an external underground cable faulted on feed A. Because the PLCs checked for voltage on the line side before starting the generators, and sensed live voltage from the disconnected feed B through shared upstream sensors, they never sent a start signal. The UPS batteries carried the load until they ran out and the data halls lost power. Power was restored from a standby utility connection. DCD says banks and delivery firms, including Lloyds, Revolut, Halifax, DPD and Royal Mail, were affected. Google says it is improving how fast it notifies customers. This is Google's own incident account as reported.

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