Amazon Web Services has released details about the root cause of the outage of one of its public-cloud’s availability zones that started in the evening on 14 June and lasted until next morning, US Pacific time.

In a note posted on the cloud’s status dashboard, the company said the outage was caused by a cable fault in the power distribution system of the electric utility that served the data center hosting the US-East-1 region of the cloud in northern Virginia.

The entire facility was switched over to back-up generator power, but one of the generators overheated and powered off because of a defective cooling fan. The virtual-machine instances and virtual-storage volumes that were powered by this generator were transferred to a secondary back-up power system, provided by a separate power-distribution circuit that has its own backup generator capacity.

But, one of the breakers on this backup circuit was configured incorrectly and opened as soon as the load was transferred to the circuit. The breaker was set up to open at too low a power threshold.

“After this circuit breaker opened … the affected instances and volumes were left without primary, back-up, or secondary back-up power,” Amazon’s note read.

Customers in this availability zone that were running multi-availability-zone configurations “avoided meaningful disruption to their applications; however, those affected who were only running in this Availability Zone, had to wait until the power was restored to be fully functional.”

Among the customers affected with downtime were a number of popular web services, including Pinterest, Heroku, Quora, Foursquare and others, according to news reports. Heroku, for example, reported widespread outages of its production and development infrastructures that lasted for eight hours.

The faulty circuit breaker opened around 9pm, Amazon says, and the failed generator was restarted around 10:20pm. Most affected VM instances recovered by 10:50pm, and most cloud-storage volumes were “returned to customers” by about 1am.

Amazon said it had completed an audit of all of its back-up power distribution circuits and found another breaker that “needed corrective action.”

“We've now validated that all breakers worldwide are properly configured, and are incorporating these configuration checks into our regular testing and audit processes,” Amazon said.