Amazon and Microsoft European cloud services were down at the weekend after a lightning strike caused power failures at their datacentres in Dublin.

The lightning strike took out the main power supply and affected part of the phase control system that synchronises the back-up generator plant, causing a disruption to service of Amazon's EC2 cloud computing platform for the second time this year and Microsoft's Business Productivity Online Suite (BPOS).

According to Amazon's service health dashboard, the back-up power sources had to be phase-synchronised manually before power could be restored.

Amazon started bringing up EC2 instances within three hours and had restored 60% of impacted instances within 12 hours.

Microsoft also begun restoring services within three hours and reported in its Twitter feed that all European BPOS services had been restored within four hours.

Dublin is a key cloud computing centre for US companies offering services to Europe because of the city's location, connectivity, climate and supply of IT workers, according to DataCenterKnowledge.com.

The latest disruption in services is bound to raise questions over the reliability of cloud services, as did the previous disruption in April, when a routine upgrade brought down customer sites for four days.

Read Computer Weekly's analysis of what risks businesses should consider in the light of such cloud computing failures.