Power cuts that left millions of people across Europe in the dark at the weekend have raised fears that the continent's electricity network is not up to the task. The BBC's Sam Wilson examines the issues.
How did the failure occur?
The German distributor E.ON admitted it caused the blackouts, by switching off a power cable across the River Ems to allow a cruise ship to pass.
This meant areas to the west were left with a power deficit, while cables in the east were overloaded.
Supplies cut out in Germany, France, Belgium, Spain, Portugal, Croatia and Italy.
The EU's Energy Commissioner Andris Piebalgs has called for the European Transmission System
Operators (ETSO) to identify the problem urgently and ensure that such a blackout does not happen again.
Can power be cut too easily?
"It is not unusual to lose a line - this should not have led to this situation," says Marcel Bial, a spokesman for the Union for the Co-ordination of Transmission of Electricity (UCTE).
"Having prepared the switch, [engineers] saw German networks were overloaded. 'Why' will be the main subject of the investigation.
"Did traders not notify them of some transactions, or was there human error?" he asks.
Switzerland, connected to the German network, managed to avoid a power cut because its engineers quickly spotted the drop in supply and boosted domestic electricity output to compensate, said Monika Walser, spokeswoman for the Swiss power grid Etrans.
Is there a problem with the network?
Anticipating demand and managing supply accordingly is one of the main difficulties with energy distribution.
Transmission networks need to keep up with generation technology
Analysts believe investment in Europe's electricity network has not kept pace with the demands placed on it.
Mr Bial says it has not been updated as new energy-generation techniques have come on stream. Wind power, for instance, can suddenly surge if the wind gets up.
This is particular relevant to northern Germany, where the power line was cut, because it has a big wind-power capacity. Demand may also have been affected by a sudden drop in temperatures.
How much energy is transmitted between countries?
The European network is highly interconnected. Every continental EU member both imports and exports electricity across its borders.
France - with an extensive nuclear power network - is Europe's biggest exporter, while Italy is the biggest importer.
How are energy transfers managed?
EU countries are committed to introducing open markets for gas and electricity - where every consumer has a choice of suppliers. The EU has set July 2008 as the deadline. While some countries achieved this goal some time ago, others are making only gradual progress.
But energy supply is not managed by the EU. Individual countries still keep a close eye on their own energy security, and interests can conflict.
France has invested widely in energy projects abroad, but has still not fully opened up its own sector to investment from abroad.
Disputes over mergers between big European energy outfits - like E.ON and Spain's Endesa - have disrupted attempts to co-ordinate European energy policy.
"On the continent there is interconnection, allowing large flows across borders. But it is not one grid, we're not there yet," says Laura Schmidt of Britain's Association of Electricity Producers.
What is the situation in the UK?
Britain has only one interconnector, with France. It can only carry about 4% of Britain's electricity flow, so a breakdown is not likely to affect the UK too badly, says Ms Schmidt.
A decision is due soon over whether to build a second interconnector, with the Netherlands.
"The idea is to make supply more stable, not less - to add security," she says.
"But in practice, [the interconnector] is often used just to buy elecricity from France when it is cheaper there."
What is the solution?
It depends who you ask.
Italian Prime Minister Romano Prodi called for a central European power authority.
"We depend on each other but without being able to help each other, without a central authority," he said.
EU Commissioner Piebalgs said: "Events in one part of Europe impact on other parts and once again confirm the need for a proper European energy policy."
But many analysts blame a lack of investment by power companies and say they must invest in their networks.
"We have found that in the 1990s, around 2.5% of the network's total values were reinvested into the grids and now it is only around 1%," said a spokesman for Germany's BDE energy consumers' group.
German Environment Minister Sigmar Gabriel said: "We have known for a while that there are bottlenecks on the power grids and that the utilities have not ensured that the grids are being expanded... now they need to deliver on their promises."