After a major power cut that left a quarter of a million people stranded in London, the company responsible for ensuring electricity reaches its destination is coming under scrutiny.
What is the National Grid?
It is the system of transporting electricity from the generators to the regional suppliers all over the country.
The grid for England and Wales was set up in the 1930s, enabling surplus capacity at some power stations to be used to supply new areas.
In Scotland, it is run by Scottish and Southern Energy and ScottishPower.
Northern Ireland has a single system of transmission and distribution.
Does it generate or supply electricity?
The privately owned National Grid is prevented by the terms of its licence from doing anything but transport the power between generators and suppliers.
It charges both for using its system, but one of its most challenging tasks is to get the balance right between supply and demand.
Electricity has to be generated as it is required - minute by minute, second by second - because it cannot be stored in any great quantity.
How does the National Grid ensure there is enough electricity available to meet demand?
An idea of how much power generators are likely to provide and likely demand is available up to a year ahead.
So the respective companies have a good indication of likely peaks and troughs.
At times of high demand, generators will expect to provide power at a higher price. The National Grid has a balancing mechanism to fine-tune supply at the last minute.
Isn't there back-up to ensure continuity of supplies even when things go wrong?
The National Grid points to its reliability record of 99.98% as evidence of the effectiveness of its back-up systems.
If it falls below strict limits, it faces financial penalties from regulator Ofgem.
It said Thursday's problems were extremely unusual, with two faults occuring in a matter of seconds.
Some individual electricity users, such as hospitals, have their own back-up generators. The chaos caused by the power cut on London Underground has been blamed on the closure of its last independent power station a year ago.
At the time Transport Minister John Spellar said transferring to the National Grid would mean the tube's power supply would be "more reliable".
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