The likely reason for the fire, the cost of which is expected to total more than pounds 10m, has emerged after the establishment of an independent inquiry by Norfolk County Council last month.
Yesterday, Hilary Hammond, the council's director of arts and libraries, said: 'It looks as if the fire was probably caused by a fault in a lighting unit above the shelving of the sort that was fairly typical in 1960s libraries such as this one.
'It seems to have developed in one of the ordinary bays of books which stood against the wall and had an extending shelf at the top to hide the fluorescent striplight which illuminated the books.'
He refused to say whether the information was a direct result of the inquiry, which will report at the end of the year, merely commenting: 'It is what I have been advised.'
The inquiry is attempting to resolve four questions: the cause of the fire, why it spread so fast, the affect of the firefighters' response on its spread, and the difference sprinklers would have made.
Loss adjusters are attempting to put a value on the damage - the fire destroyed 170,000 books and 30,000 volumes from the Norfolk studies stock which were kept in wooden catalogues.
The insurance claim will be the biggest made by a local authority, according to Mr Hammond. The cost of replacing just half the lending and reference books will come to pounds 1m. One of the mysteries of the fire is why it reduced so many books to ash rather than charring their edges, as is normal.Reuse content