BT, of Newgate Street, London, pleaded guilty to breaching Section 2(1) of the Health and Safety at Work etc. Act 1974.
It was fined £600,000 with £60,000 in costs.
Teesside Crown Court heard how two BT Open Reach engineers were working at BT's Darlington Automatic Telephone Exchange on 01 April 2010.
One of the workers was installing a cable through a hole on the first floor along a ceiling level cable tray to the Main Distribution Frame (MDF) on the ground floor.
While using a stepladder to carry out the work, the engineer felt a pain in his arm and fell from the ladder. He was taken to hospital with head and back injuries.
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The accident was not properly investigated and on the same day a second engineer started to continue the work.
However, he also fell to the ground and was taken to hospital with serious skull and back injuries.
The first worker returned to BT a year after the accident. However he had lost his sense of smell and taste and required physiotherapy for a number of years.
The second engineer sustained serious multiple fractures of the skull and spine, his sense of smell and taste had been affected, he was blinded in one eye, and has long term memory problems.
A Health and Safety Executive (HSE) investigation found the work had not been properly assessed or planned, despite workers being exposed to such serious risks as working at height close to an electrical system.
The electrical lighting system in that area was also found to have serious failings, where workers were exposed to live metal parts, some at 240 volts.
As the system was poorly built and had not been properly maintained/tested, it is most likely both injured workers received electric shocks which through them from the stepladder.
HSE inspector Laura Lyons said: "Work at height and working close to electrical systems needs to be properly assessed and planned so that adequate controls can be put in place. This duty rests firmly with the employer.
"These life changing incidents could have been avoided if BT had provided safe systems of work and ensured that the electrical systems were properly constructed, maintained and tested."
(LM)
