Teesside Crown Court heard how the two BT Openreach engineers were working at the company's Darlington Automatic Telephone Exchange.
One of the engineers was installing a cable in a ceiling level cable tray to the main distribution frame -- which connects equipment to cables -- on the ground floor.
The engineer was standing on a stepladder reaching up to the cable tray which ran beside the lighting system. He fell off the ladder and was taken to hospital with serious head and back injuries.
BT did not properly investigate the accident and later the same day the cable installation resumed. The second engineer stood on another ladder and also fell off, sustaining skull and back injuries. He was blinded in one eye and has long-term memory problems.
The Health and Safety Executive (HSE) investigated both incidents, which occurred on 1 April 2010, and found that the work was not properly assessed or planned, despite workers being exposed to such risks as working at height close to an electrical system.
Serious failings were also found in the electrical lighting system where the workers were exposed to live components, some at 240 volts. The system was poorly constructed and had not been properly maintained or tested. It is likely the engineers received electric shocks which threw them from the ladders.
BT was fined £600,000 after pleading guilty to breaching Section 2(1) of the Health and Safety at Work Act. It was fined £600,000 and ordered to pay costs of £60,000.
HSE inspector Laura Lyons said after the hearing: "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."