Mechanical fitter, David Minor, suffered a catalogue of injuries including a broken vertebra as a result of the incident, Bury St Edmunds Magistrates' Court heard last week.
Cambridge-based building and timber merchant Ridgeons Ltd, which operates from 22 locations across the east of England, was prosecuted by the Health and Safety Executive (HSE) after the incident which happened on 16 September 2008.
Mr Minor, 63, of Bury St Edmunds, was employed by Graham Parish Engineering which had been subcontracted by Ridgeons to fix an overhead crane system at its timber engineering workshop in Sudbury, Suffolk.
He was climbing a ladder to access the crane when he made contact with a live conductor. He fell 18ft and landed on the concrete floor, sustaining a fractured vertebra, a broken ankle, smashed heel, and burns to his hands.
The conductor Mr Minor touched was the main power channel to the overhead crane. It carried three phase 415v electricity but was not marked and had not been isolated prior to work commencing.
Ridgeons admitted breaching Regulation 4(3) of the Electricity at Work Regulations 1989, failing to ensure work was carried out in such a manner as to not give rise to danger. The firm was fined £5,000 and ordered to pay £4,344.70 in costs.
HSE inspector Jonathan Elven said: "It is essential for companies to ensure that work undertaken on their behalf by subcontractors is properly managed and safe systems of work agreed prior to work commencing. Ridgeons have admitted that the task was handed over to Mr Minor without discussion as to the way it was to be undertaken or any precautions that may be needed prior to it being started."