Dartford-based Kenard Engineering Company has been prosecuted for safety failings after the incident, which took place during a crane installation on 15 June 2012.
The lad fell three metres, suffering a herniated disc and numbness in his lower back. He has since recovered enough to return to work.
The Health & Safety Executive (HSE) brought the prosecution at Dartford Magistrates' Court.
Magistrates heard that the apprentice was helping a maintenance worker to move a pillar-mounted crane. When it was in place and the crane wired-up, he climbed a ladder to plug in CNC computer equipment to the cable tray socket.
When the young worker's hand touched the cable tray on top of the pillar, the tray earthed him. At the same time, the pillar he was holding on to for balance became live as a wire had been connected wrongly into the earth.
HSE found that Kenard Engineering had failed to ensure their working systems were safe by allowing someone without the proper competence and experience to wire up the crane. The firm had an electrical specialist moving and installing the CNC machines but it was customary for employees to move and wire in the cranes.
Kenard Engineering Company Ltd, of Green Road, Dartford, Kent, was fined £10,000 and told to pay £1,981 in costs after admitting breaching Regulation 4(1) of the Electricity at Work Regulations 1989.
HSE inspector Rob Hassell said after the hearing: "This was a needless injury to a young and inexperienced worker. It could have been avoided by Kenard Engineering by simply ensuring that electrical work, particularly three-phase electrics, was carried out by suitably qualified personnel.
"Working with electricity is a high risk operation and fatalities do occur. All companies need to ensure that they identify three-phase electrical work within their operations and take suitable precautions when planning maintenance work."