South Eastern Electrical has been fined £70,000 including costs after a ground worker was knocked unconscious by an electric shock in Southend.
The Hainault based firm in London was fined £50,000 with costs of £20,000 at Basildon Crown Court.
The company had been found guilty at an earlier hearing in February concerning an incident at a supermarket in Southend, Essex, on 23 July 2006.
Major refurbishment works were being carried out when the store was closed on a Sunday afternoon.
A groundworks subcontractor, from Benfleet, suffered an electric shock after coming into contact with a live cable during replacement of lighting columns in the car park.
He was unconscious for a short period and suffered burns and bruising.
The Health and Safety Executive (HSE) has warned about the importance of robust and well supervised systems of work when dealing with the isolation of electrical systems.
HSE Inspector Dominic Elliss said: “The risks of working with electricity are well known and this incident could easily have been avoided.
“Effective identification and isolation of the electrical supply, together with clear instruction and supervision from the very beginning and throughout such work, are essential in preventing such incidents from occurring.
“By failing to implement simple and well documented controls over such work, electrical contractors place not only themselves, but others who may come into contact with their work, at risk of very serious injury.”
The HSE investigation found no circuit diagrams had been available and no attempt had been made by South Eastern Electrical to produce a diagram by surveying each column to identify its point of supply.