An employee of Non Destructive Testing Services was carrying out pile testing at a construction site in Brixton, London on 29 May 2012, Southwark Crown Court was told.
The worker was inside an accommodation unit fitted to a vehicle when he was overcome by carbon monoxide fumes from one of the unit's gas appliances and died.
The Health and Safety Executive's (HSE) investigation found the unit's ventilation was inadequate. There were two gas appliances in the cabin, with pipework and gas cylinders that that been there for several years.
It said the company, which offers pile integrity, dynamic load and static load testing, had not planned how the welfare unit would be equipped when deciding to fit it to the vehicle and had failed to suitably and sufficiently inspect the cabin over a long period.
Non Destructive Testing Services pleaded guilty to breaching s 2(1) of the Health and Safety at Work Act. It was fined £70,000 and ordered to pay costs of £45,944.
HSE inspector Darren Alldis said: "[The] unit should never have been set up in this way and posed a risk throughout the whole time it was being used."
