The firms were sentenced over the incident that saw agency worker Christopher Booker from St Athan, fall 12 metres through an unprotected opening in a platform at the power station in the Vale of Glamorgan on a Sunday evening in June 2007.
Cardiff Crown Court heard that work was being carried out to install equipment within a large deep pit in the water cooling system to hold back the seawater when the tide rose. Sections of the floor gratings at the top of the pit had been removed to allow the work to proceed.
Mr Booker was working with eight other workers who had been called in to carry out urgent modification work on the equipment to ensure an effective seal of the pit.
As the natural light faded, electric lights were turned to face those doing the grinding work which left the top of the pit in near darkness. Mr Booker fell through the opening in the walkway to the floor below. He died of multiple injuries to his chest and pelvis.
An investigation by the Health and Safety Executive (HSE) showed that a large opening in the walkway was left unprotected after the floor gratings had been removed, and inadequate precautions had been taken to protect people working near it.
The investigation also identified that there was confusion and misunderstanding between RWE npower and principal contractor Amec as to who was responsible for controlling the work at the time of Mr Booker's death.
RWE npower plc pleaded guilty at Cardiff Crown Court to health and safety breaches. It was fined £250,000 and ordered to pay £30,000 costs.
Amec Group Ltd also pleaded guilty and was fined £200,000 with costs of £30,000.
After the hearing, HSE inspector Caroline Bird said: "This tragic case highlights the consequences of failing to do something as simple as adding protection to an opening in a walkway.
"Inadequate planning and a poor choice of safety control measures meant that a very obvious hazard remained.
"Both companies had a duty of care to Mr Booker that they failed to meet ' with catastrophic consequences. This awful incident could so easily have been prevented had the correct safety measures been taken.
"Employers have a duty to manage the risk of falls from height, including providing protection around the edge of openings. It is completely unacceptable this sort of risk was not managed."
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