The man, a joiner, was working on the construction of a water treatment works in Holyhead when an over-inflated pipe stopper exploded in the confined space where he was working.
The Health and Safety Executive said the incident resulted in “significant hospital treatment” for the worker.
At the judgement at Holyhead Magistrates' Court, it said there was “a catalogue of errors that led to the incident”.
HSE inspector Debbie John said: "There were a number of failings identified in the investigation into this case, including a failure to provide any rescue equipment for staff working in confined spaces, and a failure to provide safeguards.
"Incompatible equipment was used, the pipe stopper wasn't used correctly, and Galliford Try did not exercise proper control over the work to be done, resulting in confined-space rescue equipment also not being available.
"Fortunately the injured man was able to get into a position where he could be rescued by his colleagues, but he still needed treatment in hospital and is not fully recovered from his injuries more than three years later."
Galliford Try, which pled guilty to a safety breach, was also ordered to pay costs of £8,788.
Hampshire-based Selwood, which had provided the pipe stopper and inflation equipment, was also fined over the incident.
After pleading guilty to a breach of the Provision and Use of Work Equipment Regulations 1998, Selwood was fined £3,000 and ordered to pay costs of £8,798.