Schneider Electric has been ordered to pay £21,000 for releasing harmful greenhouse gas into the atmosphere.
The Environment Agency said the global energy management company failed to recover the release of sulphur hexafluoride (SF6) gas at Stanford-le-Hope in Essex in 2013.
The incident occurred when a high voltage switchgear was being installed at London Gateway Port.
A metallic strip called busbar, which joins two circuit breakers together, was found to be faulty and needed to be removed.
While removing it, up to 15kg of SF6 was released, which is said to remain in the atmosphere for “thousands of years”.
That amount of gas damages the environment the equivalent of flying a 737 jet from Heathrow to Sydney and back three times, according to the Environment Agency.
It added Schneider’s subcontractor Metricab Power Engineering, which removed the busbars, was not aware the switchgear was filled with the SF6 gas.
It was released for one to two hours before an employee of Schneider realised what was happening and raised the alarm.
Investigating Officer Claire Cox said: “This successful conviction demonstrates our commitment to ensuring compliance across all F-gas users and industries covered by these regulations.
“This particular case displays the long term environmental harm caused to the atmosphere which is likely to continue beyond our lifetime and for many generations to come.”
Schneider Electric apologised “unreservedly” for the incident.
Andy Taylor, Energy BU & UK & Ireland H&S Director added: “We conducted an immediate investigation working with all parties involved to ensure a similar incident will not be repeated. We are fully supportive of the Environment Agency policies and the action against climate change. We currently have 15 priority initiatives and related goals across six environmental domains to support the COP21 targets and +1.5°C compatibility. To this end, we saved 12,000 tonnes of CO2 through energy savings in 2015.”