Nova Chemicals has been forced to halt production for up to two weeks after the blunder at the Ontario plant.
A new steel platform was being put in when a contractor's employee hit a safety button, causing "minor damage".
A company spokesman said he felt some sympathy for the worker. "It has got to be very distressing," he said.
'Inadvertent'
An investigation has been launched into how the switch was tripped, but the worker is not expected to be penalised for the mistake, Reuters reported.
"The switch is a safety thing so if anyone sees something going wrong they have the opportunity to shut down the plant," Nova spokesman Greg Wilkinson told the agency.
"But that's not what happened here. It was not a safety issue. It was simply inadvertent."
Because of the repairs needed to rectify the damage caused by the shutdown at the ethylene plant in Corunna, 220 km (180 miles) south-west of Toronto, Nova Chemicals has declared "force majeure" on shipments of propylene and other products.
Force majeure is a legal term used when unforeseeable circumstances prevent a company from fulfilling contracts.
The company, which is based in Alberta, Canada, but has offices in the Pittsburgh in the US, estimated the shutdown and associated loss of sales would cost it about $11m.
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I bet that worker feels pretty stupid now.
They'll never hear the end of this one.
