An explosion in a transformer at a west Oakville packaging plant late yesterday afternoon forced the evacuation of the building.

Employees of Ropak Packaging, at 2240 Wyecroft Rd., said the noise came from their building's transformer, which shorted out, forcing the evacuation.

"An electrical fire in the transformer room was what started this, but we don't know what started it," said Oakville fire Chief Richard Boyes.

"It spread and breached the wall. It was contained to one small area of the plant, but we do have smoke throughout the plant and we're doing ventilation right now."

One employee twisted an ankle during the incident. Damage to the building has yet to be estimated.

Ropak produces plastic shipping containers. The Oakville Chamber of Commerce directory shows the company has more than 100 employees.