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Firefighters and three appliances descended on the Tay St campus after the explosion at 12.45pm, and students and staff were evacuated from C Block, in Forth St.
The explosion happened during the lunch break.
Students said the power went off and they heard loud bangs, followed by fire alarms.
Electrical engineering student Ray Hughes said a phase blew in the machine shop and he smelt "burning electronics'' before hearing bangs and smelling smoke.
"There was bang after bang after bang,'' another electrical engineering student, who did not want to be named, said.
Facilities manager Neil Smith said the "small explosion'' cut power to several buildings. He had not been inside the switchroom to assess the damage. An assessment was being done by electricity network management staff.
SIT chief executive Penny Simmonds said the SIT's back-up generator had kicked in for campus computers and data.