CHICAGO (CBS) — Three CTA workers were injured Tuesday morning, when they were shocked while working near an underground power station in the Loop.

Fire Department officials said there was an “arc blast” from an electrical panel in a ComEd vault while a CTA maintenance crew was working underground near State and Lake streets around 11:30 a.m.

Three workers were injured in the electrical explosion; two suffered significant burns. One worker was taken to Stroger Hospital in critical condition, one was taken to Northwestern Memorial Hospital in critical condition, and one was taken to Northwestern in good condition.

“The injuries were significant. They were serious in the first two victims,” said Deputy District Chief Michael Carbone from the Chicago Fire Department.

CTA officials said subway trains were not affected.

Fire Department officials said ComEd was investigating what caused the “arc blast.”