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PHILADELPHIA (CBS) — There were tense moments Tuesday morning when an underground transformer exploded in Center City. No one was hurt, but some businesses sustained extensive damage.

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The Philadelphia Fire Department said crews responded to 20th and Ranstead Streets, near Chestnut Street, just before 8 a.m., after a PECO underground transformer exploded.

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The explosion was so strong it sent several manhole covers flying. A 300 to 400 pound metal transformer access cover went way up in the air, too, and broke into pieces.

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A cleaner’s front window was completely blown out. Next door at Mama’s Vegetarian, owner Haviv David said his business sustained no damage, but his nerves were rattled.

“I was inside the store, and all of a sudden I heard a big explosion,” said David.

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A few businesses were evacuated due to higher than normal carbon monoxide levels.

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The cause remains under investigation.