UPDATE: Roads open after hydro vault fire closes area of Adelaide and Bay

An explosion in an underground hydro vault is snarling downtown Toronto traffic in rush hour.

Emergency crews got word of a loud explosion and white smoke coming from a hydro vault grate on Adelaide St W near Bay St just after 5 p.m Friday.

Stephan Powell a District Chief with Toronto Fire says firefighters are on scene standing-by and assessing the situation with Toronto Hydro on the way.

"Electrical fires can be very difficult and we have to ascertain that this was an electrical fire," Powell told NEWSTALK 1010 Friday. "It has occurred in a below-grade vault but until it's been investigated we can't actually say what caused the explosion or what the extent of the damages are."

Powell explains they don't want to flood the vault with water just yet because it may still be charged with water and any fire may already be out.

Adelaide was from York St to Bay St and southbound Bay is blocked between Queen St W and King St W for several hours.  It is now open.

No one has been hurt.

A hydro vault blast on King St between Bay St and Yonge St in May kept firefighters and hydro crews busy for several days. Because of soot damage, employees working in the RBC building above are not expected to be able to move back in until November.