Local media reported on Thursday that a faulty generator appeared to have caused the explosion which occurred in an area containing the stadium's offices and led to two workers suffering serious burns.
A third person had minor injuries.
One of the injured, a 31-year-old man with burns covering 70 per cent of his body, was flown to the burns unit at Barcelona's Vall d'Hebron Hospital.
Belarus trained in the late afternoon at the stadium and Spain also practised as planned with one of four light towers working and 5,000 fans in attendance.
The match between Group I leaders Spain and bottom team Belarus is due to kick off at 2000 GMT on Friday.
World champions Spain can move to the brink of qualifying for next year's World Cup finals in Brazil with a win that would put them three points clear of France with one match to play.