A 40-foot-long concrete facing broke away from a balcony of 12th-story condominium unit in Alexandria early yesterday and plunged to the ground, city officials reported.

No one was injured, according to officials of the Alexandria Knolls condominium, 6101 Edsall Rd. The facing sand eight feet into the ground, missing an electric generator and parked cars, and cut off telephone service to the building's residents for most of the day.

The incident occurred shortly before 5 a.m, according to Bill Evarngan, Alexandria Knolls property manager.

Uwe HInz, an engineer with Alexandris's building inspection department, said the precast concrete railing's concrete supports sheared off but that the cause has not yet been determined. "It appears to be an isolated case," he said.

"It appears to be a construction fault," he said, "but I'm not sure what stage of construction."

The partially enclosed balcony was left intact. An 11th-floor balcony may have been damaged by the falling railing, Hinz said.

The incident caused disquiet among some Alexandria Knolls residents. "How do I know if the rest of them are like that?" asked Ray Terry, who lives on the 18th (top) floor. "I guess I'll sit around and pray and hope someone it will probably sprinkle holy water and make it safe."

Others disagreed, saying they won't be dissuaded from using their balconies. "I think it was an isolated case," said one woman who declined to give her name.

The occupant of the unit involved could not be located yesterday.

Matin Price, who said he rents a unit on the 12th floor next to the one that lost its balcony railing, said that when the railing broke off, the poise "shook me out of my bed. It shook my Doberman and made a nervous wreck out of me." He added that he is moving "as son as my lease is up."

Alexandria Knolls occupied since January, 1975, has 190 units ranging in price from the low $40,000 range to more than $100,000, according to building officials.

Its developer is Alexandria Knolls west, and tha construction company is the Atlantic-Pacific Contruction Co., according to Larry Jankins, East Coast project manager for Atlantic-Pacific.

Jankins said engineering firms will investigate the cause of the incident.