The fire alarm always seems to go off just after Elsa Espinosa has gotten her infant son, Dominic, to sleep. The alarm rings so often that her 3-year-old daughter Renata is afraid to go to bed.

Espinosa and her husband, Robert, live in Park Plaza, a stately, 70-year-old apartment building at 1629 Columbia Rd. NW that has been plagued by mysterious fires -- residents say as many as 48 of them -- since the beginning of the year.

Although none of the fires has resulted in injuries, they have become a constant source of anxiety for the residents. Floor watches have been organized, security guards have been hired, meetings have been held and many hours have been spent with fire investigators. But the fires have not stopped -- the smell of smoke still lingers in many of the hallways.

"The basic reaction is not to react anymore," Elsa Espinosa said. Now, when the alarm rings, she checks the hallway for smoke before rousing her children. But still, she says, "every time the bell goes off, it raises your blood pressure."

"You used to be able to set your clock by them the fires -- between 10:30 and 12:30 at night ," said Yvonne Kelly, manager of the 276-unit, eight-story building.

Most of the fires have been contained inside trash chutes in the building, and in an attempt to foil the culprit a few weeks ago, the staff began locking the trashroom doors at 10 p.m. Security guards also monitor the hallways at night, but the fires have continued and many tenants say they are convinced that the person responsible is a neighbor.

"It's got to be someone who's knowledgeable about the building," insists Harry Quintana, a member of Advisory Neighborhood Commission 1C and a resident of Park Plaza since 1977. He said there are rug burns near the trash rooms on three floors. "Surely this could not be an accident." Quintana spearheaded the effort to organize "floor captains" to keep an eye on the trash rooms.

Police arson detective Robert M. Glover told a gathering of worried Park Plaza residents last week that, "We are doing the best that we can to alleviate the problem." Glover and three other detectives have spent many hours talking to residents and poking around the basement trashroom for clues. So far, they say, they have drawn no conclusions.

"This kind of thing happens more often than we like," said Glover's partner, James D. Evans. Restless juveniles have been suspected in many of the chronic torching cases investigated in the past, but that is not necessarily the case at Park Plaza, he said.

The city's fire department has a record of 17 fires since January, according to Inspector Murdo L. MacLeay. He said that number includes only blazes that are considered suspicious. Many have not been reported because they were extinguished by personnel in the building, he said.

While MacLeay says he does not believe that all of the fires have been intentionally set, he is convinced that a lot of them are the work of an arsonist. "

William (Shorty) Shelton, a maintenance man in the building, said the fires have been started by tossing lighted rags, papers and, once, a telephone book down the trash chute, often igniting trash that has gotten stuck between floors. Shelton has taken to watering down the refuse in the bottom of the chute before he goes home at night.