The severity of the Los Angeles office building fire last week was almost without precedent, because serious fires so rarely occur when there are workers around to detect the initial signs of a fire, a building owners trade group said.

In a prepared release, the Building Owners and Managers Association International, said there were no serious office building fires in the United States last year. The Washington-based group based its findings on an extensive survey of 1,524 office buildings representing 618 million square feet of office space.Fires in U.S. office buildings have been rare in recent years, said Noel Leary, executive vice president of BOMA International. In fact, fewer than 4 percent of the office buildings in North America experienced any sort of fire in 1987.

The survey also revealed that 79 percent of high-rise office buildings have either full or partial sprinkler systems currently in operation.

No deaths occurred in office building fires last year, and only six injuries - one fire fighter and five building service employees - were reported in the BOMA survey. Typically, those injuries were in service areas rather than in hallways and tenant spaces.

Among other findings, the leading cause of office fires was electrical (40 percent), followed by careless workers (19 percent), smoking (12 percent) and arson (10 percent). Building occupants discovered 59 percent of office fires last year, with an additional 22 percent detected by smoke alarms.

BOMA's 1988 Tenant Study shows that 85 percent of office workers are

satisfied with information about their buildings' fire and medical emergency procedures.