At the Big N deli on L Street NW, one day last week, a customer pulled out his wallet for some cash. Right next to his American Express card was his Kastle card.

Kastle card?

After Washington Shopping Plate, Sears Roebuck and Central Charge, its about the most popular plastic card in the region. But Kastle card dosen't buy merchandise. Like some 150,000 other Washingtonians, the man in line at Big N works in one of 148 major office complexes in the metropolitian area that are provided building security services by a unique, computer-operated detection cards provide entry to these buildings at night and on weekends. The system is the brainchild of a Washington entrepreneur, A. Gene Samburg, who heads one of the region's emerging growth companies.

Samburg has experienced many of the roadblocks to new ideas and new businesses in the U.S. economy. But Samburg's Kastle Systems Inc., in Arlington, has become a success story in less than a decade. "we've grown from nothing with something that everyone in the business said couldn't be done. They said you could'nt secure a building with computers," he said in an interview.

Although his main operations still are within the Beltway, Kastle Systems has begun to attract national and international attention because its service is the only one of its kind in the country.

Investors in such cities as Houston, Los Angeles and San Francisco are seeking franchises to offer similar security operations to their office complexes. A national firm, Walter Kidde & Co., is buying computer programs that Kastle developes to sell as part of its own building management and security services as far away as Australia.

Samburg's main problem is managing growth -- in business volume, employes, space for his operations and computer capacity. Kastle Systems brought in revenues of $74,000 in 1972, in first year, and business is now running at an annual rate of $3 million a year. The initial workforce of 3 has mushroomed to 86 and is growing.

The Oliver T. Carr Co. prefers off hour guards for its many Washington buildings and is not among Kastle's customers. But virtually all other major Washington area builders are converts. More than 23 million square feet of area offices in 162 buildings are under surveillance by Kastle computers and personnel, and a picture (or blank space for one) of every single building lines Samburg's office wall and provides what he said is his best selling tool for prospective clients who stop by.

A combination of factors has led to Samburg's success:

His unusual idea for a custom security system, monitored 24 hours a day and sicne expanded to include such building management functions as turning on air-conditioners, responding to persons in stalled elevators and fire protection. Because of Kastle monitoring, burglars have been caught and fires have been stopped in very early stages.

The increased need for building protection in an era when terrorists and others have sought access to many types of business and government facilities. Kastle computers print out a record of every person who enters a building during the off-hours, by name.

Inflation, which has led many building operators to assess their guard costs as well as to refine energy management. Nighttime and weekend wages for guards are about $37,000 a year for a typical Washington building. The Kastle system has a top one-time charge of about $40,000, and the money buys more service than the individual can provide.

Samburg, a Cornell engineering graduate, credited another Washingtonian, Stanely Westreich of Westfield Realty Inc., with the initial push into today's exploding business venture. While working for Westinghouse, Samburg secured the White House. Westreich said if he could do that, why couldn't he develop a similar system for commercial office buildings?

"I spent a lot of time designing it and went to Westinghouse and they said it couldn't be done, that there would never be any volume in such a business," Samburg recalled.

But Westrich persisted and offered Samburg a contract of some of his buildings. Samburg, now 38, quit Westinghouse and hasn't looked back.

Indeed, Samburg is anticipating that the emphasis of his business will be quite different in future years. Currently, Kastle has four different levels of service. The initial phase service replaces a guard and provides access control to buildings by use of the electronically encoded plastic cards, issued to persons who work in the buildings involved.

A second level provides similar alarm services for each tennent's suite, by use of security keys, an electronic Kastle card or a digital combination unit. Kastle machines in Arlington, watched at all hours, receive a signal whenever an office is entered. Forced entries sound alarms.

The third phase offers 24-hour service for tennents who prefer special arrangements and the fourth service is monitoring of such devices as pump operaters, flood detectors, heat and somke detectors and telephones. For each building and tennent, Kastle computers have on record a list of persons to contact whenever alarms sound to reflect developing problems.

Today, these services account for a third of annual revenues. A second operation, energy management services (now in 14 buildings), controls such functions as air conditioning (automatic on and off at specified outside temperatures), when to turn on garage fans and utilities such as lighting. This also accounts for a third of business volume today but Samburg said this is the growing sector and "will probably be much bigger than security, because everyone has this problem and not everyone has guards."

Kastle's third operation is developing the compute programs (called software) to operate the intricate energy and security management needs of thousands of individual tennents and dozens of building managers.

Currently, Samburg is working on possible sales of his services for government buildings here. He said he is no hurry to franchise his idea for other cities but will probably do so within a year. Kastle will continue to concentrate on the Washington region, since Samburg dosen't think he could adequately manage the complex business in many areas at once.