After being enforced for almost a year, a Prince George's County law requiring owners of burglar alarms to buy permits has reduced the number of false alarms by about 20 percent.

According to Charles Deegan, acting director of the office of licenses and permits, the law was passed in August 1981 to cut down on the number of false alarms reported to police, a figure that had risen to an average of 196 each day.

Last year, county police estimated that responding to an average of 50 false alarms per day cost the county $547,500.

The law, which the county began enforcing in January, requires all owners of burglar alarms to purchase permits costing $10 initially, and $5 yearly to renew. Failure to register may result in a $200 penalty.

Owners are allowed three false alarms each year at each registered location, and each alarm after that may cost a $30 fine.

On the fifth false alarm, an owner may be called in for a hearing and ordered to have the system repaired or removed.

In September 1982, 2,282 false alarms were reported, according to Deegan, compared with 2,767 the previous September.

Deegan, who estimates the number of alarm systems at 10,0000 to 20,000 throughout the county, said he expects the number to drop still further when the licensing system is computerized in November.

False alarms may be triggered by the weather, vibration, animals, or a property owner's failure to turn off his alarm before entering a building.

In addition to heightening citizen awareness, the law is also intended to make "the police a little more sure there's an actual break-in or holdup when they go to respond," Deegan said.

No one has received the $200 penalty yet, according to Deegan. So far, enforcement efforts have centered on education, he said.

His licensing office, which is signing up 15 to 20 owners each week, recruits owners by sending applications along with permits for electrical work, and by scrutinizing a weekly list of addresses of false alarms provided by police.

Deegan said that no decision has been made on whether to fine town and county government offices. The worst offender, he added, is the department of education. Its buildings account for 20 percent of all false alarms, he said.