Time spent on "nonproductive" activities by the U.S. Postal Service's mail-processing employees has tripled in the last 20 years, a trend experts say has nullified some of the gains in productivity attributed to the service's recent $5 billion investment in automated mail-sorting equipment.

Officials of the Postal Service, which spends 83 percent of its budget on labor and is struggling to reduce a $900 million deficit, say they cannot explain the increase. It "is to some extent puzzling," Deputy Postmaster General Michael S. Coughlin said this week, adding that "we may be seeing some changing relationships."

Last year, nearly 19 percent of a worker's day was spent on nonproductive activities, such as taking breaks, clocking in and out and moving around empty trays and carts, according to Postal Service figures. In 1969, nonproductive time accounted for 5.8 percent of a worker's day.

Under criticism from business mailers fighting a proposed 19 percent increase in average postage rates, the service has produced several "working hypotheses" about the growth in nonproductive time on the job, according to its testimony before the Postal Rate Commission.

One hypothesis is that automation leads to more frequent and prolonged breaks. Another is that new no-smoking policies force employees who smoke to leave work areas for longer periods. A third is that increased hand-washing time may be adding to break time.

Other hypotheses include the possibility that break time is greater in larger offices because it takes longer to walk out of a work station. However, the service's favorite hypothesis appears to be that the amount of nonproductive time has not changed; instead, according to this theory, the method used to collect data has been modified, and the observers who record behavior are more keen-eyed than they were two decades ago.

But if the comparisons are correct, the cost of the tripling of nonproductive time on the job is clear. In 1989, the service paid $1.82 billion in wages for nonproductive time for clerks and others who process mail, compared to $118 million in 1969. The dollar amount has increased 15 times because hourly wages have gone up as well.

Neither postal officials nor customers appear to begrudge employees breaks or the other activities that make up nonproductive time, but they say they are befuddled by the steep increase in such time.

"It's inexcusable that there should be this increase in break and personal need time without a clear explanation," said Charles W. King, an economics consultant who testified before the commission on behalf of Dow Jones & Co. Inc. "I see that as a kind of lapse in management."

An official with the American Postal Workers Union, the largest postal union, said he was at a loss to explain the increase. Blaming employees, however, "is absolutely baloney," he said.

The answer to the nonproductive time riddle is important now because, say postal officials and industrial experts, not enough is known about what effect increasing automation will have on the humans who have to adjust to it and, in turn, on overall productivity.

Frank R. Heselton, assistant postmaster general for rates and classification, said that the increase in nonproductive time has not affected productivity and that employees may need added break time because they get fatigued working with machines that move mail at breakneck speed.

Business mailers argue that the data suggests something entirely different -- that managers have allowed unacceptable levels of unscheduled breaks, that productivity gains would be greater if nonproductive time had not risen so much and that low productivity is the reason why the Postal Service has asked for the rate increase.

The Postal Service has said it needs the rate increase, which would raise the cost of a first-class stamp from 25 to 30 cents, because its costs are rising.

In an effort to boost productivity, postal officials this week launched a nationwide campaign to get the public to address its mail correctly so that new machines can read and sort it. About 30 percent of the mail "has some flaw," said Postmaster General Anthony M. Frank. Mail that is rejected by the machines must be sorted and processed by hand.

At a cost of $7.5 million, the Postal Service will send each household and business a guidebook showing how to address envelopes and packages. "We think this will help a lot," Frank said.