"Thanks for writing about TV commercial interruptions," says a letter I received this week:

"I know there are federal regulations on how many interruptions there can be in a program and how much total time can be devoted to commercials, but nobody seems to be enforcing these rules. Can you please find out what the limits are and publish them so that the public will be able to monitor compliance?"

In all, I received 16 letters about TV after I complained that Channel 5 had interrupted Victor Borge in midsyllable to broadcast a series of commercials. In seven of the 16 letters, readers indicated a belief that there are federal limits on commercials. With one minor exception, this is not so. Federal limitations on commercials apply only to children's programs.

The Communications Act has been on the statute books for 44 years, but it wasn't until recently that the Federal Communications Commission ventured to put any kind of limit on commercials. And when it finally did, it limited commercial time on children's programs only.

There is still no federal rule governing total commercial time or frequency of interruptions on programs that fall outside the "children's" category.What does exist is the Television Code put out by the National Association of Broadcasters. Many stations subscribe to this voluntary code and make an honest effort to operate under its terms.

The FCC is barred by law from censoring what goes on the air. Its mandate is to see that the public's airwaves are used "in the public interest, convenience or necessity" - and only Heaven and the Supreme Court know what that includes or doesn't include.

One can only speculate as to whether the FCC will some day expand its role and begin regulating all commercial interruptions. The trend appears to be in that direction. The FCC is now getting into matters that 20 years ago it wuldn't have touched with an 11-foot pole.

However, the National Association of Broadcasters has for decades been aware that voluntary restraints can be better than federal supervision - better for the industry and better for the viewing public as well.

The NAB has worked hard to make its Television Code a respected standard and therefore an effective bulwark against federal intrusions that could quietly evolve into government control over program content.

The current Television Code says that a network station should not carry more than 9 1/2 minutes of "non-program material" in each hour of prime time, nor more than 16 minutes per hour in the remainder of its broadcast schedule. In prime time, the code says there should be no more than two interruptions in a 30-minute program, four in a 60-minute program, or five in a 60-minute variety show. In time other than prime time, the number of interruptions shall not exceed four in any 30-minute program period.

For non-network stations like Channel 5, the code limits commercials to 7 minutes in each 30-minute segment of prime time and 8 minutes out of 30 at all other times. The number of interruptions permitted is a bit complicated: If the station shows no commercial during the "station break" between programs, it may interrupt the following program four times in 30 minutes. If the station does use a commercial in the break between shows, the code says that it must thereafter limit itself to three interruptions in each 30 minutes of program time. Children's programs, news and sports are governed by other rules.

What actually happened to Victor Borge, says Channel 5 program director Stan Rudick, is that he was victimized by one of the automated systems that control modern TV broadcasting. The Merv Griffin show is syndicated to non-network stations in film form. The syndicate supplies each station with a "time sheet" that is supposed to state the precise length of each segment - a "segment" being any interval between two of Merv's promises to "be right back.

The TV station thereupon programs its automated system to show a series of commercials at precise intervals in the syndicated show. But when the timing sheet is off or the automated system glitches, people like Victor Borge are cut off in midsyllable.

"It is certainly contrary to our policy to have things like this happen," Stan says, "but we live in an automated world that sometimes doesn't work perfectly."

How true. As my friend George Kidwell is wont to observe at times. "Progress is our biggest problem."