In December 1976, Prince George's County Executive Winfield M. Kelly Jr. signed into law a measure that required landlords to install smoke detectors in all county apartment units by July of this year.

Yesterday, as fire inspectors were making the rounds to see whether the law was being obeyed, they came across an apartment building in Temple Hills that had not yet complied. The name of the building is Alpine Apartments. The name of one of the owners is Winfield M. Kelly Jr.

If the letter of the smoke detecor law were strictly followed, Kelly and his partners in the partment enterprise would be smacked with fines of $21,000 or prison sentences of 21 year - $500 or six months for every day since July 1, when the measure took effect. Such penalties are enough to make the average man sweat, let alone a man who is seeking reelection this year.

But Kelly was not sweating yesterday. When informed that Alpine Apartments was breaking the law, the county executive responded calmly, "Gee, that's unfortunate."

Kelly said he did not know anything about the day-to-day operations of the 170-unit apartment building on St. Moritz Drive, in which, according to his 1978 financial statement, he owns his 1978 financial statement, he owns a $370,000 interest. "It and all my other holdings have been in a blind trust since 1970, when I became an elected official," he said. "I put everything in the hands of the trustees."

One of those trustees, accountant Joseph W. Poole, has served as the apartment's manager in recent years. Poole said he has been trying to get the smoke detectors installed for some time without success.

"We contracted with a master electrician to have them installed 60 days stacked up over at the apartment, but he apparently hasn't had time to do it yet. There are hundreds of thousands of apartment units around here that need smoke detectors and only so many electricians. We're all at their mercy."

Although Pooole was served written notice yesterday by fire inspectors that he must have the detectors installed within the next 10 days, County Fire Capt. Duncan Muro, who is in charge of enforcing the law, said he doubted that anyone would fined or jailed for late installation.

"We're not interested in busting apartment owners," said Capt. Munro. "We're interested in making sure the smoke detectors get in. If we push people too much, we might end up with shoddy installation or price gouging by the electricians."

Most of the hundreds of smoke detector models now on the market run by battery and are installable by even the most unmechanically-minded person. But the County Council - after hearing testimony that some models emit low levels of radiation - decided when drafting the law that the apartments should have radiation-free detectors that are hooked-up to the electrical system. That requires the work of a master electrician.

"It's a monumental task for the owners to get them installed, and it's a monumental task for us to inspect them all," said Capt. Munro. "We've had our regular engine personnel inspect about two-thirds of them so far. If someone asks for more time, we'll usually give it to them."