President Carter yesterday raised expectations about a future halt to all nuclear testing, but Congress also was asked to support two treaties that only limit nuclear explosions, as "a hedge" against negotiations for a total ban.

The President praised an agreement in Geneva by the United States, the Soviet Union and Britain to move up from exploratory test ban talks to formal negotiations, starting Oct. 3. This is essentially a procedural step, although the President treated it as politically significant. U.S.-Soviet relations have been strained since March.

Carter opened his news conference by describing the two-week talks in Geneva as "sufficiently promising" for "the possible negotiation of a comprehensive ban against the testing of nuclear weapons or peaceful nuclear devices."

In Geneva the American, Soviet and British negotiators agreed that preliminary talks were serious, but they did not minimize the long road ahead for an agreement.

In 1963, after negotiating since 1958 on a total test ban, the United States, the Soviet Union and Britain compromised on halting all nuclear testing in the atmosphere, in space and underwater. That shifted their tests underground, where the nuclear arms race continued.

British Minister of State Lord Goronwy-Roberts told the standing 30-nation Geneva Disarmament Conference yesterday that:

"Since the partial test ban treaty came into force in 1963 there have been over 500 underground nuclear tests, as well as a number of tests in the atmosphere, now only conducted by one country." He was referring to China, a non-signer of the ban along with France, which later ceased above-ground tests.

At the 1974 summit meeting between Soviet leader Leonid I. Brezhnev and President Nixon, a new treaty was signed to prohibit underground nuclear weapons tests with explosive power above 150 kilotons (the equivalent of 150,000 tons of TNT).

With the Soviet Union insisting on using nuclear explosions for nonmilitary purposes, that treaty was tied to agreement on a parallel treaty limiting "peaceful nuclear explosions" underground.

The second treaty, completed in May, 1976, during the Ford administration, also limits peaceful explosions to 150 kilotons. Both treaties have been heavily criticized for setting limits too high to check nuclear arms competition. The new limit is 10 times greater than the American bomb dropped on Hiroshima at the start of the nuclear age.

President Carter in his campaign joined in criticizing the two pending treaties on those grounds. The Carter administration, decided to support them nevertheless as an interim step, partly because the treaty on peaceful explosions could permit the first on-site inspection of Soviet testing.

In testimony before the Senate Foreign Relations Subcommittee on Arms Control yesterday, Under Secretary of State Philip C. Habib and Paul C. Warnke, director of the U.S. Arms Control Agency, urged Senate consent to the two treaties.

Warnke said that in the recent Geneva talks on a total test ban, Soviet negotiators strongly pressed for U.S. ratification of the treaties. Habib similarly emphasized "the political benefits" of approving the treaties, to "provide a positive diplomatic climate" for negotiating a total ban on nuclear tests.

Failure to ratify the treaties. Warnke said, "could raise doubts in the minds of the Soviets about the reliability of the United States as a negotiating partner . . ."

Although the United States hopes to conclude a total test ban "at an early date," Warnke said, approving the pending treaties can serve as "a hedge against the possibility that the negotiating process will take longer than we would hope . . ."

Subcommittee Chairman Claiborne Pell (D-R.I.) and other senators present indicated they will support the treaty, many without enthusiasm. Sen. Frank Church (D-Idaho) said the 150-kiloton level is so high that "it hasn't very much meaning."

Warnke agreed the level is high, but said in principle to negotiate on the suggestion the treaties can provide a base for a total ban. A major obstacle to a ban on all testing, Warnke said, is Soviet insistence on "peaceful explosions" for oil exploration, earth moving and other purposes. The United States, he said, hopes "to persuade" the Soviet Union that these benefits "are offset by the risks."

Another long-standing Soviet obstacle to a total test ban treaty, the Kremlin's insistence that China join it, at the outset, is no longer a large problem.Warnke indicated to reporters after the hearing.

The Soviet Union reportedly has agreed its principle to negotiate on the suggestion first made publicly by President Carter in January, for an agreement limited in years among nations now willing to stop all tests, with other nations asked to join later.