The plan to save Social Security agreed to over the weekend would for the first time create a partial means test for the payment of benefits under the giant system.

In another break with the principles that have governed the system in the past, the compromise also would use general federal revenues--income tax money--much more explicitly than ever before to finance retirement, survivors and disability benefits.

And under certain circumstances it also would depart from the accepted government practice of using the consumer price index (CPI) to index payments, or tie them to inflation. Many experts say the CPI has overstated inflation, and the compromise provides for use of a wage index instead when Social Security costs go up faster than revenues and prices are rising faster than wages.

From its inception nearly 50 years ago, Social Security has been portrayed as similar to insurance, with tax payments called "contributions" and benefits based in part on what each individual had "contributed."

A person had to retire to qualify for benefits, and still does, so there is a limit on how much earned income most beneficiaries can have. But there is no limit on other types of income--from investments, for example. In other words, there has been no means test, such as is required for welfare.

Technically, if the reform package is accepted by Congress as expected, that would still be true. But for a single individual with income over $20,000, one-half of Social Security benefits would become subject to the income tax beginning in 1984. For a married couple filing a joint return, this cutoff would be $25,000.

According to commission officials, benefits would be taxable on this basis regardless of whether they were being received by retired or disabled workers, their spouses or their survivors, including minor children.

The commission estimates that between 1984 and 1989 about $30 billion would be raised by taxing these benefits and adding the revenue to the Old Age and Survivors Insurance Trust Fund and the Disability Insurance Trust Fund. The taxes collected technically would go into the Treasury's general fund, but would be paid automatically into the trust funds.

Since only beneficiaries with incomes above a certain level would be subject to the tax, a number of Social Security experts said they regarded the change as a reduction in benefits linked to the recipient's total income.

By using the income tax system to apply this means test indirectly--political considerations apparently ruled out using it directly--the compromise also may have made the means test less fair than it might have been.

No other federal program uses a means test that counts income only if it is taxable. Thus, a Social Security recipient with a substantial amount of tax-free income, say, from municipal bonds, could find the effective level of benefits reduced less than one with the same income from taxable sources.

For many years, the Treasury has been using general revenues to support Social Security in limited and special ways.

There has been strong political opposition to greater use of general revenues on grounds such a step would erode the contributory nature of the system and remove an important discipline on it.

The compromise uses general revenues explicitly but indirectly. It calls for a refundable personal income tax credit for one year, 1984, to offset a 0.3 percentage point increase in the employe's share of the payroll tax. The $4 billion tax increase is not scheduled to take effect until 1985. Workers would pay the Social Security tax increase but then would be reimbursed through the income tax credit. In the end, general revenues would bear the burden.

General revenues also would be used to offset a portion of a proposed Social Security tax increase for the self-employed. At present, they pay the employe's share and half the employer's share, or 75 percent of the current total rate for retirement, survivors and disability coverage. Beginning next year they would pay 100 percent, but would be allowed to deduct one-half of the payment as a business expense in calculating personal income taxes. None can be deducted now.