Federal workers and retirees who try to save money by purchasing a low-premium health insurance plan to protect them next year could wind up paying thousands of dollars out-of-pocket if they are hit with major medical or dental bills not covered by the plan they pick.

On the other hand, people who pay top premiums (of up to $1,600 next year) for Cadillac-style coverage could waste money if they have big medical expenses that could be covered adequately by a plan that has much lower premiums.

So what do you do?

Feds and retirees here have about 20 health plans to choose from between now and Dec. 23 when the open season enrollment period ends.

No one plan is best.

The "best" plan for a 68-year-old retiree who lives alone isn't necessarily the best for a 27-year-old computer programmer with two children.

The cost of health premiums (which will jump an average 24 percent next month) is important. But that may not be your full cost. A plan with low premiums is of little value of it doesn't cover a substantial portion of your medical expenses.

In previous columns we dealt with premiums and listed plans rated as best overall buys by the Washington Consumers CHECKBOOK. Today and tomorrow, with the help of the CHECKBOOK experts, we'll be looking at the best buys for specific kinds of coverage.

You are supposed to be able to examine brochures of the different plans at your agency's health insurance office. And retirees are supposed to have received cards telling them how to write the Office of Personnel Management for additional brochures. But many people say they still haven't gotten the word. And time is running out. So if you want to see a particular plan's brochure, call the individual health plans direct for a copy.

All but two of the plans have local telephone numbers. Look for them in the phone book. The Government Employees Hospital Association (GEHA) plan can be reached by calling (800) 821-5244. The Mail Handlers health plan is at (800) 621-8443.

CHECKBOOK's editors suggest the following plans for consideration:

Best overall coverage at lowest cost: GEHA; Postmasters high option; Blue Cross-Blue Shield's standard option; Mail Handlers (high and low options); Postal Supervisors; American Federation of Government Employees; low-option Aetna plan, and the various Health Maintenance Organizations (HMOs).

Keep in mind, however, that all health plans have some areas where their coverage is weak. For example, GEHA, one of the top-rated plans, does not have dental benefits coverage.

Catastrophic Illness: GEHA; National Alliance of Postal and Federal Employees; National Association of Government Employees; Postal Supervisors; Rural Letter Carriers (postal workers only); National Treasury Employees Union; and GEBA (for National Security Agency workers only); and Foreign Service plan (limited to Foreign Service workers and retirees).

Also, the National Association of Letter Carriers; American Postal Workers Union; and Blue Cross-Blue Shield's high-option plan. Those last three, however, have some exclusions from full catastrophic coverage that you should check.

Maternity Benefits: The HMOs really shine in covering most costs associated with having a baby. Most of them do not have deductibles for this kind of health care, nor a requirement that the covered individual pay a percentage of the total bill (copayment). CHECKBOOK editors say that employes can save $500 or more in maternity benefit costs, if they choose an HMO.

Mental Health Benefits: Blue Cross-Blue Shield's high option plan generally offers the best coverage, with an annual limit of 50 visits to a doctor and/or psychiatrist.

Tomorrow's column will cover plans that offer some of the best coverage for things like dental benefits, and also plans that retirees should consider. Retirees with Medicare can usually get by with less expensive, low-option plans. Those who do not have Medicare should consider the more costly high-option plans that offer greater protection. Details tomorrow.