Credit-card frauds are up dramatically over the past couple of years, but consumers haven't been much alarmed. Under the law, you can't lose any more than $50 if a crook charges merchandise to your account. If you report a missing card before it's used, you should not be penalized at all.

But debit cards are something else.

About 30 million debit cards are now in consumer hands, and the law doesn't give you much protection against their fraudulent use. If someone gets hold of your debit card or card number, it could cost you a lot of money.

You own a debit card if you use an automatic-teller machine. The card authorizes automatic withdrawals from your account. In some cities, you may also be able to use a debit card to pay for merchandise. (A debit card charges your account immediately--whereas if you pay with a credit card you have 30 days or more to cover the bill.)

Some brokerage houses issue debit cards to holders of their cash-management accounts. These cards work like checks, only faster. The money is deducted immediately from your bank account.

Here's what can happen if someone fraudulently uses your debit card:

* If you report the loss within two days of learning that your debit card is gone, or within two days of finding an unauthorized withdrawal on your monthly statement, your penalty cannot exceed $50. The law provides no way for you to escape penalty-free, although your bank may, if it wishes, decide not to charge you.

* If you delay more than two days in reporting the theft or fraud, you can be charged for up to $500 of the loss.

* If you don't report an unauthorized withdrawal within 60 days of the time the bank mailed your statement, you can be charged for every penny that is stolen from your account. Your losses could equal the amount you had in the account plus the total loan that can be charged automatically to your card.

You don't have to lose your debit card to have money fraudulently lifted from your account. A thief might get access to your funds by learning your personal-identification number (pin) which tells the bank's computer that it's okay to issue cash. "The numbers are supposed to be secret," H. Spencer Nilson of the industry newsletter, The Nilson Report, told my associate, Virginia Wilson, "but they are easy to get." If someone does tap into your account, you have to prove to the bank that you are innocent, which may not be easy.

Your pin number does not appear on your debit card, so if you lose it your money should, theoretically, be safe. But here's the scam: the thief may call you up, claiming to be a police officer or bank employe who has just found your card. You'll be asked to give your pin for identification. As soon as you do, the thief can steal every cent in your account.

It is never necessary to identify yourself by giving out your pin, so be suspicious of anyone who asks. Bankers also warn debit-card users not to write their pins on their cards, or carry the numbers in the same wallet or handbag that contains the card.

Most thefts from accounts today are connected with family feuds. A rebellious son, for example, might clean out his parents' account and take off. The institution will normally consider this your loss, even if you notify it of the unauthorized withdrawal within two days.

Other thefts have taken place at gunpoint in front of an automatic-teller machine, especially in lonely areas or at lonely times of night.

Counterfeiters are not yet widely active in the debit-card field. They are just now expanding into credit cards. "But they will be attracted to this area as the opportunity presents itself," Nilson says.

Anyone with a debit card should open the monthly statement as soon as it arrives and double-check all withdrawals. If you delay more than two days, you may be on the hook for $500 in losses, and possibly more. Your bank can waive the penalty, but doesn't have to.

"I have no advice for debit-card owners on how to protect their accounts from fraud," Nilson says. "I really don't know how to prevent it, and I don't intend to use a debit card."