A consortium of 30 large and small financial institutions with offices in the District, Maryland and Virginia announced the formation yesterday of the region's largest shared automatic teller machine network.

The new ATM network, called EFT Group, is scheduled to become operational in September.

With more than 500 ATMs available in the system initially, EFT Group will offer the most extensive network of remote teller machines in the region to customers of member banks, savings and loan associations and credit unions.

A "switch," or computer system, operated by NCR Corp. will allow customers of one institution to complete transactions at ATMs of other institutions in the network.

EFT Group is the fourth ATM network to be formed by financial institutions in the Washington region.

Network Exchange, a consortium of 13 banks and S&Ls, is expected to become operational next Tuesday. Customers of the 13 institutions will be able to withdraw cash, check balances or transfer funds from one account to another through the use of plastic cards inserted in ATMs operated by Network Exchange in the District and the two adjoining states.

EFT Group, organized originally as MX Group, describes its ATM program as the first phase of a much broader system that is expected to revolutionize the way consumers handle funds in this region.

Initially, the network will link electronically members' 500 existing ATMs, enabling customers to obtain cash and checking account balances at remote units from Maryland's border with Delaware and Pennsylvania to the Virginia-North Carolina border. Later, customers will be able to transfer funds between accounts, obtain balances and, where permitted by law, deposit money in their accounts.

"We think we're laying the foundation for a lot of things that will be happening," said Warren Minami, president of EFT Group and a vice president at American Security Bank in the District.

He said that the next phase in the program will connect several retail outlets to the system. EFT Group is expected to announce next month that several merchants in the region will participate.

Although no merchants have been identified, EFT Group is considering "obvious locations where checks are cashed and where you improve customer convenience through electronics," said Minami. Locations also might include service stations that would be equipped with point-of-sale terminals to process electronic payments, he added.

In addition to American Security, other member banks in EFT Group are NS&T Bank, McLachlen National Bank and Women's National Bank, all in the District; Bank of Bethesda, Equitable Trust Co. of Baltimore, First National Bank of Southern Maryland, State National Bank of Rockville, and Union National Bank of Westminister, all in Maryland; and Dominion Bankshares Corp. of Roanoke, Bank of Virginia in Richmond, Jefferson Bankshares Inc. of Charlottesville, Fauquier National Bank of Warrenton, and Town and Country Bank of Reston, all in Virginia.

S&Ls include Perpetual American Federal Savings and Loan Association, Columbia First Federal Savings and Loan Association, Independence Federal, and OBA Federal, all in the District; Citizens Savings and Loan Association of Silver Spring, Continental Federal Savings and Loan of Fairfax, Equitable Federal of Wheaton, Fairfax Savings and Loan of Baltimore, Liberty Savings and Loan of Warrenton, Mount Vernon Savings and Loan of Alexandria, Municipal Savings and Loan of Towson, and Piedmont Federal of Manassas.

Other members are Civil Service Employes Federal Credit Union, Department of Justice Federal Credit Union and State Employes Credit Union of Maryland Inc.