At 2 p.m. Eastern time on Wednesday all television channels and radio stations in the United States will be interrupted by a brief test of the nation’s Emergency Alert System.

Viewers and listeners are accustomed to hearing the tones and reminders — “this is just a test” — when the systems are activated locally each week by broadcasters. But government officials say the national system has never been tested before as a whole, nor has it ever been used in an emergency, allowing the president to address the public during a national emergency.

Officials said the nationwide test should last about 30 seconds as it digitally ripples across the country. Government agencies and media companies have sought to spread the word about the test so that it does sneak up on, and potentially scare, the public.

Michael Powell, the head of the Cable and Telecommunications Trade Association, wrote on Twitter, “No one wants a ‘War of the Worlds’ sequel!”

In a blog post, the trade association wrote: “Our message is simple: This is just a test of the system, and no action is required.”

The Federal Communications Commission and the Federal Emergency Management Agency are leading the effort to carry out what is formally called the Presidential Emergency Action Notification. The test “will allow
emergency personnel to assess and improve our alerting capabilities in the event of a crisis,” the White House wrote Tuesday in a blog post.

During the test, the on-screen text set up by the government will say, “This is an Emergency Action Notification.” It will not specify that the notification is only a test because officials want the test to duplicate actual alert conditions “as closely as possible,” according to an F.C.C. planning document. But voiceovers and other on-screen graphics will indicate that there is no need for alarm.

Planning for the test has not been without its hiccups. Last month, the cable trade association asked the government to postpone it because some cable systems appeared to be unable to include their won “this is just a test” graphics. Perhaps to alleviate their concerns, the government shortened the test to 30 seconds, after having planned for it to last for up to three minutes.

Television and radio stations will have to report back to the agency about whether they received the test message and whether they rebroadcast it. “We’re confident that the vast majority of local radio and TV stations will participate in the E.A.S. test, and that those tests will be successful,” a spokesman for the National Association of Broadcasters said.

Satellite distributors like DirecTV and cable distributors like Comcast will participate along with over-the-air stations. DirecTV said its customer service agents are prepared to answer questions from customers about the interruption, adding, “we expect the test to go smoothly.”

Internet connections are not included in the test.

Notifications about the test have been hard to miss — though some people surely will be surprised by it anyway. Public service announcements and graphics have run on local stations, messages have appeared on cable customer bills and Web sites and news segments have informed people about the plan.

What is now called the emergency alert system was first authorized in 1951 by President Harry Truman. It was first intended to inform Americans about an impending nuclear attack and was called CONELRAD, short for “Control of Electromagnetic Radiation.” The system was superseded by the emergency broadcast system, which was used primarily for local weather alerts and was replaced by the current emergency alert system in 1997.

The current system has never been turned on nationwide. After the terrorist attacks of Sept. 11, 2001, President George W. Bush addressed the nation through the major television networks without activating the emergency alert system. The networks were able to transmit Mr. Bush’s statements live on their own.

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