Don't panic. It's a test, only a test.On Wednesday, at 2 p.m. EST, U.S. radio and television stations will break away from regular programming to broadcast emergency warnings, but it'll be a practice exercise. The U.S. Federal Emergency Management Agency (FEMA) and Federal Communications Commission will conduct the first ever nationwide test of the Emergency Alert System in an effort to try the service on a large scale.The alert system, about 50 years old in various forms, is tested frequently by local stations, but never on such a wide basis. The test will last for about 30 seconds and will look and sound similar to past system tests.The Emergency Alert System allows the U.S. president, as well as state and local authorities, to broadcast warnings about disasters, severe weather and other emergencies."The various disasters our country has faced this year underscore the need for effective and well-tested emergency alert and warning systems that could be used in a time of real emergency, at a moment's notice," FEMA Administrator Craig Fugate and FCC Chairman Julius Genachowski wrote in their letter to participants last week. "The purpose of the test is to allow FEMA and the FCC to assess how well the Emergency Alert System would perform its primary function: alerting the public about a national emergency."The test will happen on all U.S. broadcast radio and television stations, cable television systems, satellite radio and television systems, and wireline video service systems. The test will not involve landline or mobile phones or Internet services.The FCC announced the test in June, after several months of planning. The agency has already found that the system is not built to support captioning services and translations, the FCC said. "Consequently, many viewers, particularly cable television subscribers, will see the emergency alert on the screen that is accompanied by an audio explanation that 'this is only a test,' but may not see a corresponding visual message that 'this is only a test,'" the agency said in a notice.FCC and FEMA educational efforts have focused on letting the public know what to expect during the test. Grant Gross covers technology and telecom policy in the U.S. government for The IDG News Service. Follow Grant on Twitter at GrantGross. Grant's e-mail address is email@example.com.