“This is a test. This is only a test.”

When millions of Americans hear that warning at 2 p.m. ET Wednesday, the words will sound familiar, but the occasion will be historic. It will mark the first-ever nationwide test of the Emergency Alert System, a system with Cold War roots that enables the president to address the American public within 10 minutes from any location at any time.

Although state and local communities regularly use the system to notify communities about tornadoes, child abductions and other events, all previous tests of the system have been local or regional, and involve the voluntary compliance of broadcasters. Wednesday’s test is the first national test featuring a live “presidential” alert code, which instructs TV and radio stations that the alert takes priority over all other programming.

“I think the biggest reason nobody ever tested it was because of all the concerns of what could happen and what could go wrong,” Federal Emergency Management Agency chief Craig Fugate told CNN Tuesday. “We take a different approach. If we don’t test it, we don’t know what we need to fix.”

Officials at FEMA and at the Federal Communications Commission, which ordered the test, expect things to go smoothly.

But there are concerns. Chief among them: cable television systems are worried the test will unduly alarm deaf and hard-of-hearing viewers. That is because some cable systems, unlike broadcast and satellite TV, cannot superimpose a visual warning saying “This is a test” over the government’s graphic. Instead, they will see only this scrolled message: “This is an Emergency Action Notification,” which gives no visual indication it is just a test.

Some critics also said the test is a stark reminder that — 10 years after the September 11 terrorist attacks — the nation is still living with an antiquated warning system in a digital world.

The test will air on every broadcast, cable and satellite TV station in the nation, as well as every AM, FM and satellite radio station. But it will not be sent to landline or cell phones, pagers or computers, which millions of people monitor for news and information.

The country has made strides in developing an integrated warning system featuring mobile devices, but for most Americans, those systems are years away.

And some experts question whether the warning system has much value at the national level.

In a local emergency such as a gas leak, emergency management expert Art Botterell notes, the system enables authorities to tell the homeowners to stay inside or to evacuate. “But if the message is nationwide, what is the protection message that applies to everyone?” he said.

The system was not used during the September 11 terrorist attack, Botterell noted. But it remains, at a national level, a symbol of the president’s authority and responsibility, he said.

“I think the bottom line is everybody prays to God that we don’t have to use it, because if we do, whether it works or not will probably be the least of everybody’s worries.”

The test will be brief — 30 seconds from start to finish.

In the event of a real emergency, the test would originate from wherever the president is. For test purposes, it will originate from a FEMA operations center. The president will not be involved and will not speak.

But to duplicate real-world conditions, FEMA will initiate the test by sending a “live” presidential alert code, a code that would be used in an actual emergency, to a number of radio stations known as Primary Entry Point (PEP) stations.

Those in turn relay the information in a daisy chain fashion to other radio and television stations.

When a station receives the alert, it must discontinue normal programming and make its transmission lines available for the presidential message.

Under the Emergency Alert System agreement, participating stations agree to carry the message, but they have the option of signing off the air.

In some systems, the presidential alert automatically interrupts programming; at other stations, a switch is made manually.

At the end of the message, a termination signal is sent to stations allowing them to return to normal programming.

After Wednesday’s test, stations must report the results to the FCC within 45 days. The FCC says it will not release test data to the public, saying broadcasters worry that potentially embarrassing results could discourage participation in future tests, and test data could reveal security vulnerabilities.

READ MORE: CNN