Millions of mobile phone users are about to receive a "presidential alert". Here is what you need to know.

From 14:18 Wednesday, Eastern Time, Americans from all over the country will be interrupted by an urgent notification on their cell phone.

"Presidential Alert" will read the message. "THIS IS A TEST of the national wireless emergency alert system. No action is necessary. "

As it was launched by cell towers across the country in 30 minutes, the message is expected to reach some 225 million people in an unprecedented federal fiscal year.

Many mobile phone users may be aware of the alerts they receive as warnings about floods or missing children. But these emergency weather alerts and AMBER alerts can only be sent on a regional or national basis, not at the national level, but simultaneously.

Wednesday's wireless emergency alert system test will be the first time emergency management officials have used national alert capabilities reserved for the president's office.

Unlike other types of emergency alert, the Presidential Alert is designed to allow the White House to inform the entire country almost instantly of serious public emergencies, such as that a terrorist attack or an invasion, according to the Federal Agency for Emergency Management. And while Americans may choose not to receive weather alerts and AMBER, they can not withdraw from presidential alerts.

Despite his name, Wednesday's presidential warning will not be triggered directly by President Trump. But in general, the process requires the president – or his representative – to authorize FEMA to send an alert on behalf of the White House.

Prior to displaying on cell phones, the alert must be routed to wireless operators such as AT & T and Verizon via an online system called Integrated Alert System. public warning or IPAWS.

The mobile phone alert will be accompanied by a similar test of the emergency alert system, an older system that manages radio and television.

The law requires the federal government to test its warning systems every three years. FEMA tested the EAS in 2016 and 2017 and the first wireless emergency alerts were sent out in 2012.

However, not all mobile phones can receive Wednesday's alert. You will not get it if you are not in range of a cell phone tower, for example, or if your phone is off. But this should only be the case for about a quarter of the country's mobile phones.

The warning is probably a reflection of our hours of permanent connection with technology. Yet the same technology that has connected millions of people also has the potential to cause chaos at Internet speeds.

In January, US emergency management technologies made headlines when a national official mistakenly alerted Hawaiian residents to warn him of a ballistic missile attack. The report was false, but he pushed the Hawaiians to rush to hide, to seek refuge in gyms, mountain bunkers and even in manholes.

A subsequent investigation by the Federal Communications Commission revealed that the staff member responsible for the message had thought that an attack was actually underway, having misinterpreted an exercise.

FEMA officials said the Hawaii incident had no role in planning Wednesday's tests.