NAPERVILLE, Ill. (CBS) — If you’re in Naperville on a clear, sunny day and an air raid siren begins to wail, don’t worry. It’s only a test.

On Monday, contractor Braniff Communications Inc. began testing the 25 outdoor warning sirens in Naperville activated to test their functioning. They will each be set off sometime between 8 a.m. and 5 p.m. sometime before July 15.

Each siren includes a radio that receives commands and reports the status of the unit to public safety crews. Dan Nelson, the city’s emergency management coordinator, said the testing process encompasses the radios as well as the emergency alert equipment.

Meanwhile, another DuPage county town has changed its policy on siren use, after a tornado touched down in the town with no public outdoor warning.

The tornado touched down in Downers Grove on June 21. The sirens in the town never went off, because Downers Grove required a funnel cloud actually to be spotted for the sirens to be activated.

No funnel cloud was actually seen within five miles of the village.

The new policy, put into effect on July 1, is similar to that of surrounding towns that did activate their sirens. Under the new policy, the sirens would be activated without a visible sighting of a tornado.

Sirens would be activated if a tornado warning has been issued for the area, or if top village officials determine there’s an imminent threat from a tornado or severe winds.

The Sun-Times Media Wire contributed to this report.