What is this?

[iwire]

Hm...I guess it would be used AFTER the fire, as in case of fire the fans are usually shut down.

That would depend on the conditions.

Many of the occupancies I have worked on have either just smoke evac or smoke evac combined with positive pressure systems. Sometimes the fans are dedicated to the purpose, sometimes the buildings existing HVAC system is used or a combination of both.

For example if a large store connected to a shopping mall has a fire or smoke condition the smoke evac may be used to keep the smoke out of the mall. Or if the there is fire in the mall the system goes into positive pressure mode keeping the smoke in the mall and out of the store.

Most of the systems I have been involved with are directly controlled the fire panel with override controls for the FD located at a location that suits the FD.

Along with large stores I see these systems in multi-floor multi-family dwelling units.