They are released by a detection system rather than by heat at a sprinkler head.

Wet and dry valves respond to a head opening; these respond to detectors.

That moves the decision from the head to the electronics.

Release can be electric, pneumatic or hydraulic depending on the system.

It also means the detection system becomes part of the fire protection rather than merely reporting on it.

Its design and maintenance therefore matter as much as the valve's.

Where fire would spread across an area almost instantly - flammable liquids, transformer bays, aircraft hangars, some process plant.

Deluge uses open heads with no thermal element, so every head discharges at once.

Waiting for individual heads to operate would be far too slow in those hazards.

The whole protected area is wetted simultaneously.

It is completely wrong anywhere else, because an accidental release floods everything.

Foam-water deluge is a common variant where flammable liquids are involved.

Accidental discharge from a damaged pipe or a broken sprinkler.

In a data hall, archive or cold store, water damage can cost more than the fire would.

Single interlock requires detection to release the valve, then a head to open before discharge.

Double interlock requires both detection and a head to operate before water even enters the pipe.

A physically damaged pipe therefore does not flood the room.

The cost is complexity - detection, release and air supervision can all fail in ways a wet system cannot.

Whether water enters the pipework on detection alone, or only when detection and a head both operate.

Single interlock fills the pipe on detection, so water is waiting at the heads when one opens.

That reduces delay but means a subsequent pipe failure discharges water.

Double interlock keeps the pipe dry until both events occur, giving maximum protection against accidental discharge.

It also adds delay, because water only starts moving after both conditions.

Double interlock is common in freezers, where the pipe must stay dry for a second reason.

Very much - a detection system slower than the fire is a preaction system that arrives late.

Detector type, spacing and response all affect when the valve releases.

Aspirating and high-sensitivity detection is common where early release matters.

Detection in high airflow environments such as data halls needs specific attention.

Because detection is now part of the protection, its maintenance is not optional.

A detection fault should be treated as a sprinkler system fault, which is not always how it is handled organisationally.

End to end - detection, release, valve and water delivery - because tripping the valve manually proves only part of it.

A manual release test says nothing about whether the detectors would have released it.

Full functional testing exercises the whole chain from detector to water.

That is disruptive, which is why it is periodic and carefully planned.

Air supervision and supervisory alarms are tested separately.

The applicable standard governs how often this is done, by what method, and what counts as a pass.

Detection failure, release failure, air supervision loss and false release - none of which exist in a wet system.

A detection system out of service takes the sprinkler protection with it.

Release solenoids and their power supplies are additional single points of failure.

Air supervision on a double interlock system introduces the same leak management as a dry system.

False release from a detection fault floods the room the system was protecting from water.

That extra complexity is accepted deliberately in exchange for what preaction protects.