Because it is otherwise entirely passive - it will fail silently and a defective system looks identical to a healthy one.

The pipework gives no indication of whether it is charged, isolated or flowing.

Switches convert the system's state into information at the alarm panel.

Without a flow switch, a sprinkler can discharge unnoticed until the water is seen.

Without a tamper switch, a closed valve is invisible.

They are the only routine feedback the system provides.

Either a vane in the pipe that moves with flow, or a pressure switch on the alarm line.

Vane-type switches sense water movement directly through a paddle in the pipe.

Pressure-type switches sense the alarm line pressurising when the system valve operates.

Both include a retard function so brief pressure surges do not cause false alarms.

The retard introduces a short deliberate delay, which the design accepts.

It is what turns a sprinkler operation into a fire alarm rather than a discovery.

Because a closed control valve is the most serious defect a system can have and it is completely invisible.

The installation looks entirely normal and will do nothing.

Valves get closed for maintenance and not reopened, or are closed deliberately.

No inspection short of physically checking every valve would find it.

Most regimes require supervision of valve position rather than merely recommending it.

The switch signals a supervisory condition as soon as the valve moves off fully open.

An alarm means something is happening now; a supervisory signal means the system's readiness has been impaired.

Waterflow is an alarm - water is moving and a fire may be in progress.

Tamper and low pressure are supervisory - the system is less ready than it should be.

They demand different responses and should present differently at the panel.

Muddling the two during commissioning is a recurring problem.

A supervisory signal treated as a nuisance is how systems end up isolated for months.

Supply pressure, air pressure in dry and preaction systems, or the alarm line depending on where they are fitted.

On a dry system, low air pressure is an early warning of a leak before the system trips.

High air pressure can indicate a regulator fault.

Supply pressure monitoring reveals problems with the water source or pumps.

Alarm line pressure switches perform the waterflow alarm function on some systems.

Each has its own expected range, and the switch settings are part of the commissioning record.

Functionally, end to end - a switch installed and never connected is a common defect that only functional testing finds.

Waterflow switches are tested through the inspector's test connection.

Tamper switches are tested by partially closing the valve and confirming the signal appears.

Pressure switches are tested by varying pressure or by the manufacturer's test method.

Testing has to confirm the signal reaches the panel and displays correctly.

Frequencies and record-keeping are set by the local standard rather than by the device.

It crosses trades, and work that crosses trades is where installations are most often left unfinished.

The switches are sprinkler components; the panel and cabling are usually alarm contractor scope.

That boundary is where devices get installed and never connected.

Commissioning should prove every device through to the panel rather than assuming the interface.

Cause and effect documentation states what each signal should produce.

Later alterations to either system are a frequent source of disconnected or misattributed signals.