Potter Releasing Systems - Manual (Page 56)

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Pre-Action (Double-Interlock) Systems
The double interlock system is designed for applications such as refrigerated areas, requiring maximum
protection against inadvertent operation.
Double interlock systems use closed sprinklers attached to a piping system containing supervisory air pressure.
The water is held back by an electrically operated valve. The valve is connected to a compatible releasing panel.
The panel is monitoring initiating devices such as smoke or heat detectors, and manual stations that are located in
the same area as the sprinkler heads. These devices are connected to the releasing panels alarm initiating zones.
The supervisory air pressure in the piping system is also monitored. A low air switch on a double interlock
system may perform two functions. One is to send a supervisory signal if the air pressure drops to a certain
point such as if an air compressor failed. This would be connected to a supervisory zone on the release panel.
The other would be an alarm condition if the air pressure dropped farther such as from a sprinkler opening due
to fire. This would be connected to an alarm initiating zone on the release panel and is one of the two events
necessary for the panel to operate the release circuit. The other event is a smoke or heat detector going into
alarm. On some release panels the low air switch may only cause a supervisory condition not an actual alarm
condition even though it is still one of the two events necessary to operate the release circuit.
In order to trip the release circuit, two independent events must occur. The sprinkler system piping must lose
air pressure due to the operation of one or more heads which causes the low air pressure switch to trip, and the
release panel must detect an alarm from an initiating device such as a smoke or heat detector.
If the system pressure drops due to an open sprinkler, damaged pipe, or compressor failure, the pressure switch
will cause a supervisory and/or an alarm condition on the release panel. If a smoke or heat detector or manual
station has not operated, the release circuit will not trip because only one event has happened
If a smoke or heat detector operates or a manual station is activated, the release panel will go into alarm. If the low
air switch has not tripped, the release circuit on the panel will not operate because only one event has happened.
Typical System Configuration (PFC-4410RC Program #11)
Initiating (inputs)
Zone #1 - Conventional Alarm Zone - Smoke or Heat detectors
Zone #2 - Conventional Alarm Zone - Low Air Alarm switch
Zone #3 - Waterflow Zone - Pressure type waterflow switch
Zone #4 - Manual Station Zone - Pull Stations
Sup #1 - Supervisory Zone - Valve tamper and low air supervisory switch
Sup #2 - Supervisory Zone - Valve tamper and low air supervisory switch
Indicating (outputs)
#1 - Alarm - General alarm. Bell, horn, strobe, etc.
#2 - Alarm - Waterflow bell
#3 - Release - Solenoid on valve
#4 - Supervisory - Bell or horn to indicate supervisory condition
Theory of Operation
When both zones 1 and 2 are in alarm at the same time or if the manual station zone is
activated, the release circuit and the general alarm outputs, will operate.
Activation of zone 1 or zone 2 will operate the general alarm output.
Activation of the waterflow zone will operate the waterflow output.
Activation of the supervisory zone will operate the supervisory output.