Application

Sequence of Operation
Alarms
34
Siemens Industry, Inc.
Application Note, App 6752
140-1322
2015-07-07
Even if the alarm level is set to zero, the ventilation alarm will still turn on if:
 SUP AIR VOL stays below the currently active supply minimum, for a time at least
equal to VENT ALM DEL.
and/or
 GEX AIR VOL stays below the currently active general exhaust box minimum, for a
time at least equal to VENT ALM DEL.
Pressurization Alarm
The pressurization alarm, VOL DIF ALM indicates that the difference between supply
and exhaust flow is not what it should be, or that the controller can't calculate the flow
difference, VOL DIFFRNC, because it has lost a flow signal. The Figure Failure Mode
Sequence
lists reasons why VOL DIFFRNC may fail.
The pressurization alarm point is turned on when at least one of the following
conditions occurs:
 VOL DIFFRNC has a status of Failed.
 VOL DIFFRNC stays below VOL DIF STPT – DIF ALM DBD for a time at least
equal to DIF ALM DEL.
 VOL DIFFRNC stays above VOL DIF STPT + DIF ALM DBD for a time at least
equal to DIF ALM DEL.
WARNING
To ensure that VOL DIF ALM turns on before the pressure in the room changes sign,
DIF ALM DBD must be less than the absolute value of VOL DIF STPT.
For example, if negative pressure is desired and VOL DIF STPT equals 70 cfm and
DIF ALM DBD is 200 cfm, then the room could go positive by almost 130 cfm without
the pressure alarm turning on. In this case, if you want the alarm to turn on before the
room changes sign, then you must set DIF ALM DBD to be less than 70 cfm.
The pressurization alarm point is turned off when all of the following conditions occur:
 VOL DIFFRNC has a status of Normal.
 VOL DIFFRNC stays above VOL DIF STPT – DIF ALM DBD for a time at least
equal to DIF ALM DEL.
 VOL DIFFRNC stays below VOL DIF STPT + DIF ALM DBD, for a time at least
equal to DIF ALM DEL.
 DIF ALM DBD and DIF ALM DEL can be configured to prevent nuisance alarms.
Local Annunciation
ALARM ENA is an analog point whose value determines whether or not a particular
alarm activates ALARM DO7.
For ALARM ENA, the terms enabled and not enabled do not mean that a particular
alarm is enabled or not. It means whether or not a particular alarm will or will not
activate ALARM DO7. For example, if ALARM ENA is set to
1 (Vent Alarm Enabled)
and a ventilation alarm occurs, then both VENT ALM and ALARM DO7 will turn on.
However, if ALARM ENA is not Vent Alarm Enabled and a ventilation alarm occurs,
VENT ALM will turn on, but ALARM DO7 will not.