Application

Sequence of Operation

Fail Mode Operation

Siemens Industry, Inc.

BACnet LCM-OAVS, Application 6760

140-1326

2015-07-07

Fail Mode Operation

If one of the controller’s accessories (inputs) fails, a failure mode sequence is initiated

that leads to the failure of VOL DIFFRNC. The following figure shows the order in

which points will fail.

* If MAX HOOD VOL is set to 0, a “Failed” status of HOOD VOL will not initiate a failure

in TOTL EXHAUST or VOL DIFFRNC. See

Fume Hood Flow Input

Air Velocity Sensors – If one or both of the LCM air sensor signals (SUP AIR VOL,

GEX AIR VOL) are out of range (for example, improper wiring to/from the Offboard Air

Module(s), tubing not connected or connected backward), then the actions of the

supply and general exhaust Venturi Air Valves will depend on the value of AVS

FAILMODE. Once GEX AIR VOL and SUP AIR VOL are normal, the supply and

general exhaust Venturi Air Valves return to normal operation.

See

Operation of AVS FAILMODE

for more information.

Fume Hood Flow – If MINHOODVOLTS is 1 Volt or higher, HOOD VOL will fail when

the LCM receives an invalid (less than .4 Vdc) fume hood flow signal or when the fume

hood controller (FHC) loses power or when it loses its flow sensor. If HOOD VOL fails

and if VOL DIF STPT is greater than or equal to 0 (negative or neutral pressurization

required), the supply and exhaust loops assume a hood exhaust value of 0 cfm and

continue to maintain user-defined pressurization. If HOOD VOL fails and VOL DIF

STPT is less than 0 (positive pressurization required), the supply and exhaust loops

assume the hood's exhaust value is equal to MAX HOOD VOL and continue to

maintain user-defined pressurization. When the LCM is being used with fume hoods,

and MAX HOOD VOL is set above its default of 0 cfm, a failure of HOOD VOL causes

TOTL EXHAUST and VOL DIFFRNC to fail in succession. When VOL DIFFRNC fails,

the pressurization alarm point, VOL DIF ALM, turns on automatically. If any of these

points have been defined as alarmable and unbundled at the field panel, an alarm will

be annunciated across the network.

If MINHOODVOLTS is set to a value less than 1 (including 0), the fail voltage sensing

at .4 Vdc as described above is disabled. However, if there is a break (open circuit) in

the wiring, both the AI 3 point and HOOD VOL point will indicate failure.