Application

Sequence of Operation

Fail Mode Operation

Siemens Industry, Inc.

Application Note, App 2926

140-1308

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 the LCM receives an invalid (less than 1 Vdc) fume hood flow

signal, or the fume hood controller (FHC) loses power or loses its flow sensor, HOOD

VOL will fail. 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

(see figure). 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.