Detcon 12B - Manual (Page 11)

Model 12B
12B Control Module Instruction Manual Rev. 1.4 Page 7 of 12
8.0 FAULT CIRCUIT FUNCTIONS
Model 12B control modules feature fail safe supervisory circuits designed to assure maximum reliability in
system performance. A fault condition will cause the fault LED to illuminate and the fault relay to actuate. The
exception to this is if the fault relay is jumper-programmed as normally energized, a loss of external 24 VDC
power or the internal 5 VDC supply will cause the relay to de-energize but the LED will not illuminate.
Below is a list of conditions that will cause a fault alarm.
1) Placing the alarm disable switch in the disable position
2) Loss of 24 VDC power to the 12B control module (only if relay is jumper programmed as normally
energized).
3) Open power loop from the control module to the sensor.
4) Open signal loop from the control module to the sensor.
5) When used with a Detcon TP series solid state H2S sensor — an open heater on the sensor chip
6) When used with a Detcon FP series combustible sensor — an open catalytic bead or bridge.
7) Loss of the internal 5 VDC supply (only if relay is jumper programmed as normally energized).
8) Signal level drift below 2.4 mA from the sensor.
8.1 Detector Load Bypass Jumper
When a sensor other than the Detcon TP series H2S, or FP series LEL is used with the 12B control module, a
load bypass jumper must be placed in the appropriate location to prevent a false fault alarm. This jumper’s
purpose is to bypass the heater/bridge fault detection circuitry which is not needed when used with Detcon
oxygen or toxic gas sensors.
Another application that will require the load bypass jumper is if the sensor is being powered by a power
supply other than the supply that is coming from the 12B control module. In this instance, the load bypass
jumper will prevent a false fault condition that would be caused by the 12B control module’s monitoring of the
power loop. An example of this type of application is a sensor that is part of a sample draw system. In this
instance, the sensor is an integral part of the sample draw system and is typically powered by that system. If
desired, gas conditions at the sensor can be monitored by simply terminating the 4-20 mA signal output from
the sensor to the 12B control module. Since the control module is not powering the sensor, it will detect a
power loop fault and will require that the load bypass feature be used.