Document: 553632
(ICSTT-RM448_EN_P) Issue: 08:
3-15
The suggested values for R1 and R2 are as follows:
R1 = 15K Ω 1%, 1W (maximum power dissipated is 47mW at 26.4V)
R2 = 3K9 Ω 1%, 1W (maximum power dissipated is 182mW at 26.4V)
Suggested threshold values for both of the above circuits are as follows:
Threshold ID Value (mV)
Maximum Allowed = 32000
SHORT CIRCUIT
Threshold 8 = 19000
Threshold 7 = 18500
ON (nominal 16V)
Threshold 6 = 11000
Threshold 5 = 10500
INDETERMINATE
Threshold 4 = 6500
Threshold 3 = 6000
OFF (nominal 8V)
Threshold 2 = 3500
Threshold 1 = 3000
OPEN CIRCUIT
Assumptions:
Loop supply voltage = 24V ± 10%
Maximum Field Cable Line Resistance: < 100 Ω total; this means < 50 + 50 Ω for
the two cables.
Minimum Isolation is 0.75M Ω between the field loop conductors.
These values will allow the input to detect more accurately different voltage levels
that represent OPEN CCT - OFF - ON - SHORT CCT and will also detect Over
Voltage and an input which is neither ON or OFF. The values ensure that a line
fault will be declared before it becomes possible for a false declaration of On and
Off states due to a combination of resistor value drift and loop voltage variation.
The F&G circuit will also allow two devices to be in alarm without reporting short
circuit.
All of the input circuits are suitable for simplex, dual and TMR configurations.
The F&G circuit assumes that the devices are volt-free contacts.
For further information, please refer to application note AN-T90001 Field Loop
Configuration. This also includes advice for fire detectors which are not simple volt
free contacts.
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