Rockwell Automation T3419 ICS Regent Fire Detector Input - Manual (Page 11)

Fire Detector Input Modules (T3419)
P D - 6 0 3 2 M a r - 0 6
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Circuit and Short Circuit should be examined. Each of these
points should fall within the boundaries determined by the
input module thresholds. If necessary the thresholds can be
adjusted during configuration of the module using
W
INTERPRET .
For example, consider a fire detector that has an alarm
characteristic of 300Ohm with a 4Vdc Zener and a line
termination resistor of 3K Ohms. When the fire detector is in
the normal state (not tripped) the input load is only the 3K
Ohm termination resistor. This is illustrated in Figure 4 as
line A. The point at which this line intersects the load-line
represents the input voltage and loop current when the fire
detector is in the normal state. This point is located well
between the default Open Circuit current threshold (3 mA)
and the default On/Off Alarm threshold (10.5 mA).
Next, the fire detector alarm state should be examined. The
alarm resistance of the fire detector is 300 Ohms and this is
illustrated as Line B in the figure. However, the 4 Volt Zener
of the fire detector offsets the operating point by 4 Volts as
shown by Line C. The point at which Line C intersects the
load-line represents the input voltage and load current when
the fire detector is in the alarm state. This point should be
between the On/Off threshold and the Short Circuit threshold.
In this case the alarm point is rather close to the Short Circuit
threshold so it may be necessary to modify the Short Circuit
threshold to a slightly higher value, like 17 mA.
Finally, the Open Circuit and Short Circuit conditions must
be examined. In this example, the Short Circuit condition is 0
Ohms (or a small amount of resistance representing the field
wire resistance to the point at which the short occurs). This
would place the Open Circuit point near to the point where the
load line intersects the x-axis in the Figure. Similarly, the
Open Circuit is infinite Ohms which would place the operating
point where the load-line intersects the y-axis.
In many applications you may also connect and intrinsic
safety barrier in the field loop. In these applications the
resistance and voltage drops of the barrier must also be
included in determining the operating points of the input
circuit under all four operating conditions. If necessary,
Note: