Woodward MicroNet TMR 5009, Installation/Hardware Manual

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5009 Installation/Hardware Manual 85580V2
48 Woodward
4. If the developed load voltage is greater than the load’s drop-out voltage, it is
recommended that latent fault detection be disabled, or that a resistor be
connected in parallel (shunt) with the load. A correctly sized resistor
connected in parallel with the circuit load will decrease the developed load
voltage below the load’s drop-out voltage level. Using the corresponding LFD
graph and the load’s minimum drop-out voltage, perform the above
procedure in reverse (See Step #2) to determine an acceptable shunt
resistance. When selecting a shunt resistor also verify that its voltage and
wattage ratings meet that of the circuit.
LFD Verification Example:
Circuit Power = 110 Vac
Load Resistance = 200
Ω
Load drop-out voltage = 25 Vac
Using the graph in Figure 4-11, the intersection point between the 200
Ω load
resistance line and the 110 Vac line was found. From this intersection point it
was determined that the voltage developed across the load due to leakage
current (when the relay is open) is approximately 7.5 Vac. This voltage level is
lower than the load’s 25 Vac drop-out voltage, thus Latent Fault detection can be
used with this example circuit.
If, however, the load resistance was 1200
Ω , the intersection would be
approximately 29.5 Vac too high for LFD. By following the graph along the 25
Vac line to the 110 V line, a total load resistance of 900
Ω is needed. By placing
a properly rated 3600 Ω resistor in shunt with the load, (1200//3600 Ö 900) LFD
can be used.
Figure 4-10. Latent Fault Detection Verification Graph—18–32 Vdc Circuitry