Woodward MicroNet TMR 5009, Installation/Hardware Manual

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Manual 85580V2 5009 Installation/Hardware
Woodward 47
When a relay output is open, the contacts of all six relays are open. Because of
the series-parallel configuration that the relays are in, the failure of any one relay
will not cause the output to be closed. The relay output would continue to be
open. Once a relay output is open, the output’s individual relays are periodically
closed and re-opened one by one, to ensure that they were in the correct state,
and that they change state. Position readback circuitry allows the state of each
relay contact to be detected. Any failures are annunciated, and further testing is
disabled without affecting the state of the relay output contact or control
operation.
Latent fault detection (LFD) is not usable with all applications or circuits. The
control’s LFD logic can only work with circuits using voltages between 18–32
Vdc, 100–150 Vdc, or 88–132 Vac. For LFD to work, a small leakage current is
passed through the circuit’s load. See Figure 4-15. Depending on the size of the
load, the leakage current may be enough to cause a load to be on or active,
when a relay contact is open. In this case, the individual relay’s LFD logic may be
disabled, eliminating the leakage current.
If LFD is desired, but the leakage current is too great for the load, an external
resistor may be connected in parallel with the circuit’s load to shunt some of the
leakage current away from the load. To prevent failure of a load to de-energize,
careful consideration should be given, to ensure that the voltage developed
across the load due to leakage current is below the load’s drop-out voltage.
With LFD, when a relay contact is closed, no difference in operation is
experienced; the relay output appears as a closed contact. However, when a
relay contact is open, it appears to the interfaced circuit as a large resistor
instead of an open contact. Thus a small amount of current is leaked to the load,
resulting in a developed voltage across the load. In most cases this has no
bearing on the customer’s circuitry, because such a small amount of voltage is
developed across its load. However, when a relay output is used with a very high
resistance load (low current load), enough voltage may be developed across the
load to prevent it from de-energizing.
To verify if Latent Fault Detection can be used with a relay output:
1. Verify that circuit the relay output is used with has a voltage level of 18–32
Vdc, 100–150 Vdc, or 88–132 Vac. If circuit voltage is not within these
ranges, disable the relay output’s latent fault detection by placing the relay
jumpers in their disable state. See Figure 4-13.
2. Use the graph below (Figures 4-10, 4-11, or 4-12) which corresponds to the
circuit’s voltage level to determine if the voltage developed across the load
(due to the leakage current) is lower than the load’s drop-out voltage level.
•
Acquire the resistance of the load (relay, motor, solenoid, etc.) to be
driven by the relay.
•
Acquire the load’s minimum drop-out voltage.
•
From the bottom of the graph, follow the line corresponding to the load’s
resistance, up until it intersects the circuit power line. At this point the
corresponding voltage level (on the left of the graph) is the level of
voltage that will be developed across the load due to leakage current.
3. If the developed load voltage (from the graph) is less than the load’s drop-out
voltage, latent fault detection can be used with the circuit.