Dual Power Source

The MOSCAD RTU uses a dual source of operating power. The primary power source is a power supply
connected to the ac power mains. The primary power source provides a nominal 12 Vdc operating voltage to the
modules, the communication device, and to other active elements within the RTU. A rechargable battery is also
present that will power the modules, communication device, and certain other active elements of the RTU when
ac main power fails. The battery is connected to these elements through the power supply; the power supply acts
as the battery charger. The power supply/battery interface provides zero-transfer-transient performance.
Low-current and high-current power supplies are available. The low-current power supply provides 3 amps
at 12 Vdc and is included in MOSCAD RTU models that have no radio or low-power radios. The high-current
power supply provides 8 amps at 12 Vdc and is included in MOSCAD RTUs with high-power radios or in the
model with an interface to an external radio. The V261 option will replace the 3 amp power supply with it’s 8
amp equivalent when the extra current capacity is needed. The V274 option replaces the power supply and battery
with a cable that may be connected to a clean external source of 12 Vdc power; the V251 option changes the
input voltage from 117 Vac 50/60 Hz to 230 Vac 50/60 Hz.
The battery capacity is 5 A-h and is recharged by the power supply. The V326 option adds a second battery
to achieve 10 A-h capacity. Battery life is dependent upon transmitter usage, etc; refer to Chapter 3 for a discussion
of current drain & battery life. A lithium battery is also provided in the CPU module that is active only when ac main power fails and the backup battery disconnects. The lithium battery keeps the RAM and real-time clock
circuits functioning to help achieve an effortless RTU restart following voltage reconnect.
The power supply contains a low voltage disconnect circuit that prevents the battery from an over-discharge
condition which would destroy the battery. If a disconnect should occur following an ac power failure, the power
supply will maintain the disconnect state following power restoration until a minimum level of battery recharge
has occurred (provide the full output of the power supply to the battery). The power supply provides an ac fail
signal to the CPU module; this signal may initiate an ac fail message to be sent to the SCADA Manager and may
cause some RTU startup activities to occur following reconnect.
The CPU module and the rackmount Expansion module both contain a 5 volt power source that can supply
up to 2 amps to the 1-to-15 I/O modules in the module rack. Refer to Appendix A for help in calculating the
current drain from the 5 volt supply.