Rockwell Automation 1758-RTU202 DataSite Hardware User Manual - Manual (Page 15)

Publication 1758-UM001D-EN-P - June 2011
Installing the Controller 15
Power Distribution
There are some points about power distribution that you should know:
• The master control relay must be able to inhibit all machine motion by
removing power to the machine I/O devices when the relay is
de-energized. It is recommended that the controller remain powered
even when the master control relay is de-energized.
• If you are using a DC power supply, interrupt the load side rather than
the AC line power. This avoids the additional delay of power supply
turn-off. The DC power supply should be powered directly from the
fused secondary of the transformer. Power to the DC input and output
circuits should be connected through a set of master control relay
contacts.
Periodic Tests of Master Control Relay Circuit
Any part can fail, including the switches in a master control relay circuit. The
failure of one of these switches would most likely cause an open circuit, which
would be a safe power-off failure. However, if one of these switches shorts
out, it no longer provides any safety protection. These switches should be
tested periodically to assure they will stop machine motion when needed.
Power Considerations
The following explains power considerations for the DataSite controller.
Loss of Power Source
The power supply is designed to withstand brief power losses without
affecting the operation of the system. The time the system is operational
during power loss is called program scan hold-up time after loss of power.
The duration of the power supply hold-up time depends on the type and state
of the I/O, but is typically between 10 milliseconds and 3 seconds. When the
duration of power loss reaches this limit, the power supply signals the
processor that it can no longer provide adequate DC power to the system.
This is referred to as a power supply shutdown. The processor then performs
an orderly shutdown of the controller.