Rockwell Automation 1770 Industrial Automation Wiring and Grounding Guidelines - Manual (Page 14)

Industrial Automation Wiring and Grounding Guidelines 14
Publication 1770-4.1 – February 1998
Isolation Transformer — For applications near excessive
electrical noise generators, an isolation transformer (for the second
transformer) provides further suppression of electromagnetic
interference from other equipment. The output actuators being
controlled should draw power from the same ac source as the
isolation transformer, but not from the secondary of the isolation
transformer (Figure 9).
Constant-Voltage Transformer — In applications where the ac
power source is especially “soft” and subject to unusual variations, a
constant-voltage transformer can stabilize the ac power source to the
processor and minimize shutdowns. The constant-voltage
transformer must be of the harmonic neutralizing type.
If the power supply receives its ac power through a constant-voltage
transformer, the input sensors connected to the I/O chassis should
also receive their ac power from the same constant-voltage
transformer. If the inputs receive their ac power through another
transformer, the ac source voltage could go low enough that
erroneous input data enters memory while the constant-voltage
transformer prevents the power supply from shutting down the
processor. The output actuators being controlled should draw power
form the same ac sources as the constant-voltage transformer, but not
from the secondary of the constant-voltage transformer (9).
Ground Connection
When ac power is supplied as a separately derived system through
an isolation/step-down transformer, you can connect it as a grounded
ac system or an ungrounded ac system. For a grounded ac system,
connect one side of the transformer secondary to the ground bus as
in Figure 7. For an ungrounded ac system, connect one side of each
test switch for the ground-fault-detector lights to the ground bus as
in Figure 8. We do not recommend an ungrounded system. Follow
local codes in determining whether to use a grounded system.
When bringing ac power into the enclosure, do not ground its
raceway to the ground bus on the back-panel. Connecting the
raceway to the ground bus may cause the processor to fault by
introducing emi into the grounding circuit. Local codes may provide
an exception for permitting isolation from the raceway. For
example, article 250-75 of the National Electrical Code has an
exception that explains the conditions under which this isolation
from the raceway is permitted.