Siemens SIMATIC TD 200, User Manual

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B-8
SIMATIC TD 200 Operator Interface
C79000-G7076-C272-01
B.3 CPU Grounding and Circuit Reference Point Guidelines for Using
Isolated Circuits
The following items are CPU grounding and circuit guidelines for using isolated
circuits:
You should identify the reference point (0 voltage reference) for each circuit in
the installation, and the points at which circuits with possibly different
references can connect together. Such connections can result in unwanted
current flows that can cause logic errors or can damage circuits. A common
cause of different reference potentials is grounds that are physically separated
by long distances. When devices with widely separated grounds are connected
with a communication or sensor cable, unexpected currents can flow through
the circuit created by the cable and the ground. Even over short distances, load
currents of heavy machinery can cause differences in ground potential or can
directly induce unwanted currents by electromagnetic induction. Power supplies
that are improperly referenced with respect to each other can cause damaging
currents to flow between their associated circuits.
When you connect CPUs with different ground potentials to the same PPI
network, you should use an isolated RS-485 repeater.
S7-200 products include isolation boundaries at certain points to help prevent
unwanted current flows in your installation. When you plan your installation, you
should consider where these isolation boundaries are provided, and where they
are not provided. You should also consider the isolation boundaries in
associated power supplies and other equipment, and where all associated
power supplies have their reference points.
You should choose your ground reference points and use the isolation
boundaries provided to interrupt unneeded circuit loops that could allow
unwanted currents to flow. Remember to consider temporary connections which
may introduce a new circuit reference, such as the connection of a
programming device to the CPU.
When locating grounds, you must also consider safety grounding requirements
and the proper operation of protective interrupting devices.
In most installations, you will have the best noise immunity if you connect the
CPU sensor supply M terminal to ground.
Multiple CPU Configurations