Grounding And Circuit Referencing Guidelines For Using Isolated Circuits - Siemens Simatic S7-200 CPU 210 System Manual

Grounding and Circuit Referencing Guidelines for Using Isolated Circuits

The following items are 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 possible different references can connect
together. Such connections can result in unwanted current flows that can cause logic
errors or damage circuits. A common cause of different reference potentials is grounds
which are physically separated by long distances. When devices with widely separated
grounds are connected with a 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 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.
The CPU 210 includes 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, 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.
The following descriptions are an introduction to general isolation characteristics of the
CPU 210, but some features may be different on specific products. Consult the data sheet in
Appendix A for your product for specifications of which circuits include isolation boundaries
and the ratings of the boundaries. Isolation boundaries rated less than 1500 VAC are
designed as functional isolation only and should not be depended on as safety boundaries.
CPU logic reference is the same as DC Sensor Supply M.
CPU logic reference is the same as the input power supply M on a CPU with DC power
supply.
CPU logic is isolated from ground to 100 VDC.
DC digital inputs and outputs are isolated from CPU logic to 500 VAC.
Relay outputs and AC inputs are isolated from CPU logic to 1500 VAC.
Relay output groups are isolated from each other by 1500 VAC.
AC power supply Line and Neutral are isolated from ground, the CPU logic, and all I/O to
1500 VAC.
S7-200 Programmable Controller, CPU 210
C79000-G7076-C235-01
Installing the S7-200 CPU 210
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