Creating An Ungrounded Reference Potential On A Cpu - Siemens CPU 313C Hardware And Installation Manual

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If you install an S7-300 with ungrounded reference potential, any interference
currents that occur are dissipated via an RC system to the ground conductor/local
ground integrated into the CPU.
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An S7-300 with a CPU 31xC cannot be configured ungrounded.
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In extended systems, the S7-300 may require configuration with grounded
reference potential due to ground-fault monitoring. This is the case, for example, in
chemical industry and power stations.
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The diagram illustrates an S7-300 configuration with floating reference potential
Figure 5-8 Creating an ungrounded reference potential on a CPU
 If you implement an ungrounded reference potential in your CPU: Use a
screwdriver with 3.5 mm blade width and push the grounding slide contact forwards
in the direction of the arrow until it snaps into place.
Ground of internal CPU circuitry

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S7-300 Automation System, Hardware and Installation: CPU 31xC and CPU 31x
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