Siemens S7-300 Installation Manual page 48

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If the jumper is not installed, the reference potential of the S7-300 is connected
internally to the ground conductor via an RC system and the rail. This discharges
radio-frequency interference current and avoids static charge.
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To create a floating reference potential, remove the jumper from the CPU between
the M terminals and functional ground.
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In configurations with isolated modules, the reference potentials of the control
circuit (M
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Use isolated modules for:
• All AC load circuits
• DC load circuits with a separate reference potential, for example:
DC load circuits whose sensors have different reference potentials (for
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example if grounded sensors are located at some considerable distance
from the control system and no equipotential bonding is possible)
DC load circuits with grounded positive pole (L+) (battery circuits).
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You can use isolated modules, regardless of whether or not the control system's
reference potential is grounded.
5-22
) and the load circuit (M
internal
S7-300 Automation System, Hardware and Installation: CPU 312IFM - 318-2 DP
) are galvanically isolated.
external
A5E00203919-01