Siemens CPU 313C Hardware And Installation Manual page 46

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If at all possible, set up the ungrounded reference potential before mounting on the
rail.
If you have already installed and wired up the CPU, you may have to detach the
connection to the MPI interface before you pull out the grounding slide contact.
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When isolated modules are installed, the reference potentials of the control circuit
(M ) and load circuit (M
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Use isolated modules for:
• All AC load circuits
• DC load circuits with separate reference potential
Examples:
DC load circuits whose sensors have different reference potentials (for
–
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.
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) are electrically isolated.
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S7-300 Automation System, Hardware and Installation: CPU 31xC and CPU 31x
A5E00105492-03