Guidelines For Dc Installation - Siemens SIMATIC S7-200 System Manual

Installing an S7-200 PLC

Guidelines for DC Installation

The following items are general wiring guidelines for DC installations. Refer to
Figure 2-12.
[a] Provide a single disconnect switch that removes power from the CPU, all
input circuits, and all output (load) circuits.
[b] Provide overcurrent devices to protect the CPU power supply, [c] the
output points, and [d] the input points. You can also fuse each output point
individually for greater protection. External overcurrent protection for input
points is not required when you use the 24 VDC sensor supply from the
Micro PLC. This sensor supply is current limited internally.
[e] Ensure that the DC power supply has sufficient surge capacity to maintain
voltage during sudden load changes. External capacitance may be required.
[f] In most installations, you will have best noise immunity by connecting all DC
power supplies to ground. Equip ungrounded DC power supplies with a
resistor and a capacitor in parallel [g] from the power source common to
protective earth ground. The resistor provides a leakage path to prevent static
charge accumulations, and the capacitor provides a drain for high frequency
noise. Typical values are 1 MΩ and 4,700 pf.
[h] Connect all S7-200 ground terminals to the closest available earth ground
to provide the highest level of noise immunity. It is recommended that all
ground terminals be connected to a single electrical point. Use 14 AWG or 1.5
2
mm wire for this connection.
Always supply 24 VDC circuits from a source that provides safe electrical
separation from 120/230 VAC power and similar hazards.
The following documents provide standard definitions of safe separation:
PELV (protected extra low voltage) according to EN60204-1
Class 2 or Limited Voltage/Current Circuit according to UL 508
2-14
S7-200 Programmable Controller System Manual
A5E00066097-02