Siemens Simatic S7-200 System Manual page 25

Guidelines for DC Installation
The following items are general wiring guidelines for isolated DC installations. Refer to
Figure 2-13.
Provide a single disconnect switch (1) that removes power from the CPU, all input
circuits, and all output (load) circuits.
Provide overcurrent devices to protect the CPU power supply (2), the output points (3),
and the input points (4). 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.
Ensure that the DC power supply has sufficient surge capacity to maintain voltage during
sudden load changes. External capacitance (5) may be required.
Equip ungrounded DC power supplies with a resistor and a capacitor in parallel (6) 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. You can also create a grounded
DC system by connecting the DC power supply to ground (7).
Connect all S7-200 ground terminals to the closest available earth ground (8) 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 mm
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
L1
N
PE
24 VDC
Figure 2-13
S7-200 Programmable Controller System Manual
C79000-G7076-C230-02
Floating (6) or Grounded (7)
(1)
AC
(6)
DC
(5)
(2)
(3)
(4)
L+ M
Isolated DC System Installation
Installing an S7-200 Micro PLC
2
wire for this connection.
(8)
(7)
DO PS
DO
S7-200 EM 222
DI
DC/DC/DC DC
DI
EM 221
DC
2-11