Siemens Simatic S7-300 CPU 31xC Series Installation And Operating Instructions Manual page 233

Lightning Conductor for Signal Modules
You can use standard surge protection components for the digital I/O modules. However,
please note that these only permit a maximum of 26.8 V for a rated voltage of 24 VDC. If the
tolerance of your 24 VDC power supply is higher, use surge protection components with 30
VDC rating.
You can also use Blitzductor VT, type AD 24 V. Note that input current can increase if
negative input voltages are generated.
Low-voltage protection elements for 1 <-> 2
For the transition points between lightning protection zones 1 <-> 2 we recommend the
surge protection components listed in the table below. This low-voltage protection must be
used in S7-300 for CE compliance.
S7-300, CPU 31xC and CPU 31x: Installation
Operating Instructions, Edition 08/2004, A5E00105492-05
A.3 Lightning and surge voltage protection
Appendix
A-23