Appendix
A.3 Lightning and surge voltage protection
S7-300, CPU 31xC and CPU 31x: Installation
A-28 Operating Instructions, Edition 08/2004, A5E00105492-05
A.3.6 How to protect digital output modules against inductive surge voltage
Inductive surge voltage
Overvoltage occurs when inductive devices are switched off. Examples are relay coils and
contactors.
Integrated surge arrester
S7-300 digital output modules are equipped with an integrated surge arrester.
Additional overvoltage protection
Inductive devices require additional surge arresters only in following cases:
• If SIMATIC output circuits can be switched off by additionally installed contacts (e.g. relay
contacts).
• If the inductive loads are not controlled by SIMATIC modules
Note: Request information on relevant surge protection rating from the supplier of inductive
devices.
Example: EMERGENCY-OFF relay contact in the output circuit
The figures illustrates an output circuit requiring additional overvoltage protectors.
Contact in output circuit
Inductance requires a circuit
CPU
PS
SM
SM
SM
SM
SM
SM
Refer also to the rest of the information in this section.
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