Siemens Simatic M7-400 Reference Manual page 216

Digital Modules
Note
Use a suppressor circuit in environments with high humidity and where sparks
might occur at the relay contacts. This will increase the life of the relay contacts.
To do this, connect an RC element or a varistor parallel to the relay contacts or to
the load. The dimensions depend on the size of the load (see Chapter 4 of the
installation manual).
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S7-400, M7-400 Programmable Controllers Module Specifications
A5E00069467-07