Siemens DI overvoltages protection

How can we protect our siemens DI modules against to overvoltages that occurs on any input chanel.

thanks.

We sometimes use barriers on input signals.
In particular interface signals from 3rd parties can be risky ("Oh, you need a potentialfree contact ! I thought that 230VAC was standard for exchanging signals.").
The barrier can be as simple as a plain 24VDC relay that has to be activated by the 3rd party's potentialfree contact. The relay then activates the input. For fast signals consider optocouplers.

Another thing is that I really like the ET200S i/o that we use as our standard today. It makes a big difference if it is a 4-channel ET200S module or a 32-channel S7-400 module that has to be replaced.