Why is the Maximum Allowable Common Mode Voltage Lower for Direct Input Cables in the DL850 Isolation Modules?
The isolation module specifications for maximum allowable common mode voltage using direct input is 42V(DC+ACpeak).
The maximum allowable common mode voltage using the 701901(1:1) safety cable is 400V or more.
The specification written as "Direct input or cable not complying with the safety standard can be interpreted as directly inputting a cable, which is not complying with the safety standard, to the BNC input of the DL850 isolation modules. The 42V specification is not a limitation of the module, but it is a human safety limitation to prevent harm to the user.
A metallic connector is an example of direct input, which is not complying with the safety standard, or a cable which is not complying with the safety standard. A metallic connecter connected to the isolated input module can potentially induce a high voltage on the metallic connecter.
Therefore the maximum common mode voltage must be less than the voltage by which we are injured from electric shock.