When does channel-to-channel isolation go from nice to have to *must* have?

[SeanDonner]

So from my understanding of the various isolation types, having all channels isolated from each other is always something one should strive for but you tend to sacrifice channel density to achive this gold standard.  If you look at just about all of NI's offerings of Analog Input modules, the ones that are channel-to-channel isolated have about 1/4 the channel count compared to the ones that are simply channel-to-earth isolated.

So now the question in my mind is when does channel-to-channel isolation become a necessity over channel-to-earth isolation?  The only scenario I could come up with is if you needed the same multi-channel analog module to monitor analog inputs from different sources with wildly different common mode voltages; although just how wildly different they need to be to make channel-to-channel isolation a necessity, I'm not sure.

Can somebody shed a bit more light on this subject?

Thanks

[fromm8]

    Hi Sean,

-          Please take a look to these articles below:

-          http://digital.ni.com/public.nsf/allkb/CF71091043A6193B862573D70077138B

-          http://www.ni.com/white-paper/3410/en#toc2

-          http://www.ni.com/white-paper/3546/en#toc3

  So to summarize, that depends on the requirements of your application. Sometimes we need to ensure that a product meets a safety standard. Or it could depend on our needs, like the information in the third link (Needs for           isolation).

Regards