well... with only 40 fans... could be a gaming rig

 

Beyond this post - I refuse to make further contributions until the TS comes back and actually clarifies what they are trying to do. Other than detect a failed fan - - - .

 

The TS won't be coming back. He overheated.

 

If by any chance the fans are inside a test stand that uses gasoline, there is a very simple, FULLY CERTIFIED module that can monitor those contacts quite safely. It is called an intrinsicly safe isolater. It uses a very low voltage and low current so that any possible spark will not have enough energy to ignite vapors.
But a vastly cheaper scheme would be to use opto-interupter devices having an LED and a photo-transistor to detect the beam blockage.
The problem is that many small motors are impedance protected and so if they stall nothing happens.

 

Oriental Motor is one manuf that produces models with a built-in alarm circuit that the host controller can recognize if the fan speed drops