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Is it normal to see voltage between SC and I1 I2 I3 I4 on power supply option board (900-2RS, 900-4RL, 900-4R) terminals?

Yes, it is normal to see floating voltage between the Signal Common (SC) and Input terminals on the power supply option board.  The Input terminals are not Ground terminals, so they are not a valid reference point for measuring voltage. 
 
Measure all voltages with respect to power supply ground.
 
SC will always measure around (or slightly lower than) 24VDC with respect to ground.
 
Input terminal I1 - I4 will measure 0V when not active, and 24V when active.   Again, this is with respect to power supply ground.

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