Hi Naushad,
Pin 20 of the SCXI-1166 connects directly to the SCXI chassis ground. You can use Pin 20 for signal wire shielding purposes. Remember, the usual rule is to only ground one end of the shield. If it is grounded at both ends then a difference in ground potential could cause a current flow through the shield and hence introduce noise.
Let me know if you have further questions!
Chad Erickson
Switch Product Support Engineer
NI - USA