Dell PowerEdge M420 Reference Manual page 1416

NOTE:
The maximum distance VCT can function is 120 meters.
Examples
The following example results in a report on the cable attached to port 1/0/3.
console#test copper-port tdr 1/0/3
Cable is open at 64 meters
The following example results in a failure to report on the cable attached to
port 2/0/3.
console#test copper-port tdr 2/0/3
Can't perform the test on fiber ports
1412
PHY Diagnostics Commands