The DC offset in fault current causes conventional over-current (OC) relays to :

• A. sometimes under-reach and sometimes over-reach
• B. under-reach
• C. reach remains unaffected due to DC offset
• D. over-reach

Answer: (D)

The tendency of a Reach of Distance Relay to operate at impedance larger than its setting value is known as overreach.
Overreach is the presence of d.c. offset in the fault current wave, as the offset current has a higher peak value than that of a symmetrical wave for which the relay is set.