This arrangement would be ideal if the CTs always reproduced the primary currents accurately.
Actually, however, the CTs will not always give the same secondary current for the same primary
current, even if the CTs are commercially identical. The difference in secondary current, even under
steady-state load conditions, can be caused by the variations in manufacturing tolerances and in
the difference in secondary loading, i.e. unequal lengths of leads to the relay, unequal burdens of
meters and instruments that may be connected in one or both of the secondaries. What is more
likely, however, is the ‘error’ current that can occur during short-circuit conditions. Not only is
the current magnitude much greater, but there is the possibility of DC offset so that the transient
response of the two CTs will not be the same. This difference in secondary current will flow in
the relay. An overcurrent relay must then be set above the maximum error current that can flow
during the external fault; yet it must be set significantly below the minimum fault current that can
accompany a fault that is restricted due to winding or fault impedance.