The reach and operating time of the over-current relay depend upon the magnitude of fault current. The fault current that would result in case of a fault at a particular location depends upon:1) The type of fault (ground fault or phase fault) 2) The source impedance.
Since neither the type fault nor the source impedance is predictable, the reach of the over-current relay keeps on changing depending upon the source conditions and the type of fault. Thus, even though the relays are set with great care, since their reach is subject to variations, they are likely to suffer from loss of selectivity. Such loss of selectivity can be tolerated to some extent in the low voltage distribution systems, where the only objective to be met is the continuity of supply to the consumer. However, in an EHV interconnected system (grid), loss of selectivity can lead to danger to the stability of the power system, in addition to large disruptions to loads. Therefore, over-current relays can not be relied upon as a primary means of protection in EHV systems.
Another principle of relaying, known as Distance Measurement, offers a much more accurate reach, which is independent of source conditions and type of fault.