The original HRG system developed in the 1950’s and 1960’s provided process continuity, however, they were unable to solve potential safety issues, such as a second ground fault, leaving the ground on the system too long and monitoring of the neutral path.
There is a concern that the first ground will be left on the system and ignored. Since the zero-sequence current transformers identify the faulted feeder, the relay has the ability to begin a (user programmable, usually in hours) timer when the ground fault first occurs. Unless the ground fault is removed or the timer is reset, the faulted feeder is shunt-tripped offline. The purpose is to continually remind maintenance personnel to either remove the ground fault or to reset the timer every so many hours.
In the event that a second ground fault occurs prior to removing the first ground fault, a phase-to-ground-to-phase or phase-to-phase fault can occur. When this occurs on the original HRG, it would cause both feeder circuit breakers and possibly the main circuit breaker to trip.
However, the modern relay can be programmed to prevent this and only shunt-trip the lesser priority feeder, leaving the more important feeder on-line.