GE MiCOM P40 Agile Technical Manual page 123

P14x Chapter 6 - Current Protection Functions
time reset characteristic has been selected, because in this case the reset time is determined by the time dial
setting (TDS).
This feature may be useful in certain applications, such as when grading with upstream electromechanical
overcurrent relays, which have inherent reset time delays. If you set the hold timer to a value other than zero, the
resetting of the protection element timers will be delayed for this period. This allows the element to behave in a
similar way to an electromechanical relay. If you set the hold timer to zero, the overcurrent timer for that stage will
reset instantaneously as soon as the current falls below a specified percentage of the current setting (typically
95%).
Another situation where the timer hold facility may be used to reduce fault clearance times is for intermittent
faults. An example of this may occur in a plastic insulated cable. In this application it is possible that the fault
energy melts and reseals the cable insulation, thereby extinguishing the fault. This process repeats to give a
succession of fault current pulses, each of increasing duration with reducing intervals between the pulses, until the
fault becomes permanent.
When the reset time is instantaneous, the device will repeatedly reset and not be able to trip until the fault
becomes permanent. By using the Timer Hold facility the device will integrate the fault current pulses, thereby
reducing fault clearance time.
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