Toshiba GRZ100 B Series, Instruction Manual, Page: 431 / 447

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Programmable Reset Characteristics
The overcurrent stages for phase and earth faults, OC1 and EF1, each have a programmable reset
feature. Resetting may be instantaneous or definite time delayed.
Instantaneous resetting is normally applied in multi-shot auto-reclosing schemes, to ensure correct
grading between relays at various points in the scheme.
The definite time delayed reset characteristic may be used to provide faster clearance of intermittent
(‘pecking’ or ‘flashing’) fault conditions. An example of where such phenomena may be experienced is
in plastic insulated cables, where the fault energy melts the cable insulation and temporarily
extinguishes the fault, after which the insulation again breaks down and the process repeats.
An inverse time overcurrent protection with instantaneous resetting cannot detect this condition until
the fault becomes permanent, thereby allowing a succession of such breakdowns to occur, with
associated damage to plant and danger to personnel. If a definite time reset delay of, for example, 60
seconds is applied, on the other hand, the inverse time element does not reset immediately after each
successive fault occurrence. Instead, with each new fault inception, it continues to integrate from the
point reached during the previous breakdown, and therefore operates before the condition becomes
permanent. Figure O-1 illustrates this theory.
Figure O-1
TRIP LEVEL
TRIP LEVEL
Inverse Time Relay
with Instantaneous
Reset
Intermittent
Fault Condition
Inverse Time Relay
with Definite Time
Reset
Delayed Reset