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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, definite time delayed, or, in the case of IEEE/US curves,
inverse 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. On the other hand, the inverse reset
characteristic is particularly useful to provide correct co-ordination with an upstream induction disc type
overcurrent relay.
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
A-1 illustrates this theory.
Intermittent
Fault Condition
Inverse Time Relay
with Instantaneous
Reset
Inverse Time Relay
with Definite Time
Reset
Delayed Reset
Figure A-1
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