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Chapter 7 Page 27 of 64
2.3.2 Reset Delay
Faults in plastic insulated cables or compound-filled joint boxes can be intermittent or ‘flashing’ faults – the
insulant melts and temporarily reseals the fault for a short time after which the insulation fails again.
The repeating process of the fault often causes electromechanical disc relays to “ratchet” up and eventually trip
the faulty circuit if the reset time of the relay was longer than the time between successive flashes.
To ensure time grading is maintained with other relays on the system a DTL or shaped (ANSI only) reset
characteristic can be selected for all overcurrent and earth fault elements.
Where the substation feeds an outgoing overhead line network, particularly where reclosers are installed,
instantaneous resetting is desirable to ensure that, on multiple shot reclosing schemes, correct grading between
the substation incomer relays and the relays associated with the reclosers is maintained.
Figure 2-16 Overcurrent Reset Characteristics
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