Specifications

7SR242 Duobias Applications Guide
©2010 Siemens Protection Devices Limited Chapter 7 Page 28 of 56
2.3.2 Reset Delay
ables or compound-filled joint boxes can be intermittent or ‘flashing’ faults – the
ventually trip
SI only) reset
cularly where reclosers are installed,
Faults in plastic insulated c
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 e
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 (AN
characteristic can be selected for all overcurrent and earth fault elements.
Where the substation feeds an outgoing overhead line network, parti
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.
FAULT
Clashing
conductors or
re-sealing cable
R1
R2
R3
Electro-mechanical Relay
Time
TRIP
(Instantaneous.
Reset
Time
DTL Reset
TRIP
Figure 2-16 Overcurrent Reset Characteristics