Reset Delay; Figure 2.1-3 Reset Delay - Siemens 7SR210 Application Manual

OC/EF Curve Characteristic
IEC Normal Inverse (NI)
ANSI Moderately Inverse (MI)
IEC Very Inverse (VI)
ANSI Very Inverse (VI)
IEC Extreme Inversely (EI)
ANSI Extremely Inverse (EI)
IEC Long Time Inverse (LTI)
Recloser Specific
Table 2-1 Application of IDMTL Characteristics

2.1.2 Reset Delay

The increasing use of plastic insulated cables, both conventionally buried and aerial bundled conductors, have
given rise to the number of flashing intermittent faults on distribution systems. At the fault position, the plastic
melts and temporarily reseals the faulty cable for a short time after which the insulation fails again. The same
phenomenon has occurred in compound-filled joint boxes or on 'clashing' overhead line conductors. The
repeating process of the fault can cause electromechanical disc relays to "ratchet" up and eventually trip the faulty
circuit if the reset time of the relay is longer than the time between successive faults.
To mimic an electromechanical relay the relay can be user programmed for an ANSI DECAYING characteristic
when an ANSI operate characteristic is applied. Alternatively a DTL reset (0 to 60 seconds) can be used with
other operate characteristics.
For protection of cable feeders, it is recommended that a 60 second DTL reset be used.
On overhead line networks, particularly where reclosers are incorporated in the protected system, instantaneous
resetting is desirable to ensure that, on multiple shot reclosing schemes, correct grading between the source
relays and the relays associated with the reclosers is maintained.
©2011 Siemens Protection Devices Limited
Generally applied
Used with high impedance paths where there is a significant difference
between fault levels at protection points
Grading with Fuses
Used to protect transformer earthing resistors having long withstand times
Use when grading with specific recloser

Figure 2.1-3 Reset Delay

7SR210 & 7SR220 Applications Guide
Application
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