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Applications
5.4 Overcurrent Characteristics
5.4
Overcurrent Characteristics
Selection of Overcurrent Characteristics
5.4.1
Select the characteristic curve shape of the same type as other relays on the same circuit or to grade with
items of plant. For example, fuses or grounding resistors.
The following table summarizes the application of IDMTL characteristic.
Table 5-3
OC/EF Curve Characteristic
IEC Normal Inverse (NI)
IEEE/ANSI Moderately Inverse (MI)
IEC Very Inverse (VI)
IEEE/ANSI Very Inverse (VI)
IEC Extreme Inversely (EI)
IEEE/ANSI Extremely Inverse (EI)
IEC Long Time Inverse (LTI)
Reset Delay
5.4.2
The increasing use of plastically insulated cables both conventionally buried and aerial bundled conductors
have raised the number of flashing intermittent faults on distribution systems. At the fault location, the plastic
melts and temporarily reseals the faulty cable for a short time after which the insulation fails again. The same
phenomenon occurs in compound-filled joint boxes or on clashing overhead line conductors. If the reset time
of the relay is longer than the time between successive faults, the repeating occurrence of the fault can cause
electromechanical disc relays to ratchet up and eventually trip the faulty circuit.
To mimic an electromechanical relay, the relay can be user-programmed for an IEC/ANSI DECAYING character-
istic when an ANSI operate characteristic is applied. Alternatively, a DTL reset (0 s to 60 s) can be used with
other operating characteristics.
For protection of cable feeders, Siemens recommends using 60 s DTL reset.
On overhead line networks, particularly where circuit breaker with automatic reclosing functions are incorpo-
rated 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 circuit breaker with
automatic reclosing functions are maintained.
[dw_7SR46_reset-delay, 1, en_US]
76
Selection of Overcurrent Characteristics
Application
Applied
Used with high-impedance paths where there is a significant
difference between the fault levels at protection points
Grading with fuses
Used to protect transformer earthing resistors having long
withstand time
C53000-G7040-C103-1, Edition 10.2022
Reyrolle, 7SR46 Device, Manual
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