Selection Of Overcurrent Characteristics - Siemens 7SR45 Argus User Manual

Figure 1-3
To increase sensitivity, dedicated Earth fault elements are used. There should be little or no current flowing to
earth in a healthy system so such relays can be given far lower pick-up levels than relays which detect excess
current ( > load current) in each phase conductor. Such dedicated earth fault relays are important where the fault
path to earth is a high-resistance one (such as in highly arid areas) or where the system uses high values of
earthing resistor / reactance and the fault current detected in the phase conductors will be limited.
1.2

Selection of Overcurrent Characteristics

The characteristic curve shape is selected to be the same type as the other relays on the same circuit or to grade
with items of plant e.g. fuses or earthing resistors.
The application of IDMTL characteristic is summarised in the following table:
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)
©2018 Siemens Protection Devices Limited
1000.00
OPERATE
100.00
10.00
1.00
0.10
0.01
1 10
IEC NI Curve with Minimum Operate Time Setting Applied
Table 1-1 Application of IDMTL Characteristics
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
Chapter 7 - 7SR45 Applications Guide
ZONE
Min Operate Time = 4sec
IEC NI Curve: TM = 1
100 1000
Current (x Is)
Application
Chapter 7 Page 8 of 10