Selection Of Over-Current Characteristics - Siemens 7SR18 Solkor Manual

7SR18 Applications Guide
Figure 2.1-2 IEC NI Curve with Minimum Operate Time Setting Applied
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.
2.1.1

Selection of Over-current Characteristics

Each pole has two independent over-current characteristics. Where required the two curves can be used: -
To produce a composite curve
To provide a two stage tripping scheme
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)
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)
Table 2.1.1-1 Application of IDMTL Characteristics
Unrestricted Page 14 of 63
1000.00
100.00
10.00
1.00
0.10
0.01
1 10
Current (x Is)
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
100 1000
Application
©2018 Siemens Protection Devices Limited