7SR18 Applications Guide
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Current (x Is)
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)
Generally applied
IEC Very Inverse (VI)
ANSI Very Inverse (VI)
Used with high impedance paths where there is a significant difference
between fault levels at protection points
IEC Extreme Inversely (EI)
ANSI Extremely Inverse (EI)
Grading with Fuses
IEC Long Time Inverse (LTI) Used to protect transformer earthing resistors having long withstand times
Table 2.1.1-1 Application of IDMTL Characteristics
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