[dw_7SR5_IECcurve withminoperatetime, 1, en_US]
Figure 5-25
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 lower pickup levels than relays which detect current
above 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.
Selection of Overcurrent Characteristics
Each pole has 2 independent overcurrent characteristics. Where required the 2 curves can be used:
•
To produce a composite curve
•
To provide a 2 stage tripping scheme
•
Where one curve is to be directionalized in the forward direction the other in the reverse direction
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 summarized in the following table:
Protection and Automation Functions
5.17 51 Time-Delayed Overcurrent – Phase
260 Reyrolle 7SR5, Overcurrent Protection Device, Device Manual
C53000-G7040-C014-1, Edition 11.2019
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