Selection Of Overcurrent Characteristics; Figure 2.1-2 Iec Ni Curve With Minimum Operate Time Setting Applied - Siemens 7SR21 Manual

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 Overcurrent 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
Where one curve is to be directionalised 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 summarised in the following table:
©2010 Siemens Protection Devices Limited
1000.00
100.00
10.00
1.00
0.10
0.01
1 10
Current (x Is)
7SR21 & 7SR22 Argus Applications Guide
100 1000
Chapter 7 Page 11 of 39