ABB RELION REB500 Technical Manual page 99

1MRK 505 406-UEN B
I-Setting
Delay
Over or
undercurrent
Number of
phases
Setting I-Setting
The current setting
false signals under normal load conditions, but should be low enough to detect the lowest
fault current that can occur. The margin which has to be allowed between the maximum short-
time load current and the setting must allow for:
• the tolerance on the current setting
• the reset ratio
The maximum short-time load current has to be determined according to the power system
conditions and must take switching operations and load surges into account.
IEC19000443 V1 EN-US
Figure 39: Operating characteristic of the definite time over-current function
Compensating any difference between the rated currents of CT I
recommended. This is achieved with the aid of the reference value of the A/D channel or by
correcting the overcurrent setting.
For example, for I
A should be:
I
GN
1.5
I
N
CurrentInp
An interposing CT in the input is essential for current settings lower than 0.2 I
Bay protection functions REB500
Technical manual
I-Setting
Delay
MaxMin
NrOfPhases
I-Setting must be sufficiently high to avoid any risk of false tripping or
I
I - Setting
I
I
N
0
18000038-IEC19000443-1-en.vsdx
= 800 A and I
GN
800 A
 
1.5 1.2
1000 A
1
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Delay
t
= 1000 A, the setting for a pick-up current of 1.5 I
N1
Bay protection functions
and protected unit I
N1 GN
= 1200
GN
.
N
Section 5
is
93