14/1/98
Page 15
Issue H
7.3.10. Earth Fault trip Time
This setting determines the trip time following the measured earth fault current exceeding the pickup
value. A setting of 0.0 indicates instantaneous operation, i.e. 30 - 50mS at twice times setting.
7.3.11. Unbalance Current
This setting determines the pick-up level of the unbalance Current as a percentage of FLC before a
Trip occurs. This is based on the difference between the maximum and minimum measured line
currents and the following formula:-
a)
For Line currents above 25% FLC to 100% FLC (The unbalance feature is inhibited below 25%
FLC):-
b)
Between 1xFLC and 2.5xFLC
c)
Above 2.5xFLC
A setting of OFF here will disable the feature. The relay measures the difference in magnitude
between the maximum and minimum line currents as a percentage of the maximum. The equivalent
apparent negative phase sequence component is then displayed.
The time characteristics are shown in the graph below.
%
100
%
min
max
x
I
I
I
Unbalance
FLC
−
=
%
100
%
max
min
max
x
I
I
I
Unbalance
−
=
%
100
%
min
max
x
I
I
I
Unbalance
FLC
−
=
