7.4.9 WEAK-FEED INTERNAL A-G FAULTS
The RFL 9300's Weak-Feed Trip ("WFT") logic is activated when it senses a small current at one terminal and
a large current at the other terminal. This condition is an internal fault with weak infeed at one terminal. The
RFL 9300 defines "small" currents as less than 1.5 Arms. "Large" currents are defined as 1.5 amperes plus
the bias setting.
1. Connect the test circuit shown in Figure 7-10.
The test connections shown in Figure 7-10 simulate an internal A-G fault with weak in-
feed at the right terminal. The current infeed at the right terminal is set at ¼ Arms.
Since the current is <0.5 Arms there will not be any current messages transmited from
the right terminal to the left terminal. This means that the two current sources are not
required to be synchronous. The fact that the phase controllers at the right terminal de-
tect a peak current =>1/4 A will be interpreted to mean that the breaker is closed. If
there is no current in the right terminal, the phase controller will interpret this to mean
that the breaker is open and this test will generate strong-feed trips.
The RFL 9300 trips whenever both of the following conditions are met:
a. The current level at the left terminal is large enough to satisfy the "large"
threshold defined above (1.5 amperes plus the bias setting).
b. The fault detector requirement is satisfied.
AC
CURRENT
SOURCE
TB4
TB5
A
C
T
A
C
T
TB6
TB7
A
C
T
A
C
T
3I
0
I
C
I
B
I
A
1 1 1 1
2 2 2 2
TB4
TB5
A
C
T
A
C
T
TB6
TB7
A
C
T
A
C
T
3I
0
I
C
I
B
I
A
1 1 1 1
2 2 2 2
LEFT TERMINAL RIGHT TERMINAL
AC
CURRENT
SOURCE
Figure 7-10. Test connections simulating an internal A-G fault with weak infeed at the right terminal
RFL 9300 RFL Electronics Inc.
October 20, 2004 7 - 17 (973) 334-3100
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