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Figure 14. Grounding Method Description
4.5.1 Residual Current Ground
Residual current sensing is the standard mode of ground
fault sensing in Series PT units. This mode uses one current
sensor on each phase conductor and one on the neutral for
a four-wire system. This mode of sensing sums the vector
outputs of the three or four individual current sensors. If the
sum is zero, then no ground fault exists. Residual current
ground fault sensing features are adaptable to main and
feeder circuit breaker applications. If an external neutral
sensor is used with reverse feed breaker applications, the
proper polarity of the neutral needs to be considered.
An external current sensor is required when a 3-pole circuit
breaker performs residual current sensing.
•
The ground pickup current is a multiple of the
corresponding current when the rated current is less
than 1200A.
•
The ground pickup current is a multiple of 1200
(maximum by NEC) when the rated current is greater
than or equal to 1200A.
Table 6. Residual Current Ground Settings
Settings
PT20/25 Adjustment range: 0.2 – 1.0 with 0.01 interval
Rated
PT10
0.2
current In
< 1200A
Ig=
0.2x(I
)
n
>= 1200A
Ig=
0.2x1200
0.4
0.6
0.8
1.0
0.4x(I
)
0.6x(I
)
0.8x(I
)
1.0x(I
n
n
n
480
720
960
1200
4.5.2 Source and Zero Sequence Ground
The current sensor (4000:1) signal on the ground wire of the
source ground receiving device directly measures the total
ground current flowing through the ground conductor. When
the current measured by the trip exceeds the ground fault
setting, the circuit breaker will trip or alarm.
The ground circuit sensing method is usually applied when
the main circuit breaker is used for a simple one-way power
system. This method is also applicable to double-ended
systems where a mid-point grounding electrode is employed.
This protection feature can be enabled by setting the ground
fault type.
Zero Sequence sensing, also referred to as vectorial
summation, is applicable to mains, feeders, and special
schemes involving zone protection.
The Zero-sequence sensing method uses a single sensor that
contains all phase and neutral wires in a four-wire system. If
a ground fault occurs, the vector sum of all magnetic fields
will not be zero, and the sensor will produce an output
signal equivalent to the current flowing to ground. When the
current measured by the trip unit exceeds the time delay
ground fault setting, the circuit breaker trips or alarms.
The sensors used in both sensing methods are iron-core
)
CTs, connected to frame secondary wiring terminals G1 and
n
G2. The sensor ratio must be adapted to the trip device of
4000:1A.
Note: The starting current for both the return source ground
and zero sequence ground are multiples of 400.
PT UNIT USER MANUAL APRIL 2023 www.eaton.com.cn
4. PT Protection Settings
13
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