12
3. Protection functions
Power XPert® release triP unit for MagnuM PXr circuit breakers
MN013015EN January 2022 www.eaton.com
3.6.1 Residual current method
Residual sensing is the factory default method of ground
fault protection in Magnum PXR circuit breakers. This
method is used for 4-wire systems with one current sensor
on each phase conductor and one on the neutral. In this
method of sensing, the trip unit sums the outputs of the
three individual current sensors (IA, IB, IC) and the neutral
(IN) if present. If the sum is zero, then no ground current
exists. When the sum is greater than the ground fault
pickup for the time delay, the breaker will trip or alarm.
The pickup (I
g
) is expressed as a multiple of the breaker’s
current rating (I
n
) for circuit breakers less than 1200 A. For
breakers 1200 A and greater, the pickup setting is expressed
as a multiple of 1200 A (the NEC maximum) (see Table 5).
Table 5. GF residual settings.
setting
rating
PXr 25 from 0.2 to 1.0 in steps of 0.01
PXr 20 0.2
0.4
0.6
0.8
1.0
less than
1200 a
I
g
= 0.2 x (I
n
) 0.4 x (I
n
) 0.6 x (I
n
) 0.8 x (I
n
) 1.0 x (I
n
)
1200 a or
greater
I
g
= 240
480
720
960
1200
The time delay has a range from 0.05 to 1.0 seconds. Two
ground fault protection curve slopes are available: a fixed
time (flat) or I
2
t response. The slope should be chosen to
match the selective coordination requirements. The I
2
t
response provides a longer time delay for current below
0.625 x I
n
than a fixed time (flat) response.
For 3-pole circuit breakers used in a 4-wire system, an exter-
nal neutral sensor is used. It is connected to the secondar-
ies at N1 & N2. See Section 4.1 - Neutral Sensors for the
appropriate sensors.
3.6.2 Source ground and zero sequence method
By setting the ground fault type to source ground/zero
sequence, either sensing method is enabled in the trip unit.
Source ground sensing uses a current sensor on the equip-
ment’s ground-bonding jumper. The output of the sensor
directly measures the total ground current flowing in the
grounding conductor. When the current measured by the
trip unit exceeds the ground fault settings, the breaker will
trip or alarm.
Zero sequence sensing uses a single sensor that surrounds
all the phase conductors and the neutral conductor in a
4-wire system. If a ground fault occurs, the vector balance
of all the magnetic fields will not sum to zero and the sensor
will generate an output signal equivalent to the current
flowing to ground. When the current measured by the trip
unit exceeds the ground fault settings for the time delay, the
breaker will trip or alarm.
The sensor used for both sensing methods is an iron-core
CT, connected to the secondary wiring terminals at G1
and G2. See Section 4.6 - Secondary wiring terminals for
Magnum PXR for terminal numbering.
The input to the trip unit at G1 and G2 is 100 mA per unit.
To use a CT that has a 1 A per unit secondary, you must
convert the 1 A level to 100 mA by using the adapter module
PDGXGFSTM1A (Eaton PN 70C2068G02) with internal 10:1
transformer (see Instructional Leaflet IL012309EN).
Alternately, you can use a CT without the converter module
by using a transformer with the appropriate ratio. For
example, a CT with a 1000:1 ratio could be used without the
adapter module, providing a 100 mA output with 100 A on
the primary.
Figure 10. Ground fault secondary transformer module.
Select the ratio on the display or in PXPM assuming the 10:1
converter module is being used.
