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35Trip Units and Communications
May 2003
PG01301002E
Over-Temperature Trip
Magnum Circuit Breakers equipped
with integral trip units have an over-
temperature trip function provided as
a standard feature. The over-temper-
ature trip is designed to trip the
breaker when the internal tempera-
ture of the trip unit exceeds 85°C.
This is the approximate manufactur-
er’s published maximum tempera-
ture rating for the trip unit electronic
components. An 85°C temperature
inside the trip unit indicates that
there are much higher temperatures
and potentially significant heating
problems elsewhere on the circuit
breaker.
When the over-temperature function
trips the breaker, the red trip flag of
the mechanical indicator will pop-out
and the red Long Time Delay LED of
the trip unit will flash is the breaker
is so equipped.
Ground Fault Provisions and
Protective Schemes
As a standard feature, Magnum
Circuit Breakers are internally wired
with provisions for ground fault
protection. However, ground fault
protection is provided only if the
breaker is equipped with a trip unit
with optional LSIG protection.
Ground fault protection can be easily
added to breakers not originally
equipped with a ground fault trip
unit. For example, an LSI trip unit
can simply be changed out with a
like model trip unit that has LSIG
protection. Modifications to the circuit
breaker internal wiring are generally
not required as long as the trip unit
changeout is with a like model trip unit.
To provide for maximum application
flexibility, breaker ground fault provi-
sions are wired out to the breaker
secondary terminals for external use.
The secondary terminals can be
easily configured to accommodate
the most common ground fault
schemes, including:
NOTE: For detailed wiring
diagrams that describe all
ground fault sensing options
and wiring connections, refer to
the Trip Unit Instruction Leaflet.
Multiple Source/Multiple Ground
Multiple Source/Multiple Ground
sensing schemes are also possible
on systems with multiple sensors.
Zone Interlocking
Zone Selective Interlocking is a
standard feature provided on
Magnum Circuit Breakers that are
supplied with Digitrip 520i or higher
trip units. The trip unit Short Time
and Ground Fault Time Delay Zone
Interlock function is wired out to
breaker secondary terminals B8 and
B9 for customer use.
The zone interlock function provides
for hard-wired selective circuit breaker
coordination in the power distribution
system. When employed, breaker
secondary terminals B8 and B9 are
connected to establish zones of
downstream and upstream breakers
using 3 wires: Zin, Zout, and a common
wire to establish the zone.
NOTE: For detailed wiring dia-
grams that describe zone selective
interlocking schemes and wiring
connections, refer to the trip
unit instruction leaflet.
In the event that a fault current on
the system exceeds the trip unit
Short Time or Ground Fault pick-up
settings or the Short Delay Setting
2xIr is exceeded, the breaker nearest
to the fault trips instantaneously. At
the same time the trip unit sends a
logic signal to the connected
upstream trip unit to restrain it from
tripping immediately and follow its
protective delay settings. Coordination
is maintained, the breaker closest to
the fault trips first and power is only
selectively disrupted.
NOTE: If zone interlocking is not
employed, it may be defeated
by installing a shorting jumper
across breaker secondary termi-
nals B8 and B9.
Installing a jumper in this manner
defeats the zone interlock function
and tells the trip unit to follow its set
Short Time and Ground Fault Time
protective delays. If breaker sec-
ondary terminals B8 and B9 are left
as open circuit, the trip unit Short
Time and Ground Fault Time Delay
functions will trip instantaneously if
the system fault current exceeds the
pick up setting.
Residual Sensing
Residual sensing is the standard or
default wiring configurations provided.
It is also the most common mode of
ground fault sensing used in 3-pole
and 4-pole breaker applications. One
current sensor per pole on each
phase conductor is provided internal
to the circuit breaker. When 3-pole
breakers are applied on 3 phase, 4
wire systems, an external sensor
matched to the breaker sensor ratio
is mounted on the Switchgear neutral
bus and wired to the breaker secondary
contacts to connect all sensors in the
residual circuit.
Source Ground Sensing
Source ground sensing is commonly
found on 3 pole breakers applied as
a main breaker on single-ended radial
or on double-ended systems where
a mid-point grounding electrode is
employed.
To configure a breaker for source
ground sensing, a shorting jumper is
applied to breaker secondary contacts
B6 and B7 to defeat the residual
sensing mode. An external current
sensor is mounted directly on the
bonding conductor to ground to
measure the ground current flowing
in the grounding electrode conductor
and all other grounding conductors.
The sensor is then wired to the breaker
secondary contacts B4 and B5.
NOTE: The secondary current
rating of the sensors must be 1
ampere.
Zero Sequence Sensing
Zero sequence sensing is applied on
main and feeder breakers, and other
breakers employed in zone interlocking
ground protection schemes.
To configure a breaker for zero
sequence sensing, a shorting jumper
is applied to breaker secondary con-
tacts B6 and B7 to defeat the residual
sensing mode. Zero sequence sensors
with 1000:1 and 100:1 ratios are
available for mounting in the
switchgear cable compartment. The
sensor internal window dimensions
are 114 x 342 mm (4.5 x 13.5 inches)
for accommodating power cables.
The zero sequence sensors are wired
to breaker secondary contacts B4
and B5.