lever and a rotating trip lever to actuate the tripping action
of the circuit breaker.
When the Flux Shunt Trip is reset by the handle moved to
reset position, the moving core assembly is held in readi-
ness against the force of the compressed spring by the
permanent magnet. When a tripping action is initiated,
the Flux Shunt Trip coil receives a tripping pulse from the
Digitrip RMS Trip Unit. This pulse overcomes the holding
released to trigger the tripping operation via the rotating
trip bar.
5.4 GROUND FAULT PROTECTION
5.4.1 General
When the Digitrip RMS Trip Assembly includes ground
fault protection, the distribution system characteristics,
i.e., system grounding, number of sources, number and
location of ground points, etc. must be considered as well
as the manner and location in which the circuit breaker is
applied to the system.
Two modes of sensing ground fault currents are generally
used: residual and source
g
round. R-
F
rame
Circuit
Brea kers are internally prewired to accommodate
both types. A side mounted 4-point terminal block, as
shown in Fig. 4, is provided to revise connections
required to accommodate each method. A nameplate is
provided on the side of the circuit breaker that illustrates
the required connections. Applicable connection varia-
tions are illustrated in Fig. 11.
If the system neutral is grounded, but no phase-to-neutral
loads are used, the Digitrip RMS Trip Assembly includes
all of the components necessary for ground fault protec-
tion.
5.4.2 Residual Sensing
The standard mode of ground fault sensing in
R-Frame Circuit Breakers is Residual Sensing. This
mode utilizes one current sensor on each phase conduc-
tor and one on the neutral if 4-wire system. This mode of
sensing vectorially sums the outputs of the three or four
individual current sensors. As long as the vectorial sum is
zero, then no ground fault exists. The neutral sensor must
have characteristics which are identical to the three inter-
nally mounted phase current sensors. Available types of
neutral sensors are illustrated in Fig. 14. Residual ground
fault sensing means are adaptable to main and feeder
breaker applications. Available ground fault pick-up set-
tings employing Residual Sensing means are given in
Table 3.
5.4.3 Source Ground Sensing
Depending upon the installation requirements, alternate
ground fault sensing schemes may be dictated. The
ground return method is most applicable where ground
fault protection is desired only on the main circuit breaker
in a simple radial system. This method is also applicable
on double-ended systems where a mid-point grounding
electrode is employed. For this mode of sensing, a single
current sensor mounted on the equipment bonding
in the grounding electrode conductor and all other equip-
ment grounding conductors.
The values shown in Table 3 will apply when the neutral
sensors shown in Fig. 14 are employed in a source
ground sensing scheme provided the neutral sensor is
the same as the frame rating.
Note: Regardless of the mode of sensing employed,
the polarity of the sensor connections is critical.
Always observe the polarity markings on the installa-
tion drawings. To insure correct ground fault equip-
National Electrical Code requirements under Article
230-95(C).
5.4.4 Ground Fault Settings
The adjustment of the ground fault functional settings is
illustrated in the applicable Digitrip RMS Trip Unit instruc-
ground fault Time-Current curves referenced in Section 6
of this instruction book. Applicable residual ground fault
pick-up settings and current values are given in Table 3
as well as on the Time-Current curve.
5.5 Current Sensors
The three primary current sensors installed in the circuit
breaker are located internally on the lower conducts
which are normally on the load side of the main contacts.
The auxiliary current transformers are mounted as shown
in Fig. 4. The ground fault auxiliary current transformer is
supplied only when the ground fault protection function is
supplied in the Digitrip RMS Trip Unit. The Neutral Auxil-
iary Current Transformer is supplied only with 4-pole
breakers. A partial internal schematic is shown in Fig. 6
as well as in the residual diagram shown in Fig. 8.
The primary current sensors produce an output propor-
tional to the load current and furnish the Digitrip RMS Trip
Assembly with the intelligence and energy to trip the cir-
cuit breaker when functional protection settings are
exceeded.
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