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Effective 10/2004
Page 12
I.L. 70C1036H05
the circuit breaker is applied to the system. These ele-
ments are discussed in Sections 2.3.3 through 2.3.6.
The Digitrip 1150 family uses three modes of sensing to
detect ground fault currents: residual, source ground, and
zero sequence
(See Table 2.1).
The breaker secondary
contact inputs B-6 and B-7 are used to configure the trip
unit. A jumper from B-6 to B-7 programs the trip unit for
either a source ground or zero sequence configuration.
Removing the jumper will program the unit for a residual
ground fault scheme. This jumper resides on the stationary
side of the switchgear assembly. In all three schemes, the
proper current sensor input is required on the external
sensor input terminals B-4, B-5 of the breaker secondary
contacts.
Table 2.1 Digitrip Sensing Modes
2.3.2 Residual Sensing - 3-Wire or 4-Wire
Residual Sensing is the standard mode of ground fault
sensing in Magnum Circuit Breakers. This mode utilizes
one current sensor on each phase conductor and one on
the neutral for a 4-wire system
(Shown in Figures 2.2
and 2.3).
If the system neutral is grounded, and no phase
to neutral loads are used (a three wire system), the Digitrip
1150 with ground includes all of the components neces-
sary for ground fault protection. This mode of sensing
vectorially sums the outputs of the three individual current
sensors. For separately-mounted neutrals, as long as the
vectorial sum is zero, then no ground fault exists. The
neutral sensor (for a four wire system) must have charac-
teristics and a ratio which matches the three internally-
mounted phase current sensors. Available types of neutral
sensors are shown in Figure 2.4. Residual ground fault
sensing features are adaptable to main and feeder circuit
breaker applications.
CAUTION
IF THE SENSOR CONNECTIONS ARE INCORRECT, A
NUISANCE TRIP MAY OCCUR. ALWAYS OBSERVE THE
POLARITY MARKINGS ON THE INSTALLATION DRAW-
INGS. TO INSURE CORRECT GROUND FAULT EQUIP-
MENT PERFORMANCE, CONDUCT FIELD TESTS TO
COMPLY WITH NEC REQUIREMENTS UNDER ARTICLE
230-95(C).
2.3.3 Source Ground Sensing
Depending upon the installation requirements, alternate
ground fault sensing schemes may be dictated
(See
Figures 2.6 and 2.7).
The ground return method is usually
applied when ground fault protection is desired only on the
main circuit breaker in a simple radial system. This
method is also applicable to double-ended systems where
a midpoint grounding electrode is employed. For this
mode of sensing, a single current sensor mounted on the
equipment-bonding jumper directly measures the total
ground current flowing in the grounding electrode
conductor and all other equipment-grounding conductors.
In the CURRENT (protection) submenu, there is a setting
(EXT GND CT RATIO) that enables the presentation of
ground pickup in amperes and the display of ground
current in amperes when an External Ground CT is
employed. This screen is presented when jumper B-6 to
B-7 is present for a trip unit equipped with GROUND.
External Ground CT ratios of 10, 25, 50, 100, 200, 400,
800, 1000 A:1 and “none” settings are selectable, with
“none” defaulting to the plug (In) rating. With the correct
ratio selected the GF pickup settings are adjustable in
amperes.
2.3.4 Zero Sequence Sensing
Zero Sequence Sensing, also referred to as vectorial
summation
(See Figure 2.8),
is applicable to mains,
feeders, and special schemes involving zone protection.
Zero Sequence current transformers (4 1/2“ x 13 1/2“ [114
mm x 342 mm] rectangular inside dimensions) are
available with 100:1 and 1000:1 ratios. The EXT GND CT
RATIO setting described above is applicable for zero
sequence.
2.3.5 Multiple Source/Multiple Ground
A Multiple Source/Multiple Ground scheme is shown in
Figure 2.9. In this figure, a ground fault is shown which
has two possible return paths, via the neutral, back to its
source. The three neutral sensors are interconnected to
sense and detect both ground fault and neutral currents.
2.3.6 Ground Fault Settings
The adjustment of the ground fault functional settings and
their effect upon the ground fault time-current curve is
discussed in Section 4.2.1.8.
2.4 Current Sensors for Standard Breaker
The three (3-pole) or four (4-pole) primary current sensors
are located in the circuit breaker on the lower conductors
of the breaker. The current sensor rating defines the circuit
Ground
(Earth)
Fault Sensing
Method
Circuit Breaker
Secondary
Contacts Req’d
Applicable
Breakers
Figure
Reference
Digitrip GF
Sensing
Element Used
Residual
No Jumper
3 or 4 pole
2.2, 2.3, 2.5, 2.9 element R5
Source Ground Jumper B6 to B7 3 pole only
2.7
element R4
Zero Sequence Jumper B6 to B7 3 pole only
2.8
element R4
Note: This information applies to Trip Units with Ground