Effective 8/13/99
Page 9
I.L. 70C1037H02
2.3.1 General
When the Digitrip 520 family includes ground fault protec-
tion features, the distribution system characteristics (for
example, system grounding, number of sources, number
and location of ground points, and the like) must be
considered along with the manner and location in which
the circuit breaker is applied to the system. These ele-
ments are discussed in Sections 2.3.3 through 2.3.6.
The Digitrip 520 family uses three modes of sensing to
detect ground fault currents: residual, source ground, and
zero sequence (see Table 2.1). Magnum Circuit Breakers
can accommodate all three types, except for 4-pole
breakers. The breaker secondary contact inputs B-6, B-7
are used to configure the breaker cell positions for the
three schemes. No jumper from B-6 to B-7 programs the
unit for a residual ground fault scheme, while a jumper
from B-6 to B-7 programs the trip unit for either a source
ground or zero sequence configuration. If present, 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
Ground (Earth)
Fault
Sensing Method
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
2.3.2 Residual Sensing
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, but no phase
to neutral loads are used, the Digitrip 520 family of units
includes all of the components necessary for ground fault
protection. This mode of sensing vectorially sums the
outputs of the three or four individual current sensors. For
separately-mounted neutrals, as long as the vectorial sum
is zero, then no ground fault exists. The neutral sensor
must have characteristics and a ratio which are identical
to the three internally-mounted phase current sensors.
Available types of neutral sensors are shown in Fig-
ure 2.4. Residual ground fault sensing features are
adaptable to main and feeder breaker applications.
Available ground fault pick-up settings employing Re-
sidual Sensing are given in Table 2.2. Figure 2.5 shows a
4-pole breaker with Residual Ground Fault Sensing.
CAUTION
IF THE SENSOR CONNECTIONS ARE INCORRECT, A
NUISANCE TRIP MAY OCCUR. ALWAYS OBSERVE
THE POLARITY MARKINGS ON THE INSTALLATION
DRAWINGS. TO INSURE CORRECT GROUND FAULT
EQUIPMENT 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 mid-point 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.
The settings shown in Table 2.1 will apply when the
neutral sensor is not the same as the frame rating in a
ground return sensing scheme.
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
½
" x 13
½
"
[114 mm x 342 mm] rectangular inside dimensions) are
available with 100:1 and 1000:1 ratios.
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.
Call Cutler-Hammer for more details on this scheme.
2.3.6 Ground Fault Settings
The adjustment of the ground fault functional settings
(FLAT response or I
2
t) is discussed in Section 4.8. The
effect of these settings is illustrated in the ground fault
time-current curve referenced in Section 9. Applicable
residual ground fault pick-up settings and current values
are given in Table 2.2 as well as in the ground time-current
curve.
