I.B. 29C891B
Page 31
Effective 11/98
Figure
3-13
graphically illustrates how the instantaneous
pickup portion of the overall curve can be moved hori-
zontally and independently by means of the programma-
ble settings.
Discriminator
A selectable discriminator circuit is provided on all OPTIM
Trip Units. This prevents the circuit breaker from remain-
ing closed for the entire duration of the programmed short
delay time, if the breaker is closed on a high short circuit
fault with the instantaneous protective function disabled.
The discriminator circuit (high initial current release) is pre-
set at
11
times the rating plug value
(11 x I
n
)
. It is active
for approximately ten cycles following the initial current
flow through the circuit breaker, and is reset whenever the
load current falls below 10% of the circuit breaker frame or
current sensor rating.
In the event the circuit breaker is not intended to trip out
on a circuit whose current could initially be higher than
11 x I
n
, it is possible to make the discriminator inactive.
If a circuit breaker would close onto a high short-circuit
current in this situation, type LS or LSG trip units would
rely on the short delay protection function. If the fault
current exceeds the short-time withstand current capa-
bility of the circuit breaker, override protection circuitry
would trip the circuit breaker without delay.
Override (Fixed Instantaneous)
An override (fixed instantaneous) circuit is included in all
circuit breakers except DSII and DSLII circuit breakers. It
protects against a short-circuit current that exceeds the
short-time withstand current capability of the circuit
breaker. The override circuit is set to a value no greater
than the short time withstand current rating of the circuit
breaker in which the override circuitry is installed. Since
the specific values vary by circuit breaker type and rat-
ing, refer to the applicable time-current curves for the
value associated with a particular circuit breaker.
3-2.7 GROUND FAULT PROTECTION
Ground fault protection is optional with all OPTIM Trip
Units. It can be provided in a number of combinations
with long, short and instantaneous. When ground fault
trip protection is not provided, a ground fault alarm may
be specified, (Tables
3.2
and
3.3
and Figure
3-4
). The
available trip unit options by circuit breaker type are as
follows:
• Series C K, L and N-Frame and SPB and DSII Circuit
Breakers
- LSIG
(with ground fault trip)
- LSIA
(with ground fault alarm where “A” may be dis-
abled)
• Model 550 Series C K, L, and N-Frame or all SPB and
DSII Circuit Breakers
- LSI
(without ground fault trip or alarm)
Ground fault protection and other related features are as
follows:
• Ground fault alarm
• Ground fault pickup
• Ground fault delay (I
2
t Slope)
• Ground fault delay (Flat Slope)
• Zone selective interlocking (Models 750 and 1050 only)
• Ground fault memory
When an OPTIM trip assembly includes ground fault
protection, the distribution system characteristics, such
as system grounding, number of sources, number and
location of ground points, must be considered. The
manner and location in which the circuit breaker is con-
nected to the system should also be considered.
OPTIM units with ground fault protection already have
all the necessary equipment for a 3-phase 3-wire
grounded system. This basic mode for ground fault
sensing employs a residual sensing scheme which vec-
TIME
CURRENT
Note: Instantaneous
Pickup represented
by dotted lines
Figure 3-13 Typical Instantaneous Pickup Adjustment
Summary of Contents for Digitrip OPTIM 1050
Page 7: ......
