214
March 1996
Power Circuit Breakers Type DSII and
SPB with OPTIM Trip Units
Electronic Trip Units
Additional Protection Features
Digitrip RMS Electronic Trip Units are de-
signed and built with safety and reliability in
mind, both to protect the user and the equip-
ment as well as making sure the trip func-
tions within its design parameters. Following
a trip, the OPTIM trip unit should be reset via
the highly visible red “trip reset” button on
the front of the unit.
By providing a SURE Start discriminator cir-
cuit in Digitrip OPTIM 750 and 1050 trip units
that do not have an instantaneous setting,
the user is protected should an attempt be
made to close a breaker on a faulted circuit.
The discriminator (or making current release)
is set at 11 times the rating plug ampere rat-
ing and is enabled for approximately the first
10 cycles of current flow. Should a fault con-
dition exist upon closing, the breaker will trip
with no intentional time delay, protecting the
user from a potentially unsafe condition. The
discriminator function can be enabled or dis-
abled by the user.
In addition to a discriminator, an instanta-
neous override is present in all SPB insulated
case circuit breakers to provide additional
protection for the breaker. The instantaneous
override is factory set just below the breaker
withstand rating to ensure rapid clearing un-
der conditions of fault currents greater than
the short time withstand rating of the breaker
in which the trip unit is installed.
Digitrip OPTIM electronic trip units can oper-
ate reliably in ambient temperatures that
range from -20C to + 95C. In the unlikely
event that temperatures exceed this ambient,
the trip unit has a built-in overtemperature
trip to protect the trip unit from operating
outside its design parameters.
Non-Volatile Memory
After an overcurrent trip operation, the trip
unit retains information in E
2
prom. Values
indicating the cause of trip, currents through
the breaker at time of trip, time when trip oc-
curred, total number of times the trip unit has
tripped the breaker, and voltage and power
quality related data when applicable are
automatically stored. The data will remain in
memory even if 120 Vac control power to the
power/relay module is lost. These values can
be displayed when control power is restored
prior to resetting the trip unit.
Thermal Memory
Digitrip OPTIM electronic trip units incorpo-
rate powered thermal memory. The units
remember recent overcurrent events which
may have initiated the trip timing sequence
and then returned to nominal levels, halting
the sequence prior to trip initiation. If a
trip occurs, OPTIM remembers the heating
effect of this overload. In the event that the
current levels again exceed the pickup set-
point before the circuit has had time to cool
from the original pickup, the unit's memory
recalls the previous trip or near trip and auto-
matically imposes a shorter delay time. In
effect, the trip unit treats multiple time
related events as a single continuous event
thereby preventing system damage due to
cumulative overheating.
As a further enhancement, the trip units in-
corporate an unpowered thermal memory
feature. In the event that current levels cause
the breaker to trip and the breaker is then re-
closed following a loss of control power, the
trip unit remembers the previous overcurrent
trip and again imposes a shorter delay time
should a further overcurrent occur before a
sufficient cool-down period has elapsed.
Thermal Memory protects the distribution
system from cumulative overheating caused
by repeated overcurrent conditions. Both
powered and unpowered thermal memory
can be programmed in or out by the user.
System Alarms
Digitrip OPTIM electronic trip units offer a
high load alarm capability but with more flex-
ibility than was previously available. OPTIM
trip units have a high load alarm that can be
programmed to operate between 50% and
100% of I
r
.
In some cases users want to know when a
ground fault exists, but want to continue to
operate without tripping due to ground fault.
Digitrip OPTIM electronic trip units incorpo-
rate a ground fault alarm capability. Units
equipped with LSIA will alert the user to a
ground fault condition without tripping the
Fault 1
There are no interlocking signals. The
main breaker trip unit will initiate the trip
instantaneously.
Fault 2
The feeder breaker trip unit will initiate the
trip instantaneously to clear the fault; and
Zone 2 will send an interlocking signal to
the Zone 1 trip unit. The Zone 1 trip unit
will begin to time out, and in the event
that the feeder breaker in Zone 2 would
not clear the fault, the main breaker in
Zone 1 will clear the fault in 0.5 seconds.
Fault 3
The branch breaker trip unit will initiate
the trip instantaneously to clear the fault;
and Zone 3 will send an interlocking
signal to the Zone 2 trip unit; and Zone 2
will send an interlocking signal to Zone 1.
Zone 1 and Zone 2 trip units will begin to
time out, and in the event that the branch
breaker in Zone 3 would not clear the
fault, the feeder breaker in Zone 2 will
clear the fault in 0.3 seconds. Similarly, in
the event that the feeder breaker in Zone 2
would not clear the fault, the main breaker
in Zone 1 will clear the fault in 0.5
seconds.
Figure 2. Zone Selective Interlocking
Breaker Accessories Pictured From Left to Right: Power Relay Module, Potential Transformer Module, and
Auxiliary Power Module.
