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WL Circuit Breaker selection and application guide
Low Voltage Circuit Breaker
Electronic trip units
Electronic trip units (ETUs)
Power system protection is necessary to treat common types of
abnormal occurrences, such as overloads or faults that can lead
to electrical power system failure.
The methods for detecting and clearing such abnormalities
and restore to normal operation is an engineered technique.
Adequate protection requires constant measurements of certain
system quantities, such as voltages and currents, comparing
those system quantities, or some combination of the quantities,
to a threshold setting computed by a systems engineer and
set into an electronic trip unit like those available on the WL
breakers. It’s equally important for power system protection
to perform under normal operating conditions. If the above
thresholds are set too low the power may be interrupted
unnecessarily causing loss of productivity or safety provisions.
The WL circuit breaker offers a practical means of setting
power system protection through vast selectivity available in
its Electronic Trip Unit (ETU). WL ETUs have a wide range of
protective settings for implementing simple or complex
coordination schemes and configuring reliable system protection.
ETU enhanced features
• Extended Instantaneous Protection (EIP): Provides full
withstand capability and coordination up to the short-time
withstand rating regardless of the Instantaneous functional
setting. Without this feature de-rating of breakers could
reduce your system interrupt capacity by 20% or require
higher interrupt rated breakers.
• Dynamic Arc-Flash Sentry: Provides selective current reduction
settings to limit the amount of available energy downstream
from the WL breaker.
• Selectable I
4
t: ETU745 and 776 make it possible to
switchover from an I
2
t to an I
4
t inverse-time function for
overload protection. This selectivity increases optimization
of coordinated overload protection when overload fuse
protection is also required.
ETU basic functions
Long-time trip
The long-time delay adjustment is used to set the tripping delay
of the circuit breaker based on the magnitude of the overcurrent
condition (6 times l
r
). For example if the rating plug is 2000
amps and the long-time delay is set to 10 seconds, a fault current
of 12,000 amps (6 x 2000) will cause the breaker to trip after
10 seconds. Long-time is an inverse of I
2
t ramp function. This
means the higher the current, the shorter the time the circuit
breaker will remain closed. An Alarm LED indicator will flash
during the delay period and a separate “Trip L” indicator will turn
on if the breaker trips on long-time function.
Short-time trip
The short-time pickup adjustment is used to set the level of
high current the breaker will carry for a short period of time
without tripping. This adjustment is set in multiples of the
value of the rating plug (Ir). Together with the short-time delay,
this adjustment allows downstream breakers time to clear short
circuit faults without tripping upstream breakers. Short-time
delay is used to set the time interval the breaker will wait before
responding to the current value selected by short-time pickup.
There are two modes of operation: fixed and I
2
t. The I
2
t delay
has the characteristic of being inversely proportional to the
square of the magnitude of the current. This means higher
overcurrent conditions have shorter delays. An Alarm LED
indicator will flash during the delay period and a separate
“Trip S” indicator will turn on if the breaker trips on short-
time function.
Instantaneous trip
The instantaneous pickup adjustment is used to set the current
level at which the breaker will trip without an intentional time
delay. Non-delayed tripping as a result of severe overcurrent
minimizes potential damage to the electrical system and
equipment. This adjustment is set in multiples of I
r
.
An instantaneous override (OFF) function is available. It is
normally set at the short-time rating of the respective breaker
frame size. This allows the breaker to ride through high fault
currents up to the breakers short-time withstand capability,
however, it is self protecting above the short-time withstand
value. An Alarm LED indicator will flash during the delay period
and a separate “Trip I” indicator will turn on if the breaker trips
on instantaneous function.
Ground fault
The ground fault pickup adjustment is used to set the level of
ground current at which circuit interruption will be initiated.
Together with ground fault delay, this adjustment allows
selective tripping between main and feeder or downstream
breakers.
The ground fault delay adjustment is used to set the time interval
(in seconds) the breaker will wait before responding once the
ground fault pickup level has been reached. The available ground
fault delay settings available are: inverse time (I
2
t) and/or fixed
delay. An optional alarm LED indicator will flash during the delay
period and a separate “Trip G” indicator will turn on if the breaker
trips on ground fault function.
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