Datasheet

48 Miniature Circuit Breakers | US Catalog
Main
breaker
CB1 CB2 CB3 CB4
Short circuit
Selective coordination and series ratings
Definition of selective coordination
Coordination between the operating characteristics of two or more
overcurrent protection devices, so that when an over-current within
established limits occurs, the device designated to operate within
those limits trips whereas the other devices does not trip.
Example of breaker coordination
When an over-current event occurs at the branch breaker level
(CB1), and the event is within the operating characteristics of
the breaker, then the branch breaker should interrupt the circuit
(open) and the main breaker should remain closed and energized.
The chart below gives a graphical representation of a down
stream branch breaker (B curve) and a main breaker (A curve)
with coordination. The separation between the curves allows the
branch breaker to react to the fault and the main breaker remains
closed and energized.
Example of no breaker coordination
Selective breaker coordination is not achieved when there is an
overload event at the branch breaker level (MCB1) and both the
branch breaker and main breaker interrupt the circuit (open).
When there is no breaker coordination, several circuits lose
power that should remain operational during and after the over-
load event. The chart below gives a graphical representation
of a down stream branch breaker (B curve) and a main breaker
(A curve) without coordination. There is no separation between
the curves. The branch breaker will react to a fault and the main
breaker will open and de-energize all circuits down stream.
Problems in coordination occur when the branch breaker allows
the “I Square T” value of the short circuit to rise to a level that
is in the operating range of the upstream main breaker. Proper
breaker coordination is easier to achieve with the use of current
limiting breakers at the branch level.
Selective coordination and current limiting breakers
Recent improvements in ABB circuit breaker technology has
pushed the performance of breakers to the same level as fuses.
The reaction time and tripping characteristics of current limiting
breakers are now on par with fuses. This allows ABB to provide
a high level of coordination between branch breakers and the
main. A current limiting branch breaker will limit the “I Square T”
value well below the level of the operating range of the up-
stream main breaker. ABB’s current limiting branch breakers can
coordinate between the main breaker up to 35 kA.
Selective coordination and zero crossing breakers
Zero crossing breakers do not limit the “I Square T” value. They
wait for the wave form to cross zero and allow a high level of
let-through energy to pass through the system. The “I Square
T” value of a zero crossing breaker is high enough that the main
breaker will likely trip during a short circuit. With zero crossing
breakers it is extremely difficult to coordinate between branch
and main breakers. A typical zero crossing breaker’s coordination
level is below 10 kA. There are a few manufacturers that have
achieved coordination between a branch zero crossing breaker
and the main by slowing the performance (protection) of the
main breaker.