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Technical catalog for Susol & Metasol ACB
Technical catalog for Susol & Metasol ACB
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By utilizing discrimination system, continuity of electricity supply can be maximized. Fully rated system require
every breaker to have breaking capacity that can cover maximum fault current. However by applying discrimination
system, breaker that is the closest to accident point will only be operated to disconnect the fault block from its
power system.
The requirement from discrimination system is that each upstream breaker must have a short time delay trip unit.
Upstream breaker should be able to withstand heat and magnetic stress without tripping while the other breaker
that is close to accident point is activated to break fault current.
Using discrimination system, initial installment expense is relatively high to fully rated or cascading system.
However total expense for the system can be reduced to certain level if partial discrimination is applied rather than
whole discrimination. Discrimination system is the most expensive one from three of basic protecting methods.
Every breaker should have fully rated and upstream breaker should contain short time delay control system, and
also withstand heat and magnetic stress while downstream breaker operate its breaking function. Continuity of
power supply is absolutely guaranteed.
Discrimination method consists of 1)current discrimination that is based on fault current size at accident point,
2)time discrimination that is based on breaking time, 3)complete selectivity method that combines previous two
discriminations, and 4)energy-based discrimination that is based on arc energy. Choosing the system out of these
options varies according to initial power supply plan parameter.
1. Complete Selectivity
Upstream breaker requires more extensive trip
operating time than downstream breaker for all values
of the fault current, and guarantee selective discrimi
nation to all fault current. In other words, to achieve
complete selectivity, time-current curves of the two
breaker should not cross each other. And also there
should be sufficient time gap between the two curves.
This time gap should provide enough time for the
successive breaker to operate its breaking function
correctly so that upstream breaker will not activate its
trip unit.
2. Current Selectivity
Current selectivity is arranged when successive
circuit-breaker’s rating and instantaneous trip value
are lower than its upstream breaker. Current selec
tivity improves along with difference increase between
two breaker’s breaking ratings and instantaneous trip
values. If current limiting breaker were used as a
downstream breaker, selectivity can be improved
further. Furthermore, current limiting breaker used at
feeder level will reduce thermal, magnetic stress on a
upstream breaker. Current selectivity concept is
portrayed in picture 3.
Picture 3. Current Selectivity
Discrimination zone
for short-circuits
D
2
t
II
ins2 I ins1
D1
Discrimination limit
for short-circuits
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