Technical
Current Limiting Circuit Breaker Technology
Fuseless Current Limiting
Circuit Breakers
The technology of Siemens Sentron®
fuseless current limiting circuit breakers
was developed to meet the demands of
modern distribution systems. It is
not uncommon for today’s systems to
have prospective short circuit currents
approaching 200,000 amperes. Users
demanded the protection and flexibility
afforded by circuit breakers, without
the nuisance and expense of fuse
replacement.
Underwriters Laboratories, in UL4892.4A, defines a fuseless current limiting
circuit breaker as one that “does not
employ a fusible element, and that when
operating within its current-limiting range,
limits the let-through l2t to a value less
than the l2t of a half-cycle wave of the
symmetrical prospective current.”
l2t is an expression which allows
comparison of the energy available as a
result of fault current flow. As used in
current limiting discussions, l 2t refers to
the energy released between the initiation
of the fault current and the clearing of the
circuit.
Figure 26 relates the “prospective l2t” to
the energy allowed by a Sentron current
limiting circuit breaker, or “let-through
l2t”. The upper curve represents the
maximum I2 the circuit can produce,
unaltered by the presence of any
protective device. The lower curve
illustrates the reduction in energy
allowed when Sentron current limiting
circuit breakers are used.
Selection
The Sentron circuit breakers use the
“blow-apart” contact principle to
accomplish current limitation. This
principle is based on the electro-magnetic
repulsion of adjacent conductors which
carry current in opposite directions.
The contact arms are arranged to create
opposing magnetic fields. As fault current
rises, magnetic repulsion forces the
contacts to separate completely. The
higher the fault current, the faster this
“blow-apart” action occurs.
As figure 27 illustrates, the energy letthrough with the current limiting Sentron
circuit breaker is decreased significantly.
This provides better protection for
downstream equipment, and reduces
damage.
Figure 26. Reduction of l2t Let-Through
with Current-Limiting Technology
Applications and Ratings
Sentron current limiting circuit breakers
are designed for use in load centers,
power panelboards, distribution
switchboards, secondary unit
substations, and all types of individual
enclosures where the available fault
currents exceed the interrupting ratings
of heavy duty and extra-heavy duty
molded case circuit breakers.
Sentron circuit breakers have ratings of
15 through 1600 amperes, 240 through
600 volts AC, with up to 200,000
symmetrical amperes interrupting rating.
Figure 27. Current Limitation
TECHNICAL
Siemens Power Distribution & Control, SPEEDFAX™ 2007-2008 Product Catalog
18
Figure 27 illustrates how the Sentron
circuit breaker limits the energy under
fault conditions. The upper curve
illustrates the first half-cycle wave of
prospective fault current. To qualify as
truly current limiting, the circuit breaker
must prevent the current value from
reaching the maximum value that it
would reach if the circuit breaker were
not connected in the circuit.
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