Miniature
circuit breakers
15.38 Low Voltage Products & Systems
1SXU000023C0202 ABB Inc. • 888-385-1221 • www.abb-control.com
15
Circuit breaker current limitation
Current limiting definitions
All ABB Miniature Circuit Breakers are UL tested and certied as current
limiting protective devices. Current limiting circuit breakers provide
a higher level of circuit protection than a typical zero point external
breakers.
UL AC 60Hz cycle
UL denes an AC cycle as the potential energy of the wave form
traveling from Zero-to-Positive amplitude, Positive-to-Zero amplitude,
Zero-to-Negative amplitude, Negative-to-Zero amplitude 60 times in
one second. One cycle is completed every 16.6 milliseconds.
UL breaker current limiting
UL denes breaker current limitation as a breaker that interrupts
and isolates a fault in less than 1/2 of an AC cycle. 1/2 a cycle is
completed in 8.3 milliseconds.
NEC240.2 current limiting
A device that, when interrupting current in its current-limiting range,
reduces the current owing in the faulted circuit to a magnitude
substantially less than that obtainable in the same circuit if the device
were replaced with a solid conductor having comparable impedance.
IEC 60947-2 current limiting circuit breaker
A circuit breaker with sufciently short trip time to prevent the short-
circuit current from reaching the peak value which would otherwise be
reached.
ABB current limiting breakers
ABB current limiting breakers can interrupt and isolate a fault in 1/8 of
an AC cycle. The breaker fault interruption is completed in 2.3 to 2.5
milliseconds.
Zero point extinguishing breakers
A typical zero point extinguishing breaker interrupts a fault and does
not isolate the energy. The breaker allows an arc to be present between
the open contacts until the AC wave form crosses zero. When the
wave form crosses zero, the potential energy is zero and the arc
(fault) naturally extinguishes. The arc could be present for up to 8.3
milliseconds.
Current limiting breakers and electrical networks
Current Limitation
When a short-circuit condition occurs, the “ideal” current limiting circuit
breaker opens before the current waveform can reach its full potential
magnitude which occurs at ¼ cycle (4.17ms). ABB’s current limiting
breakers can interrupt a fault in about ½ cycle or 2.3ms to 2.5ms.
ABB’s current limiting breakers interrupt a short circuit in less than 1/8
cycle and limit the amount of current that can reach a circuit. Limiting
the available current on the circuit provides additional protection
against network, breaker, or bus damage and prevents the tripping of
upstream breakers (selective coordination).
IsqT
The true destructive nature of a short circuit is measured by the time
it is available combined with the peak value of the short circuit. The
IsqT (Amps Squared over Time) value represents the amount of energy
available on a network during a short circuit and is represented by the
shaded area on the graph below.
During a short circuit both magnetic forces and thermal energy
combine to damage devices on the electrical network. The level of
thermal energy and magnetic forces are directly proportional to the
square of the current. The magnetic forces vary as a square of the
peak current available and the thermal energy varies as a square of
the RMS (root mean square) current available. ABB’s current limiting
breakers will limit the let-through energy to a fraction (1/100th) of the
value which is available from the network. By comparison, a Zero
Crossing breaker would let-through approximately 100 times as much
destructive energy as the current limiting circuit breaker
[ (100,000A / 10,000A) squared – 100X].
ABB’s current limiting breakers limit the short circuit current to a
relatively small magnitude in a extremely short time, which dramatically
limits a short circuit’s destructive energy.