Circuit brakers Ampere Voltage Ratings

Circuit Breaker
Another
device used for overcurrent protection is a circuit breaker. The NEC® defines a circuit breaker as a device designed to open
and close a circuit by non automatic means, and to open the circuit
automatically on a predetermined over current without damage to itself when properly
applied within its rating.
Circuit breakers provide a manual means of
energizing and deenergizing a circuit. In addition, circuit breakers provide automatic
overcurrent protection of a circuit. A circuit breaker
allows
a circuit to be reactivated quickly after a short circuit or overload is
cleared. Unlike fuses which must be replaced when they open, a simple flip of
the breaker’s handle restores the circuit.
Ampere
Rating
Like
fuses, every circuit breaker has a specific ampere, voltage, and fault current
interruption rating. The ampere rating is the maximum continuous current a
circuit breaker can carry without exceeding its rating. As a general rule, the
circuit breaker
ampere
rating should match the conductor ampere rating. For example, if the conductor
is rated for 20 amps, the circuit breaker should be rated for 20 amps. Siemens
I-T-E® breakers are rated on the basis of using 60° C or 75° C conductors. This
means that even if a conductor with a higher temperature rating were used, the
ampacity of the conductor must be figured on its 60° C or 75° C rating. There
are some specific circumstances when the ampere rating is permitted to be
greater than the current carrying capacity of the circuit. For example, motor
and welder circuits can exceed
Conductor
ampacity to allow for inrush currents and duty cycles within limits established
by NEC®. Generally the ampere rating of a circuit breaker is selected at
125% of the continuous load current. This usually corresponds to the conductor
ampacity which is also selected at 125% of continuous load current. For example,
a 125 amp circuit breaker would be selected for a load of 100 amps.
Voltage
Rating
The
voltage rating of the circuit breaker must be at least equal to the circuit
voltage. The voltage rating of a circuit breaker can be higher than the circuit
voltage, but never lower. For example, a 480 VAC circuit breaker could be used
on a 240 VAC circuit. A 240 VAC circuit breaker could not be used on a 480 VAC
circuit. The voltage rating is a function of the circuit breakers ability to suppress
the internal arc that occurs when the circuit breakers contacts open.
Fault
Current Interrupting Rating
Circuit
breakers are also rated according to the level of fault current they can
interrupt. When applying a circuit breaker, one must be selected which can
sustain the largest potential short circuit current which can occur in the
selected application. Siemens circuit breakers have interrupting ratings from
10,000 to 200,000 amps. To find the interrupting ratings of a specific circuit
breaker refer to the Speedfax catalog.

Leave a Reply