Sunday, January 25, 2009

Circuit Breaker Ratings

Voltage Rating:
Circuit breakers are rated according to the maximum voltage they can handle. 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 breaker’s ability to suppress the internal arc that occurs when the circuit breaker’s contacts open.
Some circuit breakers have what is referred to as a “slash” voltage rating, such as 120/240 volts. In such cases, the breaker may be applied in a circuit where the nominal voltage between any conductor and ground does not exceed the lower rating and the nominal voltage between conductors does not exceed the higher rating.

Continuous Current Rating
Every circuit breaker has a continuous current rating which is the maximum continuous current a circuit breaker is designed to carry without tripping. The current rating is sometimes referred to as the ampere rating because the unit of measure is amperes, or, more simply, amps.

The rated current for a circuit breaker is often represented as In. This should not be confused with the current setting (Ir) which applies to those circuit breakers that have a continuous current adjustment. Ir is the maximum continuous current that circuit breaker can carry without tripping for the given continuous current setting. Ir may be specified in amps or as a percentage of In.

As mentioned previously, conductors are rated for how much current they can carry continuously. This is commonly referred to as the conductor’s ampacity. In general, the ampere rating of a circuit breaker and the ampacity of the associated conductors must be at least equal to the sum of any non-continuous load current plus 125% of the continuous load current.

Circuit 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.

Frame Size:

The circuit breaker frame includes all the various components that make up a circuit breaker except for the trip unit. For any given frame, circuit breakers with a range of current ratings can be manufactured by installing a different trip unit for each rating. The breaker frame size is the highest continuous current rating offered for a breaker with a given frame.

Interrupting Rating:
Circuit breakers are also rated according to the maximum level of current they can interrupt. This is the interrupting rating or ampere interrupting rating (AIR). Because UL and IEC testing specifications are different, separate UL and IEC interrupting ratings are usually provided.
When designing an electrical power distribution system, a main circuit breaker must be selected that can interrupt the largest potential fault current that can occur in the selected application. The interrupting ratings for branch circuit breakers must also be taken into consideration, but these interrupting ratings will depend upon whether series ratings can be applied. Series-connected systems are discussed later in this course.

The interrupting ratings for a circuit breaker are typically specified in symmetrical RMS amperes for specific rated voltages. As discussed in Basics of Electricity, RMS stands for root-mean-square and refers to the effective value of an alternating current or voltage. The term symmetrical indicates that the alternating current value specified is centered around zero and has equal positive and negative half cycles. Siemens circuit breakers have interrupting ratings from 10,000 to 200,000 amps.

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