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QOU250VH Datasheet(PDF) 10 Page - List of Unclassifed Manufacturers |
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10 / 32 page QOU Miniature Circuit Breakers and Switches Application Information 10 © 2005 Schneider Electric All Rights Reserved 09/2005 For example, Figure 3 shows the ambient rerating curves for QOU miniature circuit breakers. What is the continuous current capacity of a 80 A circuit breaker applied at 104° F (40° C)? A 90 A circuit breaker at 50° F (10° C)? By finding 40° C on the horizontal axis and reading up to the 80 A curve, you find that the circuit breaker will carry 80 A, which is its rated current carrying capacity. If the circuit breaker will be used on a continuous load (defined as three hours or more), Section 210-20(a) of the 2005 NEC requires that loading not exceed 80% of the rating. Here, 80 A x .80 = 64 A. Locate 50° F (10° C) on the horizontal axis and move straight up to the 90 A curve. The circuit breaker will carry 110 A. Again, if the circuit is used on a continuous load, it must be applied at 80% of its rating. In this example the equation is 110 A x .80 = 88 A. As explained in Section 210-20(a) of the NEC: “Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load. “Exception: Where the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, the ampere rating of the overcurrent device shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.” Frequency Rating The standard-rated frequency for circuit breakers is 60 Hz, but Square D circuit breakers can be applied on 50 Hz systems without thermal or magnetic rerating. Other frequencies can affect the thermal, magnetic and short-circuit tripping characteristics of circuit breakers. Applying thermal-magnetic circuit breakers at frequencies above 50/60 Hz requires special consideration of the effects of high frequency on circuit breaker tripping characteristics. Thermal and magnetic operations must be treated separately. Thermal Tripping Performance at High Frequency At frequencies below 60 Hz, the thermal rerating of thermal-magnetic circuit breakers is negligible. However, at frequencies above 60 Hz, thermal rerating is required. High-frequency operation causes abnormal heat rise in the current-carrying parts because of the skin effect. One of the most common high-frequency applications is at 400 Hz. Figure 4 indicates the thermal rerating multiplier to be used with each circuit breaker family when applied on 400 Hz systems. When applying a 100 A QOU circuit breaker on a 400 Hz system, the circuit breaker’s current carrying capacity is as follows: • Non-continuous Loads (less than three hours): Using Figure 4, the QOU circuit breaker may be applied at .78 of rating, or 78 A. • Continuous Loads (three hours of more): NEC Article 210-20(a) requires that standard circuit breaker loading does not exceed 80% of the circuit breaker’s rating when used for continuous loads. Therefore, the current- carrying capacity of a 100 A QOU circuit breaker operating under continuous load at 400 Hz would be 100 A x .78 x .80 = 62 A. . Figure 3: Ambient Rerating of QOU Circuit Breaker 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 100 A 90 A 80 A 70 A 15 A 10 A 30 A 20 A 40 A 60 A 50 A 110 A 125 A -10 (14) 0 (32) 20 (68) 40 (104) 60 (140) Ambient Temperature ˚C (˚F) 45 A Figure 4: Frequency Rerating of QOU Circuit Breaker at 400 Hz 100 200 300 0 .5 .6 .7 .8 .9 1.0 QO, QOU Current Rating in Amperes |
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