Hundreds of amperes to more than 1000 amperes of the load range, how to choose the circuit breaker

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Hundreds of amperes to more than 1000 amperes of the load range, how to choose the circuit breaker
11 04 , 2021

Molded case circuit breakers are rated from 10A to 1600A, and frame circuit breakers(ACB) are rated from 630A to 6300A. See molded case circuit breaker and air circuit breaker rated current overlap area, sometimes do not know how to choose.


There are a few principles here.

A primary distribution system in a distribution system, which has both a feed loop and a motor loop.

The protection object of the feed circuit breaker is the cable. At the same time, the feed circuit breaker must realize the protection coordination relationship with the main incoming circuit breaker of the secondary distribution system, so the feed circuit breaker must have short circuit delay S protection.

Thermomagnetic molded case circuit breaker has only two sections of protection, that is, overload long delay L parameter and short circuit instantaneous I parameter, is not suitable for the loop of the long feed cable, and to use the electronic molded case circuit breaker with three sections of protection.

For motor circuits, use a single magnetic circuit breaker, that is, only short circuit protection, no overload protection circuit breaker. Visible, this is also different from the conventional plastic case circuit breaker.

In addition, if there is an isolation transformer at the outlet of the primary distribution, because the transformer inrush current is approximately equal to the short-circuit current, the rated current of the circuit breaker can be selected according to 1.6 times the rated current of the transformer when calculating. If the isolation transformer has a large capacity, air circuit breakers are likely to be used.

For example, 250kVA 0.4kV to 0.4kV isolation transformer, impedance voltage is 6%, its rated current is:

Short-circuit current is:
We divide the short-circuit current by 10 to get 600A, so we use the circuit breaker with the rated current of 630A as usual.

However, we consider the impact time length of the excitation inrush current, we want to use short circuit delay S parameter to delay, then 630A molded case circuit breaker is not good, to use 800A frame circuit breaker, frame circuit breaker short circuit delay time longer.

In addition, when considering the external cable, it is necessary to check the thermal stability of the cable, which will increase the rated current value of the circuit breaker.

Visible, we must consider what kind of circuit breaker to choose.

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