Quick Answer
The breaking capacity of a solar DC breaker is determined by its rated current and interrupting capacity, which can be calculated using the system's maximum fault current, cable size, and number of parallel strings.
Understanding the Maximum Fault Current
The maximum fault current in a solar DC system is typically determined by the short-circuit current of the inverter, which is usually specified in the inverter’s datasheet. For example, a 5 kW inverter with a short-circuit current rating of 15 A will produce a maximum fault current of 15 A. However, this value may be higher if there are multiple inverters connected in parallel, or if the system is configured with a higher DC voltage.
Calculating the DC Breaker Size
To calculate the DC breaker size, you need to determine the maximum fault current that the breaker will need to interrupt. This can be done by multiplying the maximum fault current by the number of inverters connected in parallel, and then adding the fault currents from any other sources of DC power in the system, such as DC generators or charge controllers. For example, if you have two 5 kW inverters connected in parallel, and a 2 kW DC generator connected to the system, the maximum fault current would be 15 A x 2 + 5 A = 35 A. The DC breaker should be rated for at least 35 A to ensure it can interrupt the maximum fault current.
Selecting the Right DC Breaker
When selecting a DC breaker, it’s essential to consider not only its rated current but also its interrupting capacity. The interrupting capacity is typically specified in terms of the breaker’s ability to interrupt a specific fault current in a certain time frame, usually 5 milliseconds or less. For a solar DC system, you should choose a breaker with an interrupting capacity of at least 1.5 to 2 times the maximum fault current.
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