In rooftop solar photovoltaic (PV) systems, the choice of circuit breakers is often overlooked. Inappropriate circuit breakers can cause frequent tripping of the equipment, overheating damage, and even system fire.

This article discusses how to choose a circuit breaker in a photovoltaic system.

In a solar photovoltaic system, the choice of a series of circuit breakers depends on several factors:

1. Ambient temperature at the circuit breaker

In rooftop solar installations, the equipment is usually placed outdoors (ground-mounted or flat roof systems). Compared to installations in buildings, higher outdoor temperatures are usually assumed. Therefore, the temperature in the switchboard is usually higher. This will also affect the flow rate and operating temperature of the circuit breaker.

For the selection of circuit breakers in rooftop solar systems, temperature is the most important consideration. According to the IEC 60947-2 standard, the circuit breaker has a data sheet detailing the derating/increasing current value of the ambient temperature. The circuit breaker equipment that is compatible with the on-site ambient temperature and system current should be selected.

2. Mutual heating of circuit breakers

For large solar projects with multiple inverters, there are usually multiple circuit breakers in the switchboard, which are installed next to each other. These circuit breakers will provide the maximum current at the same time, causing the circuit breaker temperatures to influence each other more quickly and may trip prematurely.

When multiple circuit breakers are installed in parallel, the correction factor specified in the circuit breaker data sheet needs to be considered.

For example, in the case of arranging six devices, the correction factor may be 0.75. A circuit breaker with a nominal current of 15.1 amperes (A) behaves like a nominal current of 0.75 x 15.1A = 11.33A.

Through this calculation, if the current is insufficient, a circuit breaker with a higher rated current can be used. Another possibility is to increase the gap between the circuit breakers. This will dissipate more heat and prevent unnecessary trips.

If the rooftop solar system is connected to the grid, it will trip due to the current and voltage effects of the load feeder network. When choosing a circuit breaker, you need to consider the components of the load in the grid.

Examples of thermal ratings of circuit breakers in parallel operation of photovoltaic power stations:

The required technical specifications can be found in the data sheet of the Solis-1P8K-5G inverter:

The choice of cable and wiring method, ambient temperature and other potential conditions limit the maximum fuse protection of the cable.

This example assumes that the selected cable (6mm²) has ideal wiring and can withstand a nominal current of 35A.

The maximum nominal current of the cable used and the maximum fuse protection of Solis-1P8K-5G limit the maximum possible nominal current of the circuit breaker. 

Choosing the correct circuit breaker

In the same example, assuming that the load does not have motors, transformers, etc., and the calculated current is 34.7A, a 40A circuit breaker with a thermal tripping characteristic of B and no gap between the circuit breakers is selected.

In order to verify whether the selected value is appropriate, the thermal adaptability of the circuit breaker needs to be checked:

The load factor meets the specifications of the data sheet:

(In a solar project, the load can be sustained for more than 1 hour)

(If one circuit breaker is used or the distance between each other is sufficient, the coefficient is equal to 1)

The rated load current of the circuit breaker is calculated as follows:

Ibn = 40 A x 0.9 x 0.75 x 1.0 = 27A 

Since the maximum current-carrying capacity of trouble-free operation is lower than the maximum output current of the inverter used, the selected circuit breaker cannot be used in this example. The circuit breaker will trip during rated operation.

Use a 50A circuit breaker. There is enough space for heat dissipation (>10mm) between the circuit breakers, and the maximum current-carrying capacity is 40.5A

(Ibn = 50A x 0.9×0.9 = 40.5A). The circuit breaker will not trip under rated operation.

Use a 63A circuit breaker. Maximum current carrying capacity is 42.5A

(Ibn = 63A x 0.75x 0.9x 1 = 42.5A). The circuit breaker will not trip under rated operation.

This article is sponsored by the solar inverter manufacturer Jinlong Technology.