How to Select the Right Circuit Breakers for Inverters in Rooftop Solar Systems

Temperature is a key consideration while choosing circuit breakers

April 21, 2021


In rooftop solar photovoltaic (PV) systems, the selection of circuit breakers is often overlooked. An inappropriate circuit breaker can cause frequent tripping of the equipment, damage due to overheating, and even system fire.

This article discusses how circuit breakers in PV systems should be chosen.

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Types of Circuit Breaker

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

  • Electrical characteristics of the system
  • Environment
  • Loads and the requirements of the installation type

1. Ambient Temperature at the Circuit Breaker

In a rooftop solar installation, the equipment is usually placed outdoors (ground-mount or flat-roof systems). A higher outdoor temperature is usually presumed compared to installations inside buildings. Therefore, the temperature in the distribution board will typically be 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, the temperature is the most important consideration. According to the IEC 60947-2 standard, circuit breakers have a datasheet detailing the derating/increasing current value of the ambient temperature. A  circuit breaker equipment appropriate to the on-site ambient temperature and the size of the system current should be chosen.

2. Mutual Heating of Circuit Breakers

There are usually multiple circuit breakers in the distribution board for large-scale solar projects with multiple inverters, which are closely mounted next to each other. These circuit breakers will provide maximum current at the same time resulting in the temperature of the circuit breakers affecting each other more quickly, with the possibility of premature tripping.

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When multiple circuit breakers are installed parallelly, the correction factor specified in the datasheet of the circuit breaker needs to be considered.

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

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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 clearance between the circuit breakers. This allows more heat to be dissipated, thus preventing unnecessary tripping.

3. Type of Connected Devices

If a rooftop solar system is connected to the grid, it will be tripped by the current and voltage impact of the load feeder network. While choosing a circuit breaker, the components of the load in this grid need to be taken into account.

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An Example

Examples for the thermal ratings of circuit breakers in parallel operation of PV plant:

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

  • Maximum output current = 34.7A
  • Its maximum fuse protection = 50A

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 routing and can withstand a nominal current of 35A.

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

Selecting the Correct Circuit Breaker

Using the same example and assuming the load that has no motors, transformers, etc., based on the calculated current of 34.7A, a 40A circuit breaker with a thermal tripping characteristic of B and no gap between the circuit breakers is chosen.

To verify if the selected value is appropriate, the thermal adaptability of the circuit breaker needs to be checked:

The load factor meets the specifications of the datasheet:

  • Reduction at permanent load> 1 h = 0.9

(Permanent loads of more than 1 hour are possible in a solar project)

  • Reduction factor when 6 circuit breakers are directly arranged next to each other = 0.75

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

  • The increase in the nominal current in the distribution board when the ambient temperature is 40°C = 1.0

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The nominal load current of the circuit breaker is calculated as:

Ibn = 40 A x 0.9 x 0.75 x 1.0 = 27A 

Since the maximum current carrying capacity for fault-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.

Solution 1

Use a 50A circuit breaker. There is enough space (>10mm) for heat dissipation 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.

Solution 2

Use a 63A circuit breaker. The 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 Ginlong Technologies, solar inverter manufacturers.