Surge protective devices (SPDs) are commonly installed in solar combiner boxes under the following conditions:

1. When the solar power system is connected to sensitive loads. If the solar power is used to supply power to electronics, computers, or automation equipment, SPDs help protect these devices from voltage spikes in the solar system.

2. In large solar installations, especially those with long cable runs. Long cables act as antennae and are more prone to induced voltage spikes from lightning and utility grid switching. SPDs at the junction box prevent these surges from entering the system.

3. In solar systems with a battery energy storage system. Batteries can be a source of voltage spikes during charging and discharging, so SPDs protect other components from surges originating from the battery bank.

4. Where frequent thunderstorms occur. In areas with heavy lightning activity, voltage spikes caused by strikes nearby can enter a solar system through long cable runs. SPDs provide surge protection at the point where cables enter the junction box.

The main purposes of installing SPDs in a solar combiner box are:

1. Protecting inverters, charge controllers, and other electronics from damage due to high-voltage spikes.

2. Preventing interference with the operation of the solar system. Voltage surges can disrupt the functioning of inverters, charge controllers and monitoring equipment.

3. Reducing stress on and extending the lifetime of wiring, inverters, batteries, and other components. Frequent surges degrade equipment insulation and reduce operating life over time.

4. Improving the reliability of the solar system by avoiding voltage-spike-induced faults and equipment downtime.

5. Protecting sensitive loads that are powered by the solar system. Any devices powered by the solar system will benefit from the voltage regulation provided by SPDs.