Key components of a photovoltaic string combiner box: technical guide

Apr 2, 2025

In a photovoltaic solar installation, especially in systems with multiple strings of panels, the string control panel is a key component to ensure the safety, efficiency, and monitoring of the system. Often overlooked in the early design stages, this panel concentrates critical protection and control functions in direct current (DC), serving as the link between the solar modules and the inverter.

In this guide, we detail what a string control panel is, what components it comprises, and why its proper selection and assembly are essential for the optimal operation of any solar plant.

What is a String Control Panel?

The string control panel—also known as a string combiner box—is a device designed to connect, protect, and monitor multiple strings of solar panels operating in direct current, before the energy is transferred to the inverter.

Its main functions include:

Individually isolating each string.
Protecting against overloads and short circuits.
Detecting faults in the performance of specific strings.
Facilitating maintenance and supervision of the entire installation.

String control panels are particularly relevant in medium to large-scale installations, such as solar farms, industrial rooftops, or photovoltaic plants.

Components of a String Control Panel

The quality, reliability, and functionality of a string control panel largely depend on the correct selection and installation of its internal components. Below, we analyze in depth the essential elements that should be part of any well-designed string control panel.

🔌 DC Disconnect Switches

These devices allow for the manual interruption of the current from each string or group of strings to perform maintenance, inspection, or component replacement tasks without the need to disconnect the entire installation.

Key features:

Specific for direct current (DC), capable of extinguishing electrical arcs.
Nominal voltages adapted (600 V, 1000 V, or 1500 V DC).
Mandatory according to regulations to ensure safe disconnection.

🫯 String Fuses or Circuit Breakers

Each string of modules must have independent protection. This prevents a fault in one string from affecting the system or damaging the inverter.

Available options:

gPV fuses, specifically designed for photovoltaics.
DC miniature circuit breakers (MCBs), for manual reconnection after tripping.

They should be located in accessible, well-labeled, and secure holders.

⚡ Surge Protection Devices (SPDs)

Transient overvoltages can be caused by lightning, network maneuvers, or inductions. An SPD protects against these spikes.

Important features:

Specifically designed for direct current.
With status indicators (visual/remote).
Models with signaling contacts for control systems.

🔎 String Monitoring Devices

They allow measuring the current and voltage of each string individually, detecting imbalances, and optimizing maintenance.

Common systems:

Precision shunts or Hall sensors.
Voltage readers per input or group.
Integration with inverters or data loggers.

Ideal for early detection of faults due to shading, dirt, or degradation.

🧐 Data Loggers or Controllers

These devices collect and transmit data from the panel to the energy management system or SCADA platform.

Key functions:

Measurement of current, voltage, temperature, SPD status, etc.
Communication via RS-485, Ethernet, 4G, or other interfaces.
Historical analysis, alarms, and remote diagnostics.

💪 Terminal Blocks and Connection Strips

They allow for the orderly connection of input and output cables, ensuring reliable distribution.

Important requirements:

Compatibility with high DC currents.
Materials resistant to corrosion and temperature.
Secure connection and durable labeling.

📦 Enclosures and Boxes

The enclosure physically protects all the components of the panel.

Key aspects:

Protection degree minimum IP65; ideally IP66 for outdoor use.
Mechanical resistance IK08 or higher.
Materials resistant to UV rays and chemicals.
Internal space sufficient for natural or forced ventilation.

Importance of Each Component in Safety and Efficiency

Poor sizing or the absence of adequate components can lead to:

Fire risks due to undetected overloads.
Production losses due to unseen faults.
Difficulty in maintenance and diagnostics.
Regulatory non-compliance with legal consequences.

Regulations and Assembly Recommendations

The string control panel must comply with international standards that ensure safety and compatibility:

Best practices include:

Avoid loose, unlabeled, or exposed connections.
Use only components certified for DC.
Design with accessibility and space for future upgrades.
Prevent overheating with proper ventilation or cooling systems.

Conclusion

The string control panel plays a crucial role in the performance and safety of any photovoltaic installation. Far from being a simple connection point, it centralizes key functions such as string protection, DC surge suppression, performance monitoring, and safe disconnection, ensuring the long-term reliability of the system.

Each component—whether DC fuses, miniature circuit breakers, surge protection devices, or connection terminals—must be carefully selected based on the environment, current levels, and regulatory requirements.

Choosing the right string control enclosure is also essential to ensure proper insulation, protection against water and dust, and resistance to mechanical impact and UV radiation. For further technical guidance, you can consult our articles on IP protection, IK impact resistance, and UV protection grades.

At Solera, we are committed to supporting installers and engineering professionals with certified, robust solutions tailored to the demands of solar energy systems. For easier design and configuration, try our online String Control Panel Configurator.

Design smarter. Protect better. Rely on Solera’s experience and expertise.