The string box is the protection enclosure for the direct-current side of a solar system, installed between the panels and the inverter. It protects the system against surges, overcurrents and lightning, and lets you disconnect the DC side safely for maintenance. Good practice — and many utilities, when approving a project — require this protection.
What the string box is for
The string box gathers the protection devices for the inverter's DC input. It protects the equipment against lightning and voltage surges, interrupts current in case of a fault and provides a safe disconnection point. In short, it is the safety centre on the panel side, preventing inverter damage and fire risk.
Components of a string box
A complete string box usually includes:
- Surge protection device (SPD): diverts voltage spikes caused by lightning and surges to earth.
- DC fuses or circuit breakers: interrupt the current of each string in case of short circuit or overload.
- DC disconnect switch: lets you isolate the direct-current side safely.
- IP-rated enclosure: protects the components against water and dust.
How to choose a string box
Check:
- Number of inputs (strings): must match the number of strings and MPPTs of the inverter.
- Maximum voltage and current: compatible with the DC side of the system.
- SPD class: suited to the required protection level.
- IP rating: for indoor or outdoor installation.
The string box receives the solar cables coming from the panels and protects the path to the inverter.
