Quick Facts
What an SCB is
A String Combiner Box (SCB) is an electrical enclosure that combines the DC outputs of multiple solar panel strings into a single combined output that feeds the inverter. The SCB sits between the array (multiple strings) and the inverter (single DC input or a few MPPT inputs), simplifying the wiring while providing protection and monitoring functions.
For commercial and utility-scale solar plants with dozens or hundreds of strings, SCBs are essential. Without them, every string would need its own cable run to the inverter, creating massive cable bundles and complex maintenance. With SCBs, each SCB serves a cluster of nearby strings and one combined cable runs to the inverter or to a DCDB (DC Distribution Box).
A typical mid-size commercial plant has multiple SCBs distributed across the array. A utility-scale plant has dozens of SCBs feeding multiple central or string inverters.
What is inside an SCB
A standard SCB contains several functional components.
DC isolators: Switches for each string and for the combined output. Allow safe disconnection for maintenance. Rated for the DC voltage and current.
Solar-specific fuses: gPV fuses with characteristics suited for DC PV applications. Protect each string against sustained overcurrent. Typically 15A to 25A rating per string.
Surge Protection Devices (SPDs): Type 1 or Type 2 SPDs that divert transient voltage spikes (lightning-induced, switching transients) to earth, protecting downstream equipment.
Current sensors: Hall-effect or shunt sensors that measure each string’s current. The data feeds into SCADA for string-level monitoring.
Combining busbar: Copper or aluminium busbar that combines the string outputs into the combined output.
Output cable connection: Terminal block or connector for the combined DC cable running to the inverter.
Enclosure: Polycarbonate or stainless steel box with IP54 to IP65 rating for outdoor mounting. Sun-shield often included.
Earthing terminal: For connection to the plant’s earthing system.
Cable glands: Sealed entries for incoming string cables and outgoing combined cable.
The specific configuration depends on the SCB size, voltage rating, and feature set.
SCB ratings and selection
When selecting SCBs for a project, key parameters include:
Number of input strings: 4, 6, 8, 12, 16, 24, or 32 strings. Should match the array design.
DC voltage rating: 1000 V DC for commercial, 1500 V DC for utility-scale. Must exceed the maximum string Voc at coldest ambient temperature with a safety margin.
DC current rating: Per-string and combined output. Must exceed maximum operating current with a safety margin.
Fuse rating: Per-string fuses sized to protect string cabling. Typically 15A to 25A.
SPD type: Type 1 (lightning direct strike protection) or Type 2 (transient overvoltage). Application depends on lightning risk and Type 1 LPS presence.
IP rating: IP54 minimum for outdoor; IP65 preferred for harsh environments.
Material: Polycarbonate (lighter, lower cost) or stainless steel (more durable, higher cost).
Monitoring: String-level current sensing capability. Communication interface (RS-485 or Ethernet) for SCADA integration.
Sizing the SCB requires coordination with the array design, cable sizing, and inverter input specifications.
SCB and DCDB
SCB (String Combiner Box) and DCDB (DC Distribution Box) serve related but distinct functions.
SCB: Combines string-level inputs into one combined output. Located near the array.
DCDB: Distributes or controls the combined DC for safety and maintenance. Located near the inverter.
In some designs (especially smaller plants), SCB and DCDB functions are combined into a single enclosure called a Combiner-DCDB or PV Junction Box.
In larger plants, SCB and DCDB are separate enclosures:
SCBs in the field, one per array section.
DCDB near each inverter, receiving multiple SCB outputs.
Inverter then sees a single DC input from DCDB.
This architecture provides multiple disconnection points and clear demarcation between array and inverter.
SCB safety functions
SCBs incorporate several safety functions:
Fuse protection: Each string has its own fuse, so a fault in one string opens that fuse without affecting other strings.
Surge protection: SPDs absorb transient voltage spikes that could damage modules, inverters, or cables.
DC isolation: Operators can isolate the SCB for maintenance without affecting the rest of the array.
Visual inspection: Open SCB allows inspection of fuses, contacts, and connections.
Tamper protection: Lockable doors and warning labels prevent unauthorised access.
Earth fault detection: Some SCBs include earth fault detection circuits that alert SCADA to insulation breakdown.
These safety features are essential for protecting personnel, equipment, and plant performance.
SCB monitoring features
Modern SCBs include string-level monitoring:
Per-string current measurement: Sensors track each string’s current in real-time.
Communication: RS-485 (Modbus) or Ethernet output to SCADA.
Local indicators: LEDs or display showing each string’s status.
Fault flagging: Software identifies strings with abnormal current (high suggests short circuit; low suggests open or shading).
String-level monitoring is the most effective way to detect individual string failures. A 1% drop in one string out of 100 may not be visible at inverter level but is obvious at SCB level.
Common Indian SCB suppliers
Hensel India (German brand with Indian manufacturing): Premium quality SCBs.
Schneider Electric: Solar SCBs and DCDBs.
ABB: Solar combiner solutions.
Eaton: Solar combiners.
L&T Electrical and Automation: Indian-manufactured SCBs.
Polycab: Indian wires and cables company with solar SCB offering.
C&S Electric (now Siemens): Indian-manufactured SCBs.
For utility-scale projects, premium brands are standard. For smaller commercial projects, Indian-manufactured SCBs at competitive prices are common.
Common SCB mistakes
Undersizing the voltage rating. SCB must handle maximum Voc at coldest ambient, with margin.
Skipping per-string fuses. Common in low-cost designs; eliminates string-level fault isolation.
Using ordinary fuses instead of gPV solar-specific fuses. Solar DC characteristics require specific fuse design.
Ignoring SPD requirements. Lightning damage to inverters is expensive; SPDs at SCB level protect the inverter.
Mounting SCB in direct sun. The internal temperature can exceed component ratings.
Poor cable management. Crowded SCB enclosures cause heat buildup and increase fault risk.
Skipping monitoring. String-level monitoring is the most useful diagnostic in a solar plant.
Best practices
For utility-scale projects, specify SCBs with built-in string-level monitoring and communication.
Mount SCBs in shaded locations or with sun shields. Internal temperature should stay below component ratings.
Use solar-specific gPV fuses for string protection.
Include Type 2 SPDs at minimum; Type 1 if direct lightning protection is needed.
Maintain cable runs short and well-organised inside the SCB.
For new installations, integrate SCB monitoring into the SCADA system from day one.
For maintenance, inspect SCBs annually for connection torque, fuse status, and SPD condition.
Standards and references
SCBs follow IEC 60439-1, IEC 61439, and IEC 61730 (PV systems). Solar-specific fuses follow IEC 60269-6 (gPV). SPDs follow IEC 61643. Indian standards include IS 8623 and related for low voltage switchgear.
Related glossary terms
Key takeaways
A String Combiner Box (SCB) combines DC outputs of multiple solar panel strings into a single combined output for the inverter. SCBs house protection devices (fuses, surge protectors), isolators, and string-level monitoring sensors. Standard configurations combine 4 to 32 strings at 1000 V or 1500 V DC ratings. SCBs are essential for commercial and utility-scale solar plants, providing simplified wiring, protection, and monitoring. Quality SCB selection involves coordinating voltage rating, current capacity, fuse sizing, SPD specification, and monitoring features with the overall plant design.