Quick Facts
What ACDB is
ACDB (AC Distribution Box) is an electrical enclosure on the AC side of a solar PV system that houses protection devices, isolators, surge protectors, and sometimes metering equipment. The ACDB sits between the solar inverter’s AC output and the building’s electrical system (or directly the DISCOM grid).
The ACDB serves multiple functions:
Protection: MCBs (Miniature Circuit Breakers) protect against overcurrent. RCDs (Residual Current Devices) protect against ground faults. SPDs (Surge Protection Devices) absorb transient voltage spikes.
Isolation: A manual AC isolator allows the inverter to be disconnected from the grid for maintenance.
Metering: Current transformers (CTs) and energy meters can be installed in the ACDB to measure solar export.
Distribution: For larger systems with multiple inverters, the ACDB combines outputs from several inverters into a single feed.
Compliance: The ACDB provides a clearly defined boundary for grid interconnection, supporting DISCOM and code compliance.
Every grid-connected solar installation in India must have an ACDB. Without it, the installation is not safe or code-compliant.
What is inside an ACDB
A typical residential or small commercial ACDB contains:
AC isolator: Manual switch that disconnects the inverter from the grid. Allows safe maintenance.
MCB (Miniature Circuit Breaker): Rated for the inverter’s continuous AC current. Trips on overcurrent or short circuit. Typically Type C characteristic for solar applications.
RCD (Residual Current Device): Detects ground faults. Trips when leakage current exceeds threshold (typically 30 mA for personal protection, 100 to 300 mA for equipment protection). Type B RCD is required for solar applications.
SPD (Surge Protection Device): Type 2 SPD typical. Absorbs transient voltage spikes (lightning-induced or switching transients), protecting the inverter and downstream equipment.
Earthing terminal: For connection to the plant’s earthing system. Critical for safety and SPD function.
Cable entries: Sealed cable glands matching the IP rating.
Sometimes:
Current transformers (CTs) and energy meter for measuring solar export.
Status indicator lights.
Communication interfaces for SCADA integration.
Larger commercial ACDBs may include additional features like:
Multiple inverter inputs combined into one output.
Phase imbalance protection.
Reactive power compensation.
More sophisticated metering and communications.
ACDB sizing
The ACDB is sized to the inverter’s continuous AC output rating, with margin for fault currents.
For a 5 kW single-phase residential inverter:
Inverter continuous AC current: 22 A (at 230 V).
MCB rating: 32 A Type C (higher than continuous to allow surge tolerance).
RCD rating: 40 A with 30 mA trip current.
SPD: Type 2 with In = 5 kA, Imax = 10 kA.
For a 100 kW three-phase commercial inverter:
Inverter continuous AC current: 145 A (at 415 V three-phase).
MCB rating: 200 A (three-pole Type C).
RCD rating: 200 A with 100 mA trip current.
SPD: Type 2 with higher ratings.
The MCB rating is typically 1.25 to 1.5 times the inverter’s continuous current. This allows the inverter to operate at full output without nuisance tripping while still protecting against fault currents.
ACDB versus DCDB
ACDB and DCDB serve similar functions on opposite sides of the inverter:
| Aspect | ACDB | DCDB |
|---|---|---|
| Side | AC (inverter to grid) | DC (panels/SCB to inverter) |
| Voltage | 230 V or 415 V AC | Up to 1000 V or 1500 V DC |
| Protection | MCB, RCD, SPD | DC isolator, fuse, SPD |
| Isolation | AC isolator | DC isolator |
| Metering | CT + energy meter | Optional string monitoring |
Both are essential for code-compliant solar installations. Some smaller systems combine ACDB and DCDB into a single hybrid enclosure.
ACDB in residential installations
For PM Surya Ghar residential installations, the ACDB is typically:
Wall-mounted near the inverter.
Outdoor-rated (IP65) if mounted outside.
Indoor-rated (IP54) if mounted inside.
Pre-assembled by the EPC contractor.
Tested and certified per IS 13947 and similar standards.
The customer typically interacts with the ACDB only through the AC isolator (which they can use to manually disconnect the solar system if needed).
ACDB in commercial and utility installations
For larger commercial and utility-scale installations, ACDBs are more sophisticated:
Multiple inverter outputs combined.
Three-phase configuration standard.
Higher current ratings.
Integrated CT-based metering for SCADA integration.
Reactive power monitoring and (sometimes) compensation.
Phase imbalance detection.
Compliance with grid code requirements.
For HT consumers, the ACDB may also include voltage transformers (VTs) for grid monitoring.
Common ACDB mistakes
Undersizing the MCB. Nuisance tripping during normal operation.
Using Type AC RCD instead of Type B. Type B is required for solar applications because solar inverters can produce smooth DC leakage currents that Type AC misses.
Skipping SPD. Lightning damage to inverters is expensive; SPD at ACDB protects the inverter.
Improper earthing. Without low-resistance ground, SPD cannot function.
Mounting the ACDB in direct sun without shading. Internal temperature can exceed component ratings.
Confusing ACDB and DCDB. They serve different functions on different sides of the inverter.
Best practices
For new installations, ensure ACDB is properly sized to the inverter rating.
Use Type B RCD for solar applications.
Install Type 2 SPD as a minimum; Type 1 if direct lightning protection is needed.
Mount ACDB in shaded locations or with sun shields.
Verify proper earthing of the ACDB enclosure and SPD ground.
For commercial installations, integrate ACDB with SCADA for monitoring.
For maintenance, inspect ACDB annually for connection torque, MCB and RCD condition, and SPD status.
Standards and references
ACDB construction follows IEC 61439-1, IS 13947 (Indian standard for low voltage switchgear), and CEA Connectivity Regulations 2019. SPDs follow IEC 61643. RCD requirements are specified by national wiring regulations and IEC 61008.
Related glossary terms
Key takeaways
ACDB (AC Distribution Box) is the electrical enclosure between the solar inverter’s AC output and the grid that houses AC isolators, MCBs, RCDs, SPDs, and sometimes energy meters. It provides protection, isolation, metering, and compliance for grid-connected solar installations. ACDB is essential for safe and code-compliant solar; every grid-tied installation in India requires one. Proper sizing, Type B RCD for solar applications, Type 2 SPD, and reliable earthing are critical specifications. ACDB and DCDB serve similar functions on opposite sides of the inverter.