Why DCDB & ACDB Are Mandatory in Solar Power Plant Installation (With Technical Design Explanation as per IS & IEC Standards)

Why DCDB & ACDB Are Mandatory in Solar Power Plant Installation

(With Technical Design Explanation as per IS & IEC Standards)

In every professionally engineered Solar PV system — rooftop or ground-mounted — DCDB (Direct Current Distribution Box) and ACDB (Alternating Current Distribution Box) are not optional components. They are statutory safety and protection assemblies required as per Indian and international standards.

Non-compliance can lead to:
❌ Equipment failure
❌ DC arc fire hazards
❌ Inverter damage
❌ Insurance claim rejection
❌ Net-metering approval cancellation
❌ Violation of IS / IEC standards

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🔶 1️⃣ DCDB – Direct Current Distribution Box

📌 Function (As per IS/IEC Guidelines)

Installed between PV array and inverter, DCDB ensures:

• String-level overcurrent protection

• Reverse current protection (parallel strings)

• DC isolation for maintenance

• Surge protection against lightning

• Proper DC arc interruption

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📘 Applicable Standards (DC Side)

• IS/IEC 62548 – Installation & safety of PV arrays

• IS 16221 (Part 1 & 2) – Safety of power converters (inverters)

• IS/IEC 60947-2 – MCB/MCCB standards

• IEC 61643-31 – DC Surge Protection Devices

• IS 3043 – Earthing

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🔎 Technical Rating Calculation (Example – 10 kW System)

Given:

• Module = 550 W

• Voc = 49.5 V

• Isc = 13.5 A

• 10 modules per string

• 2 strings in parallel

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➤ Step 1: Maximum String Voltage

$${𝑉}_{𝑠𝑡𝑟𝑖𝑛𝑔}=𝑉𝑜𝑐\times 𝑁𝑜\mathrm{.}𝑜𝑓𝑚𝑜𝑑𝑢𝑙𝑒𝑠$$$$=49.5\times 10=495𝑉$$

As per IS/IEC 62548, multiply by temperature correction factor (≈1.2):

$$495\times 1.2=594𝑉$$

✅ Select next standard rating: 1000V DC system

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➤ Step 2: String Current Calculation

$${𝐼}_{𝑠𝑡𝑟𝑖𝑛𝑔}=𝐼𝑠𝑐=13.5𝐴$$

As per IEC, apply 1.25 safety factor:

$$13.5\times 1.25=16.8𝐴$$

✅ Select 20A DC MCB / Fuse per string

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➤ Step 3: Output Current (2 Strings in Parallel)

$${𝐼}_{𝑡𝑜𝑡𝑎𝑙}=13.5\times 2=27𝐴$$

With safety factor:

$$27\times 1.25=33.75𝐴$$

✅ Select 40A DC Output MCB

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➤ SPD Selection (DC Side)

As per IEC 61643-31:

• Voltage rating ≥ 1000V DC

• Type II SPD for standard rooftop

• Type I+II for lightning-prone zones

• Minimum discharge capacity ≥ 20kA

• Proper earthing ≤ 1–2 ohms (as per IS 3043)

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🔷 2️⃣ ACDB – Alternating Current Distribution Box

📌 Function

Installed between inverter output and Main LT Panel / MDB, ACDB ensures:

• Overcurrent protection

• Short circuit protection

• Grid isolation

• Surge protection

• Proper discrimination & selectivity

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📘 Applicable Standards (AC Side)

• IS/IEC 60364 – Electrical installations

• IS 13947 (Part 2) – MCCB/MCB

• IEC 61643-11 – AC SPD

• CEA Grid Connectivity Regulations

• MNRE rooftop solar guidelines

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🔎 ACDB Rating Calculation (10 kW, 3-Phase Inverter)

Given:

• Power = 10,000 W

• Voltage = 415 V

• Power factor = 1

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➤ Step 1: Output Current

$$𝐼=\frac{𝑃}{\sqrt{3}\times 𝑉}$$$$𝐼=\frac{10000}{1.732\times 415}$$$$𝐼=13.9𝐴$$

Apply 25% safety margin:

$$13.9\times 1.25=17.4𝐴$$

✅ Select 25A 4P MCCB

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➤ SPD Selection (AC Side)

As per IEC 61643-11:

• Type II SPD

• 3P + N configuration

• Voltage rating: 415V

• Discharge capacity ≥ 20kA

• Neutral & earth bonding as per IS 3043

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🔥 Why DCDB & ACDB Are Non-Negotiable?

✔ Prevent DC arc flash hazards
✔ Protect inverter IGBT modules
✔ Ensure selectivity & coordination
✔ Comply with MNRE & DISCOM norms
✔ Mandatory for Net Metering approval
✔ Increase plant life (25+ years)
✔ Required for insurance compliance

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🎯 Professional Engineering Rule

If protection coordination is wrong:
→ Cable sizing becomes wrong
→ Breaker discrimination fails
→ SPD fails prematurely
→ Inverter warranty void

A Solar PV Plant is not just about panels and inverter.
It is about system protection engineering.

Proper DCDB & ACDB design =
✔ Safety
✔ Compliance
✔ Reliability
✔ Long-term performance

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