Description

SPD DC Surge Protective Devices

DC Surge Protective Device (SPD) is a safety component that protects electrical equipment from over-voltage surges on DC lines — especially solar PV (photovoltaic) arrays.
When lightning strikes or there’s a sudden spike in voltage, the SPD diverts excess energy to ground so it doesn’t damage your inverter, charge controller, batteries, or other DC equipment.

In solar power systems, DC SPDs are installed between the PV panels and the inverter/charge controller.

Why DC SPDs Are Important

DC electrical systems (like solar arrays) are vulnerable to:

• Lightning-induced surges

• Grid switching surges

• Electrostatic discharge

• Equipment switching events

Without protection, these surges can:

• Damage expensive components (inverter, controller, panels)

• Cause premature failure

• Create safety hazards

SPDs help limit voltage to a safe level, preserving equipment and improving safety and reliability.

Where DC SPDs Are Installed

In a typical solar power system with MPPT controller or hybrid inverter:

PV Arrays (DC) → DC SPD → Solar Charge Controller / Hybrid Inverter → Battery Bank

DC SPDs are not installed on the AC side; those require separate AC SPDs.

Types of DC SPDs

1. Type 1 DC SPD

• Rated for direct lightning current

• Installed at service entrances or where there’s exposure to direct lightning risk

• Higher energy capacity

Use-case: Large commercial systems, rooftop installations with high lightning exposure

2. Type 2 DC SPD

• Most common in solar PV systems

• Protects against indirect surges & switching transients

• Installed at PV combiner boxes or at inverter input

Use-case: Residential and commercial solar systems

3. Type 3 DC SPD

• For point-of-use protection

• Lower energy capacity than Type 1 and 2

• Used near sensitive equipment

Use-case: Additional local protection (not primary SPD)

How DC SPDs Work

When voltage surpasses a threshold:

1. SPD conducts surge current to ground

2. Voltage is clamped at a safe level

3. Once surge ends, SPD returns to normal

Key components inside SPDs:

• Metal Oxide Varistors (MOVs)

• Transient Voltage Suppression (TVS) diodes

• Sometimes gas discharge tubes

Key Specifications to Understand

 Choosing the Right DC SPD

Here’s how to pick an appropriate SPD for a solar DC system:

✔ Match the System Voltage

• 24 V battery system → SPD rated ≥ 24 V

• 48 V → SPD rated ≥ 48 V

• PV array strings (higher voltage) → SPD rated for ≥ max PV open-circuit voltage (Voc)

Important: Ensure the SPD’s Uc (continuous rating) is higher than your highest expected DC voltage.