AC vs. DC Surge Protection: What's the Difference?

Surge protection is essential to ensuring the safety, longevity, and uninterrupted operation of modern electrical systems. Whether you are protecting a residential panel, a solar photovoltaic (PV) array, or an EV charging station, choosing the right surge protection device (SPD) requires understanding the critical differences between AC and DC systems. In this guide, we explore what sets AC surge protection apart from DC surge protection and how to choose the right SPD device for your application.

1. Introduction

Transient overvoltages, also known as power surges, are short-duration spikes in voltage that can damage or destroy sensitive electronic equipment. These surges can be caused by lightning strikes, grid switching operations, large load switching, or inductive equipment startups. SPD devices, also called surge suppressors or surge arresters, are engineered to detect and divert these spikes before they reach protected equipment.

But not all surges—or SPDs—are created equal. AC (Alternating Current) and DC (Direct Current) systems exhibit very different electrical behaviors and, as such, require distinct surge protection approaches.

2. What Is AC Surge Protection?

AC surge protection is designed for use in systems where the voltage alternates direction periodically (usually at 50Hz or 60Hz). These systems are typically found in homes, offices, factories, and commercial buildings.

Key Characteristics of AC SPDs:

• Operate within typical grid voltages: 115V, 230V, 400V, etc.
• Handle bi-directional waveforms due to AC polarity switching.
• Rely on the natural zero-crossing of the AC waveform for arc extinguishing.
• Often use MOVs (Metal Oxide Varistors) for energy absorption.
• Common in smart grid, industrial automation, and building distribution board systems

Typical Applications:

• Main electrical service panels
• Distribution boards
• Control panels in industrial environments
• Point-of-use protection for sensitive AC-powered devices

Example:

A Type 2 SPD device with a rated voltage of 230V and a discharge current of 20kA is commonly used in residential electrical panels, tested under IEC 61643-11 and UL 1449.

3. What Is DC Surge Protection?

DC surge protection is built to handle direct current systems, where voltage flows in one constant direction. DC surge protection devices are particularly relevant in:

• Photovoltaic systems
• Battery energy storage systems (BESS)
• Electric vehicle (EV) charging stations
• Wind turbines and telecom towers

Key Features of DC SPD Devices:

• Voltage ratings from 500V to 1500V DC
• Unidirectional surge handling with DC polarity stability
• No natural current zero-crossing, so they require fast-acting arc suppression
• High energy absorption capability using MOVs and thermal disconnects
• Must be TUV certified SPD or CE marked for most installations

Example:

Model FLY1-60PV is a DC surge protection device used in PV strings, with 60kA maximum discharge capacity and compliance with EN 50539-11.

4. What’s the Difference Between AC and DC Surge Protection?

4.1 Electrical Current Characteristics: AC vs. DC

AC reverses direction periodically (sine wave), allowing natural zero-crossing points. DC flows in a single direction continuously (flat waveform), making arc extinguishing much more challenging.

Implication:

AC SPDs benefit from zero-crossing for arc interruption, while DC SPDs require more robust arc suppression mechanisms due to waveform characteristics.

4.2 Surge Waveform

• AC surges often have smoother, sinusoidal waveforms.
• DC surges are shorter, faster, and more abrupt with flat top voltage

Response Time Needs:

DC SPDs typically require faster response times (≤25ns) to protect against rapid transients.

4.3 Design and Operating Principles of SPDs

• AC SPD: Designed for symmetrical voltage polarity and sine wave input
• DC SPD: Built for polarity-sensitive circuits and unidirectional current flow

Technology Used: Both AC and DC SPD devices commonly use MOVs (metal oxide varistors), thermal disconnects, and sometimes GDTs (gas discharge tubes). To understand how these components function inside a surge protection device, see our detailed guide on how an SPD works.

4.4 Voltage Ratings and Surge Capacity

• AC SPDs: Range from 115V to 600V
• DC SPDs: Range from 500V to 1500V

Example:

DC SPD FLY1-40PV handles up to 40kA; AC SPD USP2 ranges up to 120kA, depending on the model.

4.5 Application Environments

• AC SPD: Offices, homes, HVAC, control panels
• DC SPD: Renewable energy, outdoor telecom, transportation systems

Environmental Tolerance:

DC SPDs usually support wider temperature ranges and relative humidity due to outdoor applications.

4.6 Arc Extinguishing

• AC: Arc naturally extinguishes at the zero-crossing
• DC: Requires mechanical/thermal arc quenching due to lack of polarity reversal.

Why It Matters:

Inadequate arc suppression in DC surge arresters can lead to thermal runaway and fire.

4.7 Standards, Testing, and Certification

• AC SPD Standards: UL 1449, IEC 61643-11 (Type 1, 2, 3)
• DC SPD Standards: EN 50539-11, IEC 61643-31, UL 1449 (PV, BESS, EV focus)

Example:

Surge protection class must be verified via TUV-certified SPD labels for grid compliance.

5. How to Choose the Right SPD

5.1 System Voltage

• Match the SPD device voltage rating to the system requirements.
• For solar PV, use 1000V or 1500V rated DC surge protection devices

5.2 Waveform and Response Time

• MOV and varistor performance should align with sine wave vs. flat DC profiles
• Faster MOVs are preferred for DC.

Tip:

Select SPDs with ≤25ns response for DC, <50ns for general AC applications

5.3 Connection Point

• Install SPDs close to the transient source (e.g., inverter input, combiner box)
• Consider layered surge suppressor setups for distributed energy systems

5.4 Certification Requirements

• Must meet UL 1449, IEC 61643-11, or EN 50539-11, depending on system
• TUV-certified SPD preferred for industrial automation and utility-scale applications

Circuit Breaker Coordination:

Fuse or MCB sizing must support surge currents (e.g., 32A/63A) and be compatible with SPD disconnection needs

6. Conclusion

AC and DC surge protection devices may look similar, but they serve fundamentally different systems. AC SPDs are optimized for sine wave applications in buildings and grids. DC SPDs are tailored for flat voltage waveforms in renewable and high-voltage DC infrastructures.

Choosing the correct SPD is essential to safeguarding equipment and maintaining compliance with safety standards.

To ensure optimal protection:

• Always match the SPD type to the current system (AC or DC)
• Consider voltage rating, waveform characteristics, and required energy absorption capability.
• Choose certified products (TUV, UL, CE) with appropriate surge protection class ratings.

Need help selecting your SPD? Contact a certified electrical professional or surge protection provider to find the right DC surge protection or AC surge protection solution for your system.