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

Surge protection is fundamental to the safety and uptime of modern electrical systems. Choosing the right Surge Protective Device (SPD) starts with understanding how AC and DC systems behave differently under transients—especially arc extinction (AC zero-crossing vs DC no zero-crossing) and component design (bi-directional vs uni-directional MOVs). This guide explains the difference between AC SPD and DC SPD, maps the IEC/UL standards that apply, and shows how to select an SPD that meets your application and compliance needs.

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. Per IEC 61643-11/-31, an SPD limits surge voltage and diverts surge current within nanoseconds, protecting downstream equipment.

2. What Is AC Surge Protection?

AC surge protection is designed for electrical systems where voltage alternates direction periodically—typically at 50Hz or 60Hz. These systems are widely used in residential homes, commercial buildings, factories, and industrial automation networks. According to IEC 61643-11, AC SPDs are classified into Type 1, Type 2, and Type 3, each intended for a different installation point within the distribution system.

Key Characteristics of AC SPDs

• Operate within common grid voltages such as 115V, 230V, 400V, and up to 690V AC.

• Bi-directional operation to handle polarity switching of AC sine waves.

• Arc extinction through natural zero-crossings of the AC waveform, which makes disconnection safer and more efficient.

• Commonly used MOVs (Metal Oxide Varistors) to absorb surge energy; in Type 1 SPDs, MOVs are often combined with GDTs (Gas Discharge Tubes) or arc chambers to withstand high-energy lightning impulses (10/350 µs waveforms).

• Coordination with upstream protection (MCB or fuse) is required; best practice is to keep the SPD lead length as short as possible, ideally<0.5 m, to minimize residual voltage.

SPD Types for AC Applications

• Type 1 SPD – Installed at the service entrance; withstands direct lightning currents (Iimp 10/350 µs).

• Type 2 SPD – Installed at distribution boards; protects against switching surges and indirect lightning (In/Imax 8/20 µs).

• Type 3 SPD – Installed close to sensitive equipment; provides fine protection against residual surges.

Typical Applications

• Main service entrance panels (Type 1)

• Building distribution boards (Type 2)

• Control panels in industrial environments (Type 2)

• Point-of-use or socket-level protection for sensitive AC-powered devices (Type 3)

Example from Britec AC SPD Product Range

• BR-50GR (Type 1): Rated for Uc 275/320/440 V, with Iimp up to 25 kA (10/350 µs); ideal for service entrance lightning protection in TN/TT systems.

• BR-12.5M (Type 1+2 hybrid): Handles both lightning impulse and switching surges, making it suitable for industrial facilities.

• BR-40 and BR-30FU (Type 2): Rated for Uc 275 V, with In 20 kA / Imax 40 kA (8/20 µs); the BR-30FU integrates a backup fuse, simplifying installation and improving safety.

• BR-230/BR275-6 (Type 3): Designed for terminal equipment protection, with low residual voltage (Up ≤ 1.5 kV).

3. What Is DC Surge Protection?

DC surge protection is designed for direct current systems, where electricity flows in a single direction without natural zero-crossings. Unlike AC systems that benefit from periodic voltage reversals, DC systems present a unique engineering challenge: arcs do not self-extinguish. This means that DC SPDs require more robust arc quenching, thermal disconnection, and mechanical release mechanisms to ensure safety and prevent thermal runaway.

Key Features of DC SPDs

• Voltage Ratings: Typically designed for 500 Vdc to 1500 Vdc, covering solar PV arrays, battery storage systems (BESS), and DC fast-charging networks.

• Unidirectional MOV selection: To accommodate DC polarity, DC SPDs use unidirectional MOVs, ensuring correct surge clamping for one-way current flow.

• Arc Suppression: Because DC lacks a zero-crossing, DC SPDs integrate fast-acting thermal disconnects, gas discharge tubes (GDTs), or arc chambers to safely break fault currents.

• High Surge Handling: Built to absorb high-energy surges with In/Imax ratings up to 20–40 kA (8/20 µs) and Type 1+2 devices rated for Iimp lightning impulses.

• Compliance and Certification:

  • Must comply with IEC 61643-31 (for PV DC SPDs) or UL 1449 (for DC systems in North America, up to 1500 Vdc).

  • Depending on project or regional requirements, products may carry additional TÜV, CB, or UL certifications for grid compliance and safety audits.

Typical Applications of DC Surge Protection

• Solar PV arrays (string or combiner box protection, typically 1000 Vdc or 1500 Vdc).

• Battery Energy Storage Systems (BESS) for grid-scale or microgrid solutions.

• Electric Vehicle (EV) charging stations, especially DC fast charging.

• Telecom towers and renewable energy systems (wind turbine converters, hybrid DC networks).

Examples from Britec’s DC SPD Range

• BRPV3-1000 (Type 1+2 DC SPD)

  • Voltage rating: Uc = 1000 Vdc

  • Nominal discharge current: In = 20 kA

  • Maximum discharge current: Imax = 40 kA (8/20 µs)

  • Voltage protection level: Up ≤ 3.8 kV

  • Features: Thermal disconnector + remote signaling for real-time monitoring.

• BRPV3-1500 (Type 1+2 DC SPD)

  • Voltage rating: Uc = 1500 Vdc

  • Nominal discharge current: In = 20 kA

  • Maximum discharge current: Imax = 40 kA

  • Voltage protection level: Up ≤ 4.5 kV

  • Features: Arc chamber design + thermal cut-off, ensuring reliable operation in PV plants and EV infrastructure.

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

Although AC SPDs and DC SPDs look similar at first glance, their design principles, standards, and application requirements are very different. The table below summarizes the key differences between AC SPD and DC SPD:

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

Selecting the correct Surge Protective Device (SPD) is not only about voltage rating – it also involves installation position, protection coordination, and compliance with international standards. Here are the key factors to consider:

1. Match the SPD to Your System Voltage

• AC systems (IEC 61643-11 / UL 1449)

  • 230/400 V (TN/TT grids): Choose Uc 275 V models.

  • Example: Britec BR-40/30FU – Type 2 SPD, Uc 275 V, In 20 kA, Imax 40 kA, with integrated fuse for backup protection.

• PV DC systems (IEC 61643-31 / UL 1449 ≤1500 Vdc)

  • 600 Vdc, 1000 Vdc, or 1500 Vdc arrays: Select matching DC Uc rating.

  • Example: Britec BRPV3-1000 (Uc 1000 Vdc) or BRPV3-1500 (Uc 1500 Vdc) – Type 1+2 DC SPDs for PV combiner boxes and inverters.

Tip: Always check that the SPD’s Uc (continuous operating voltage) is ≥ 1.2 × nominal system voltage to avoid premature ageing.

2. Apply the Right Protection Class (Type) to Each Installation Level

• Type 1 (10/350 µs lightning current)

  • Installed at service entrance / main distribution.

  • Handles direct lightning currents.

  • Example: BR-12.5M Type 1+2 SPD, Iimp 12.5 kA.

• Type 2 (8/20 µs switching surges)

  • Installed at floor distribution boards.

  • Protects against switching surges and residual lightning currents.

  • Example: BR-40/30FU Type 2 SPD (with integrated disconnector).

• Type 3 (point-of-use)

  • Installed close to sensitive equipment (computers, servers, medical devices).

  • Protects against residual surges not clamped by upstream SPDs.

Engineering rule: If the cable length between two protection stages is >10 m, install additional SPDs at both ends to ensure coordination.

3. Ensure Coordination With Breakers & Fuses

• SPDs must be coordinated with upstream MCB/RCB/backup fuses.

• Some models, like Britec BR-40/30FU, integrate thermal disconnectors and backup protection, reducing space and cost.

4. Compliance With Standards

• AC SPDs: IEC 61643-11, UL 1449

• PV DC SPDs: IEC 61643-31, UL 1449 (≤1500 Vdc)

Always ensure the SPD carries the proper CE, UL, or IEC test certifications, especially for international projects.

Key Takeaway

Choosing the right SPD means matching the voltage, selecting the correct Type per installation level, ensuring proper coordination, and complying with IEC/UL standards.

With Britec’s portfolio (BR-12.5M, BR-40/30FU, BRPV3-1000, BRPV3-1500), you can cover residential, commercial, industrial, and PV solar applications with certified, high-performance surge protection.

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.